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1****************************
2  What's New In Python 3.4
3****************************
4
5:Author: R. David Murray <rdmurray@bitdance.com> (Editor)
6
7.. Rules for maintenance:
8
9   * Anyone can add text to this document, but the maintainer reserves the
10   right to rewrite any additions. In particular, for obscure or esoteric
11   features, the maintainer may reduce any addition to a simple reference to
12   the new documentation rather than explaining the feature inline.
13
14   * While the maintainer will periodically go through Misc/NEWS
15   and add changes, it's best not to rely on this. We know from experience
16   that any changes that aren't in the What's New documentation around the
17   time of the original release will remain largely unknown to the community
18   for years, even if they're added later. We also know from experience that
19   other priorities can arise, and the maintainer will run out of time to do
20   updates -- in such cases, end users will be much better served by partial
21   notifications that at least give a hint about new features to
22   investigate.
23
24   * This is not a complete list of every single change; completeness
25   is the purpose of Misc/NEWS. The What's New should focus on changes that
26   are visible to Python *users* and that *require* a feature release (i.e.
27   most bug fixes should only be recorded in Misc/NEWS)
28
29   * PEPs should not be marked Final until they have an entry in What's New.
30   A placeholder entry that is just a section header and a link to the PEP
31   (e.g ":pep:`397` has been implemented") is acceptable. If a PEP has been
32   implemented and noted in What's New, don't forget to mark it as Final!
33
34   * If you want to draw your new text to the attention of the
35   maintainer, add 'XXX' to the beginning of the paragraph or
36   section.
37
38   * It's OK to add just a very brief note about a change.  For
39   example: "The :ref:`~socket.transmogrify()` function was added to the
40   :mod:`socket` module."  The maintainer will research the change and
41   write the necessary text (if appropriate). The advantage of doing this
42   is that even if no more descriptive text is ever added, readers will at
43   least have a notification that the new feature exists and a link to the
44   relevant documentation.
45
46   * You can comment out your additions if you like, but it's not
47   necessary (especially when a final release is some months away).
48
49   * Credit the author of a patch or bugfix.   Just the name is
50   sufficient; the e-mail address isn't necessary.
51
52   * It's helpful to add the bug/patch number as a comment:
53
54   The :ref:`~socket.transmogrify()` function was added to the
55   :mod:`socket` module.  (Contributed by P.Y. Developer in :issue:`12345`.)
56
57   This saves the maintainer the effort of going through the Mercurial log
58   when researching a change.
59
60   * Cross referencing tip: :ref:`mod.attr` will display as ``mod.attr``,
61   while :ref:`~mod.attr` will display as ``attr``.
62
63This article explains the new features in Python 3.4, compared to 3.3.
64Python 3.4 was released on March 16, 2014.  For full details, see the
65`changelog <https://docs.python.org/3.4/whatsnew/changelog.html>`_.
66
67
68.. seealso::
69
70   :pep:`429` -- Python 3.4 Release Schedule
71
72
73
74Summary -- Release Highlights
75=============================
76
77.. This section singles out the most important changes in Python 3.4.
78   Brevity is key.
79
80New syntax features:
81
82* No new syntax features were added in Python 3.4.
83
84Other new features:
85
86* :ref:`pip should always be available <whatsnew-pep-453>` (:pep:`453`).
87* :ref:`Newly created file descriptors are non-inheritable <whatsnew-pep-446>`
88  (:pep:`446`).
89* command line option for :ref:`isolated mode <whatsnew-isolated-mode>`
90  (:issue:`16499`).
91* :ref:`improvements in the handling of codecs <codec-handling-improvements>`
92  that are not text encodings (multiple issues).
93* :ref:`A ModuleSpec Type <whatsnew-pep-451>` for the Import System
94  (:pep:`451`).  (Affects importer authors.)
95* The :mod:`marshal` format has been made :ref:`more compact and efficient
96  <whatsnew-marshal-3>` (:issue:`16475`).
97
98New library modules:
99
100* :mod:`asyncio`: :ref:`New provisional API for asynchronous IO
101  <whatsnew-asyncio>` (:pep:`3156`).
102* :mod:`ensurepip`: :ref:`Bootstrapping the pip installer <whatsnew-ensurepip>`
103  (:pep:`453`).
104* :mod:`enum`: :ref:`Support for enumeration types <whatsnew-enum>`
105  (:pep:`435`).
106* :mod:`pathlib`: :ref:`Object-oriented filesystem paths <whatsnew-pathlib>`
107  (:pep:`428`).
108* :mod:`selectors`: :ref:`High-level and efficient I/O multiplexing
109  <whatsnew-selectors>`, built upon the :mod:`select` module primitives (part
110  of :pep:`3156`).
111* :mod:`statistics`: A basic :ref:`numerically stable statistics library
112  <whatsnew-statistics>` (:pep:`450`).
113* :mod:`tracemalloc`: :ref:`Trace Python memory allocations
114  <whatsnew-tracemalloc>` (:pep:`454`).
115
116Significantly improved library modules:
117
118* :ref:`Single-dispatch generic functions <whatsnew-singledispatch>` in
119  :mod:`functools` (:pep:`443`).
120* New :mod:`pickle` :ref:`protocol 4 <whatsnew-protocol-4>` (:pep:`3154`).
121* :mod:`multiprocessing` now has :ref:`an option to avoid using os.fork
122  on Unix <whatsnew-multiprocessing-no-fork>` (:issue:`8713`).
123* :mod:`email` has a new submodule, :mod:`~email.contentmanager`, and
124  a new :mod:`~email.message.Message` subclass
125  (:class:`~email.contentmanager.EmailMessage`) that :ref:`simplify MIME
126  handling <whatsnew_email_contentmanager>` (:issue:`18891`).
127* The :mod:`inspect` and :mod:`pydoc` modules are now capable of
128  correct introspection of a much wider variety of callable objects,
129  which improves the output of the Python :func:`help` system.
130* The :mod:`ipaddress` module API has been declared stable
131
132Security improvements:
133
134* :ref:`Secure and interchangeable hash algorithm <whatsnew-pep-456>`
135  (:pep:`456`).
136* :ref:`Make newly created file descriptors non-inheritable <whatsnew-pep-446>`
137  (:pep:`446`) to avoid leaking file descriptors to child processes.
138* New command line option for :ref:`isolated mode <whatsnew-isolated-mode>`,
139  (:issue:`16499`).
140* :mod:`multiprocessing` now has :ref:`an option to avoid using os.fork
141  on Unix <whatsnew-multiprocessing-no-fork>`.  *spawn* and *forkserver* are
142  more secure because they avoid sharing data with child processes.
143* :mod:`multiprocessing` child processes on Windows no longer inherit
144  all of the parent's inheritable handles, only the necessary ones.
145* A new :func:`hashlib.pbkdf2_hmac` function provides
146  the `PKCS#5 password-based key derivation function 2
147  <https://en.wikipedia.org/wiki/PBKDF2>`_.
148* :ref:`TLSv1.1 and TLSv1.2 support <whatsnew-tls-11-12>` for :mod:`ssl`.
149* :ref:`Retrieving certificates from the Windows system cert store support
150  <whatsnew34-win-cert-store>` for :mod:`ssl`.
151* :ref:`Server-side SNI (Server Name Indication) support
152  <whatsnew34-sni>` for :mod:`ssl`.
153* The :class:`ssl.SSLContext` class has a :ref:`lot of improvements
154  <whatsnew34-sslcontext>`.
155* All modules in the standard library that support SSL now support server
156  certificate verification, including hostname matching
157  (:func:`ssl.match_hostname`) and CRLs (Certificate Revocation lists, see
158  :func:`ssl.SSLContext.load_verify_locations`).
159
160CPython implementation improvements:
161
162* :ref:`Safe object finalization <whatsnew-pep-442>` (:pep:`442`).
163* Leveraging :pep:`442`, in most cases :ref:`module globals are no longer set
164  to None during finalization <whatsnew-pep-442>` (:issue:`18214`).
165* :ref:`Configurable memory allocators <whatsnew-pep-445>` (:pep:`445`).
166* :ref:`Argument Clinic <whatsnew-pep-436>` (:pep:`436`).
167
168Please read on for a comprehensive list of user-facing changes, including many
169other smaller improvements, CPython optimizations, deprecations, and potential
170porting issues.
171
172
173
174New Features
175============
176
177.. _whatsnew-pep-453:
178
179PEP 453: Explicit Bootstrapping of PIP in Python Installations
180--------------------------------------------------------------
181
182Bootstrapping pip By Default
183~~~~~~~~~~~~~~~~~~~~~~~~~~~~
184
185The new :mod:`ensurepip` module (defined in :pep:`453`) provides a standard
186cross-platform mechanism to bootstrap the pip installer into Python
187installations and virtual environments. The version of ``pip`` included
188with Python 3.4.0 is ``pip`` 1.5.4, and future 3.4.x maintenance releases
189will update the bundled version to the latest version of ``pip`` that is
190available at the time of creating the release candidate.
191
192By default, the commands ``pipX`` and ``pipX.Y`` will be installed on all
193platforms (where X.Y stands for the version of the Python installation),
194along with the ``pip`` Python package and its dependencies. On Windows and
195in virtual environments on all platforms, the unversioned ``pip`` command
196will also be installed. On other platforms, the system wide unversioned
197``pip`` command typically refers to the separately installed Python 2
198version.
199
200The ``pyvenv`` command line utility and the :mod:`venv`
201module make use of the :mod:`ensurepip` module to make ``pip`` readily
202available in virtual environments. When using the command line utility,
203``pip`` is installed by default, while when using the :mod:`venv` module
204:ref:`venv-api` installation of ``pip`` must be requested explicitly.
205
206For CPython :ref:`source builds on POSIX systems <building-python-on-unix>`,
207the ``make install`` and ``make altinstall`` commands bootstrap ``pip`` by
208default.  This behaviour can be controlled through configure options, and
209overridden through Makefile options.
210
211On Windows and Mac OS X, the CPython installers now default to installing
212``pip`` along with CPython itself (users may opt out of installing it
213during the installation process). Window users will need to opt in to the
214automatic ``PATH`` modifications to have ``pip`` available from the command
215line by default, otherwise it can still be accessed through the Python
216launcher for Windows as ``py -m pip``.
217
218As `discussed in the PEP`__, platform packagers may choose not to install
219these commands by default, as long as, when invoked, they provide clear and
220simple directions on how to install them on that platform (usually using
221the system package manager).
222
223__ https://www.python.org/dev/peps/pep-0453/#recommendations-for-downstream-distributors
224
225.. note::
226
227   To avoid conflicts between parallel Python 2 and Python 3 installations,
228   only the versioned ``pip3`` and ``pip3.4`` commands are bootstrapped by
229   default when ``ensurepip`` is invoked directly - the ``--default-pip``
230   option is needed to also request the unversioned ``pip`` command.
231   ``pyvenv`` and the Windows installer ensure that the unqualified ``pip``
232   command is made available in those environments, and ``pip`` can always be
233   invoked via the ``-m`` switch rather than directly to avoid ambiguity on
234   systems with multiple Python installations.
235
236
237Documentation Changes
238~~~~~~~~~~~~~~~~~~~~~
239
240As part of this change, the :ref:`installing-index` and
241:ref:`distributing-index` sections of the documentation have been
242completely redesigned as short getting started and FAQ documents. Most
243packaging documentation has now been moved out to the Python Packaging
244Authority maintained `Python Packaging User Guide
245<https://packaging.python.org>`__ and the documentation of the individual
246projects.
247
248However, as this migration is currently still incomplete, the legacy
249versions of those guides remaining available as :ref:`install-index`
250and :ref:`distutils-index`.
251
252.. seealso::
253
254   :pep:`453` -- Explicit bootstrapping of pip in Python installations
255      PEP written by Donald Stufft and Nick Coghlan, implemented by
256      Donald Stufft, Nick Coghlan, Martin von Löwis and Ned Deily.
257
258
259.. _whatsnew-pep-446:
260
261PEP 446: Newly Created File Descriptors Are Non-Inheritable
262-----------------------------------------------------------
263
264:pep:`446` makes newly created file descriptors :ref:`non-inheritable
265<fd_inheritance>`.  In general, this is the behavior an application will
266want: when launching a new process, having currently open files also
267open in the new process can lead to all sorts of hard to find bugs,
268and potentially to security issues.
269
270However, there are occasions when inheritance is desired.  To support
271these cases, the following new functions and methods are available:
272
273* :func:`os.get_inheritable`, :func:`os.set_inheritable`
274* :func:`os.get_handle_inheritable`, :func:`os.set_handle_inheritable`
275* :meth:`socket.socket.get_inheritable`, :meth:`socket.socket.set_inheritable`
276
277.. seealso::
278
279   :pep:`446` -- Make newly created file descriptors non-inheritable
280      PEP written and implemented by Victor Stinner.
281
282
283.. _codec-handling-improvements:
284
285Improvements to Codec Handling
286------------------------------
287
288Since it was first introduced, the :mod:`codecs` module has always been
289intended to operate as a type-neutral dynamic encoding and decoding
290system. However, its close coupling with the Python text model, especially
291the type restricted convenience methods on the builtin :class:`str`,
292:class:`bytes` and :class:`bytearray` types, has historically obscured that
293fact.
294
295As a key step in clarifying the situation, the :meth:`codecs.encode` and
296:meth:`codecs.decode` convenience functions are now properly documented in
297Python 2.7, 3.3 and 3.4. These functions have existed in the :mod:`codecs`
298module (and have been covered by the regression test suite) since Python 2.4,
299but were previously only discoverable through runtime introspection.
300
301Unlike the convenience methods on :class:`str`, :class:`bytes` and
302:class:`bytearray`, the :mod:`codecs` convenience functions support arbitrary
303codecs in both Python 2 and Python 3, rather than being limited to Unicode text
304encodings (in Python 3) or ``basestring`` <-> ``basestring`` conversions (in
305Python 2).
306
307In Python 3.4, the interpreter is able to identify the known non-text
308encodings provided in the standard library and direct users towards these
309general purpose convenience functions when appropriate::
310
311    >>> b"abcdef".decode("hex")
312    Traceback (most recent call last):
313      File "<stdin>", line 1, in <module>
314    LookupError: 'hex' is not a text encoding; use codecs.decode() to handle arbitrary codecs
315
316    >>> "hello".encode("rot13")
317    Traceback (most recent call last):
318      File "<stdin>", line 1, in <module>
319    LookupError: 'rot13' is not a text encoding; use codecs.encode() to handle arbitrary codecs
320
321    >>> open("foo.txt", encoding="hex")
322    Traceback (most recent call last):
323      File "<stdin>", line 1, in <module>
324    LookupError: 'hex' is not a text encoding; use codecs.open() to handle arbitrary codecs
325
326In a related change, whenever it is feasible without breaking backwards
327compatibility, exceptions raised during encoding and decoding operations
328are wrapped in a chained exception of the same type that mentions the
329name of the codec responsible for producing the error::
330
331    >>> import codecs
332
333    >>> codecs.decode(b"abcdefgh", "hex")
334    Traceback (most recent call last):
335      File "/usr/lib/python3.4/encodings/hex_codec.py", line 20, in hex_decode
336        return (binascii.a2b_hex(input), len(input))
337    binascii.Error: Non-hexadecimal digit found
338
339    The above exception was the direct cause of the following exception:
340
341    Traceback (most recent call last):
342      File "<stdin>", line 1, in <module>
343    binascii.Error: decoding with 'hex' codec failed (Error: Non-hexadecimal digit found)
344
345    >>> codecs.encode("hello", "bz2")
346    Traceback (most recent call last):
347      File "/usr/lib/python3.4/encodings/bz2_codec.py", line 17, in bz2_encode
348        return (bz2.compress(input), len(input))
349      File "/usr/lib/python3.4/bz2.py", line 498, in compress
350        return comp.compress(data) + comp.flush()
351    TypeError: 'str' does not support the buffer interface
352
353    The above exception was the direct cause of the following exception:
354
355    Traceback (most recent call last):
356      File "<stdin>", line 1, in <module>
357    TypeError: encoding with 'bz2' codec failed (TypeError: 'str' does not support the buffer interface)
358
359Finally, as the examples above show, these improvements have permitted
360the restoration of the convenience aliases for the non-Unicode codecs that
361were themselves restored in Python 3.2. This means that encoding binary data
362to and from its hexadecimal representation (for example) can now be written
363as::
364
365    >>> from codecs import encode, decode
366    >>> encode(b"hello", "hex")
367    b'68656c6c6f'
368    >>> decode(b"68656c6c6f", "hex")
369    b'hello'
370
371The binary and text transforms provided in the standard library are detailed
372in :ref:`binary-transforms` and :ref:`text-transforms`.
373
374(Contributed by Nick Coghlan in :issue:`7475`, :issue:`17827`,
375:issue:`17828` and :issue:`19619`.)
376
377
378.. _whatsnew-pep-451:
379
380PEP 451: A ModuleSpec Type for the Import System
381------------------------------------------------
382
383:pep:`451` provides an encapsulation of the information about a module that the
384import machinery will use to load it (that is, a module specification).  This
385helps simplify both the import implementation and several import-related APIs.
386The change is also a stepping stone for `several future import-related
387improvements`__.
388
389__ https://mail.python.org/pipermail/python-dev/2013-November/130111.html
390
391The public-facing changes from the PEP are entirely backward-compatible.
392Furthermore, they should be transparent to everyone but importer authors.  Key
393finder and loader methods have been deprecated, but they will continue working.
394New importers should use the new methods described in the PEP.  Existing
395importers should be updated to implement the new methods.  See the
396:ref:`deprecated-3.4` section for a list of methods that should be replaced and
397their replacements.
398
399
400Other Language Changes
401----------------------
402
403Some smaller changes made to the core Python language are:
404
405* Unicode database updated to UCD version 6.3.
406
407* :func:`min` and :func:`max` now accept a *default* keyword-only argument that
408  can be used to specify the value they return if the iterable they are
409  evaluating has no elements.  (Contributed by Julian Berman in
410  :issue:`18111`.)
411
412* Module objects are now :mod:`weakref`'able.
413
414* Module ``__file__`` attributes (and related values) should now always
415  contain absolute paths by default, with the sole exception of
416  ``__main__.__file__`` when a script has been executed directly using
417  a relative path.  (Contributed by Brett Cannon in :issue:`18416`.)
418
419* All the UTF-\* codecs (except UTF-7) now reject surrogates during both
420  encoding and decoding unless the ``surrogatepass`` error handler is used,
421  with the exception of the UTF-16 decoder (which accepts valid surrogate pairs)
422  and the UTF-16 encoder (which produces them while encoding non-BMP characters).
423  (Contributed by Victor Stinner, Kang-Hao (Kenny) Lu and Serhiy Storchaka in
424  :issue:`12892`.)
425
426* New German EBCDIC :ref:`codec <standard-encodings>` ``cp273``.  (Contributed
427  by Michael Bierenfeld and Andrew Kuchling in :issue:`1097797`.)
428
429* New Ukrainian :ref:`codec <standard-encodings>` ``cp1125``.  (Contributed by
430  Serhiy Storchaka in :issue:`19668`.)
431
432* :class:`bytes`.join() and :class:`bytearray`.join() now accept arbitrary
433  buffer objects as arguments.  (Contributed by Antoine Pitrou in
434  :issue:`15958`.)
435
436* The :class:`int` constructor now accepts any object that has an ``__index__``
437  method for its *base* argument.  (Contributed by Mark Dickinson in
438  :issue:`16772`.)
439
440* Frame objects now have a :func:`~frame.clear` method that clears all
441  references to local variables from the frame.  (Contributed by Antoine Pitrou
442  in :issue:`17934`.)
443
444* :class:`memoryview` is now registered as a :class:`Sequence <collections.abc>`,
445  and supports the :func:`reversed` builtin.  (Contributed by Nick Coghlan
446  and Claudiu Popa in :issue:`18690` and :issue:`19078`.)
447
448* Signatures reported by :func:`help` have been modified and improved in
449  several cases as a result of the introduction of Argument Clinic and other
450  changes to the :mod:`inspect` and :mod:`pydoc` modules.
451
452* :meth:`~object.__length_hint__` is now part of the formal language
453  specification (see :pep:`424`).  (Contributed by Armin Ronacher in
454  :issue:`16148`.)
455
456
457New Modules
458===========
459
460
461.. _whatsnew-asyncio:
462
463asyncio
464-------
465
466The new :mod:`asyncio` module (defined in :pep:`3156`) provides a standard
467pluggable event loop model for Python, providing solid asynchronous IO
468support in the standard library, and making it easier for other event loop
469implementations to interoperate with the standard library and each other.
470
471For Python 3.4, this module is considered a :term:`provisional API`.
472
473.. seealso::
474
475   :pep:`3156` -- Asynchronous IO Support Rebooted: the "asyncio" Module
476      PEP written and implementation led by Guido van Rossum.
477
478
479.. _whatsnew-ensurepip:
480
481ensurepip
482---------
483
484The new :mod:`ensurepip` module is the primary infrastructure for the
485:pep:`453` implementation.  In the normal course of events end users will not
486need to interact with this module, but it can be used to manually bootstrap
487``pip`` if the automated bootstrapping into an installation or virtual
488environment was declined.
489
490:mod:`ensurepip` includes a bundled copy of ``pip``, up-to-date as of the first
491release candidate of the release of CPython with which it ships (this applies
492to both maintenance releases and feature releases).  ``ensurepip`` does not
493access the internet.  If the installation has Internet access, after
494``ensurepip`` is run the bundled ``pip`` can be used to upgrade ``pip`` to a
495more recent release than the bundled one.  (Note that such an upgraded version
496of ``pip`` is considered to be a separately installed package and will not be
497removed if Python is uninstalled.)
498
499The module is named *ensure*\ pip because if called when ``pip`` is already
500installed, it does nothing.  It also has an ``--upgrade`` option that will
501cause it to install the bundled copy of ``pip`` if the existing installed
502version of ``pip`` is older than the bundled copy.
503
504
505.. _whatsnew-enum:
506
507enum
508----
509
510The new :mod:`enum` module (defined in :pep:`435`) provides a standard
511implementation of enumeration types, allowing other modules (such as
512:mod:`socket`) to provide more informative error messages and better
513debugging support by replacing opaque integer constants with backwards
514compatible enumeration values.
515
516.. seealso::
517
518   :pep:`435` -- Adding an Enum type to the Python standard library
519      PEP written by Barry Warsaw, Eli Bendersky and Ethan Furman,
520      implemented by Ethan Furman.
521
522
523.. _whatsnew-pathlib:
524
525pathlib
526-------
527
528The new :mod:`pathlib` module offers classes representing filesystem paths
529with semantics appropriate for different operating systems.  Path classes are
530divided between *pure paths*, which provide purely computational operations
531without I/O, and *concrete paths*, which inherit from pure paths but also
532provide I/O operations.
533
534For Python 3.4, this module is considered a :term:`provisional API`.
535
536.. seealso::
537
538   :pep:`428` -- The pathlib module -- object-oriented filesystem paths
539      PEP written and implemented by Antoine Pitrou.
540
541
542.. _whatsnew-selectors:
543
544selectors
545---------
546
547The new :mod:`selectors` module (created as part of implementing :pep:`3156`)
548allows high-level and efficient I/O multiplexing, built upon the
549:mod:`select` module primitives.
550
551
552.. _whatsnew-statistics:
553
554statistics
555----------
556
557The new :mod:`statistics` module (defined in :pep:`450`) offers some core
558statistics functionality directly in the standard library. This module
559supports calculation of the mean, median, mode, variance and standard
560deviation of a data series.
561
562.. seealso::
563
564   :pep:`450` -- Adding A Statistics Module To The Standard Library
565      PEP written and implemented by Steven D'Aprano
566
567.. _whatsnew-tracemalloc:
568
569
570tracemalloc
571-----------
572
573The new :mod:`tracemalloc` module (defined in :pep:`454`) is a debug tool to
574trace memory blocks allocated by Python. It provides the following information:
575
576* Trace where an object was allocated
577* Statistics on allocated memory blocks per filename and per line number:
578  total size, number and average size of allocated memory blocks
579* Compute the differences between two snapshots to detect memory leaks
580
581.. seealso::
582
583   :pep:`454` -- Add a new tracemalloc module to trace Python memory allocations
584      PEP written and implemented by Victor Stinner
585
586
587
588Improved Modules
589================
590
591
592abc
593---
594
595New function :func:`abc.get_cache_token` can be used to know when to invalidate
596caches that are affected by changes in the object graph.  (Contributed
597by Łukasz Langa in :issue:`16832`.)
598
599New class :class:`~abc.ABC` has :class:`~abc.ABCMeta` as its meta class.
600Using ``ABC`` as a base class has essentially the same effect as specifying
601``metaclass=abc.ABCMeta``, but is simpler to type and easier to read.
602(Contributed by Bruno Dupuis in :issue:`16049`.)
603
604
605aifc
606----
607
608The :meth:`~aifc.aifc.getparams` method now returns a namedtuple rather than a
609plain tuple.  (Contributed by Claudiu Popa in :issue:`17818`.)
610
611:func:`aifc.open` now supports the context management protocol: when used in a
612:keyword:`with` block, the :meth:`~aifc.aifc.close` method of the returned
613object will be called automatically at the end of the block.  (Contributed by
614Serhiy Storchacha in :issue:`16486`.)
615
616The :meth:`~aifc.aifc.writeframesraw` and :meth:`~aifc.aifc.writeframes`
617methods now accept any :term:`bytes-like object`.  (Contributed by Serhiy
618Storchaka in :issue:`8311`.)
619
620
621argparse
622--------
623
624The :class:`~argparse.FileType` class now accepts *encoding* and
625*errors* arguments, which are passed through to :func:`open`.  (Contributed
626by Lucas Maystre in :issue:`11175`.)
627
628
629audioop
630-------
631
632:mod:`audioop` now supports 24-bit samples.  (Contributed by Serhiy Storchaka
633in :issue:`12866`.)
634
635New :func:`~audioop.byteswap` function converts big-endian samples to
636little-endian and vice versa.  (Contributed by Serhiy Storchaka in
637:issue:`19641`.)
638
639All :mod:`audioop` functions now accept any :term:`bytes-like object`.  Strings
640are not accepted: they didn't work before, now they raise an error right away.
641(Contributed by Serhiy Storchaka in :issue:`16685`.)
642
643
644base64
645------
646
647The encoding and decoding functions in :mod:`base64` now accept any
648:term:`bytes-like object` in cases where it previously required a
649:class:`bytes` or :class:`bytearray` instance.  (Contributed by Nick Coghlan in
650:issue:`17839`.)
651
652New functions :func:`~base64.a85encode`, :func:`~base64.a85decode`,
653:func:`~base64.b85encode`, and :func:`~base64.b85decode` provide the ability to
654encode and decode binary data from and to ``Ascii85`` and the git/mercurial
655``Base85`` formats, respectively.  The ``a85`` functions have options that can
656be used to make them compatible with the variants of the ``Ascii85`` encoding,
657including the Adobe variant.  (Contributed by Martin Morrison, the Mercurial
658project, Serhiy Storchaka, and Antoine Pitrou in :issue:`17618`.)
659
660
661collections
662-----------
663
664The :meth:`.ChainMap.new_child` method now accepts an *m* argument specifying
665the child map to add to the chain.  This allows an existing mapping and/or a
666custom mapping type to be used for the child.  (Contributed by Vinay Sajip in
667:issue:`16613`.)
668
669
670colorsys
671--------
672
673The number of digits in the coefficients for the RGB --- YIQ conversions have
674been expanded so that they match the FCC NTSC versions.  The change in
675results should be less than 1% and may better match results found elsewhere.
676(Contributed by Brian Landers and Serhiy Storchaka in :issue:`14323`.)
677
678
679contextlib
680----------
681
682The new :class:`contextlib.suppress` context manager helps to clarify the
683intent of code that deliberately suppresses exceptions from a single
684statement.  (Contributed by Raymond Hettinger in :issue:`15806` and
685Zero Piraeus in :issue:`19266`.)
686
687The new :func:`contextlib.redirect_stdout` context manager makes it easier
688for utility scripts to handle inflexible APIs that write their output to
689:data:`sys.stdout` and don't provide any options to redirect it.  Using the
690context manager, the :data:`sys.stdout` output can be redirected to any
691other stream or, in conjunction with :class:`io.StringIO`, to a string.
692The latter can be especially useful, for example, to capture output
693from a function that was written to implement a command line interface.
694It is recommended only for utility scripts because it affects the
695global state of :data:`sys.stdout`.  (Contributed by Raymond Hettinger
696in :issue:`15805`.)
697
698The :mod:`contextlib` documentation has also been updated to include a
699:ref:`discussion <single-use-reusable-and-reentrant-cms>` of the
700differences between single use, reusable and reentrant context managers.
701
702
703dbm
704---
705
706:func:`dbm.open` objects now support the context management protocol.  When
707used in a :keyword:`with` statement, the ``close`` method of the database
708object will be called automatically at the end of the block.  (Contributed by
709Claudiu Popa and Nick Coghlan in :issue:`19282`.)
710
711
712dis
713---
714
715Functions :func:`~dis.show_code`, :func:`~dis.dis`, :func:`~dis.distb`, and
716:func:`~dis.disassemble` now accept a keyword-only *file* argument that
717controls where they write their output.
718
719The :mod:`dis` module is now built around an :class:`~dis.Instruction` class
720that provides object oriented access to the details of each individual bytecode
721operation.
722
723A new method, :func:`~dis.get_instructions`, provides an iterator that emits
724the Instruction stream for a given piece of Python code.  Thus it is now
725possible to write a program that inspects and manipulates a bytecode
726object in ways different from those provided by the :mod:`~dis` module
727itself.  For example::
728
729    >>> import dis
730    >>> for instr in dis.get_instructions(lambda x: x + 1):
731    ...     print(instr.opname)
732    LOAD_FAST
733    LOAD_CONST
734    BINARY_ADD
735    RETURN_VALUE
736
737The various display tools in the :mod:`dis` module have been rewritten to use
738these new components.
739
740In addition, a new application-friendly class :class:`~dis.Bytecode` provides
741an object-oriented API for inspecting bytecode in both in human-readable form
742and for iterating over instructions.  The :class:`~dis.Bytecode` constructor
743takes the same arguments that :func:`~dis.get_instruction` does (plus an
744optional *current_offset*), and the resulting object can be iterated to produce
745:class:`~dis.Instruction` objects.  But it also has a :mod:`~dis.Bytecode.dis`
746method, equivalent to calling :mod:`~dis.dis` on the constructor argument, but
747returned as a multi-line string::
748
749    >>> bytecode = dis.Bytecode(lambda x: x + 1, current_offset=3)
750    >>> for instr in bytecode:
751    ...     print('{} ({})'.format(instr.opname, instr.opcode))
752    LOAD_FAST (124)
753    LOAD_CONST (100)
754    BINARY_ADD (23)
755    RETURN_VALUE (83)
756    >>> bytecode.dis().splitlines()       # doctest: +NORMALIZE_WHITESPACE
757    ['  1           0 LOAD_FAST                0 (x)',
758     '      -->     3 LOAD_CONST               1 (1)',
759     '              6 BINARY_ADD',
760     '              7 RETURN_VALUE']
761
762:class:`~dis.Bytecode` also has a class method,
763:meth:`~dis.Bytecode.from_traceback`, that provides the ability to manipulate a
764traceback (that is, ``print(Bytecode.from_traceback(tb).dis())`` is equivalent
765to ``distb(tb)``).
766
767(Contributed by Nick Coghlan, Ryan Kelly and Thomas Kluyver in :issue:`11816`
768and Claudiu Popa in :issue:`17916`.)
769
770New function :func:`~dis.stack_effect` computes the effect on the Python stack
771of a given opcode and argument, information that is not otherwise available.
772(Contributed by Larry Hastings in :issue:`19722`.)
773
774
775doctest
776-------
777
778A new :ref:`option flag <doctest-options>`, :data:`~doctest.FAIL_FAST`, halts
779test running as soon as the first failure is detected.  (Contributed by R.
780David Murray and Daniel Urban in :issue:`16522`.)
781
782The :mod:`doctest` command line interface now uses :mod:`argparse`, and has two
783new options, ``-o`` and ``-f``.  ``-o`` allows :ref:`doctest options
784<doctest-options>` to be specified on the command line, and ``-f`` is a
785shorthand for ``-o FAIL_FAST`` (to parallel the similar option supported by the
786:mod:`unittest` CLI).  (Contributed by R. David Murray in :issue:`11390`.)
787
788:mod:`doctest` will now find doctests in extension module ``__doc__`` strings.
789(Contributed by Zachary Ware in :issue:`3158`.)
790
791
792email
793-----
794
795:meth:`~email.message.Message.as_string` now accepts a *policy* argument to
796override the default policy of the message when generating a string
797representation of it.  This means that ``as_string`` can now be used in more
798circumstances, instead of having to create and use a :mod:`~email.generator` in
799order to pass formatting parameters to its ``flatten`` method.  (Contributed by
800R. David Murray in :issue:`18600`.)
801
802New method :meth:`~email.message.Message.as_bytes` added to produce a bytes
803representation of the message in a fashion similar to how ``as_string``
804produces a string representation.  It does not accept the *maxheaderlen*
805argument, but does accept the *unixfrom* and *policy* arguments. The
806:class:`~email.message.Message` :meth:`~email.message.Message.__bytes__` method
807calls it, meaning that ``bytes(mymsg)`` will now produce the intuitive
808result:  a bytes object containing the fully formatted message.  (Contributed
809by R. David Murray in :issue:`18600`.)
810
811The :meth:`.Message.set_param` message now accepts a *replace* keyword argument.
812When specified, the associated header will be updated without changing
813its location in the list of headers.  For backward compatibility, the default
814is ``False``.  (Contributed by R. David Murray in :issue:`18891`.)
815
816
817.. _whatsnew_email_contentmanager:
818
819A pair of new subclasses of :class:`~email.message.Message` have been added
820(:class:`.EmailMessage` and :class:`.MIMEPart`), along with a new sub-module,
821:mod:`~email.contentmanager` and a new :mod:`~email.policy` attribute
822:attr:`~email.policy.EmailPolicy.content_manager`.  All documentation is
823currently in the new module, which is being added as part of email's new
824:term:`provisional API`.  These classes provide a number of new methods that
825make extracting content from and inserting content into email messages much
826easier.  For details, see the :mod:`~email.contentmanager` documentation and
827the :ref:`email-examples`.  These API additions complete the
828bulk of the work that was planned as part of the email6 project.  The currently
829provisional API is scheduled to become final in Python 3.5 (possibly with a few
830minor additions in the area of error handling).  (Contributed by R. David
831Murray in :issue:`18891`.)
832
833
834filecmp
835-------
836
837A new :func:`~filecmp.clear_cache` function provides the ability to clear the
838:mod:`filecmp` comparison cache, which uses :func:`os.stat` information to
839determine if the file has changed since the last compare.  This can be used,
840for example, if the file might have been changed and re-checked in less time
841than the resolution of a particular filesystem's file modification time field.
842(Contributed by Mark Levitt in :issue:`18149`.)
843
844New module attribute :data:`~filecmp.DEFAULT_IGNORES` provides the list of
845directories that are used as the default value for the *ignore* parameter of
846the :func:`~filecmp.dircmp` function.  (Contributed by Eli Bendersky in
847:issue:`15442`.)
848
849
850functools
851---------
852
853The new :func:`~functools.partialmethod` descriptor brings partial argument
854application to descriptors, just as :func:`~functools.partial` provides
855for normal callables. The new descriptor also makes it easier to get
856arbitrary callables (including :func:`~functools.partial` instances)
857to behave like normal instance methods when included in a class definition.
858(Contributed by Alon Horev and Nick Coghlan in :issue:`4331`.)
859
860.. _whatsnew-singledispatch:
861
862The new :func:`~functools.singledispatch` decorator brings support for
863single-dispatch generic functions to the Python standard library. Where
864object oriented programming focuses on grouping multiple operations on a
865common set of data into a class, a generic function focuses on grouping
866multiple implementations of an operation that allows it to work with
867*different* kinds of data.
868
869.. seealso::
870
871   :pep:`443` -- Single-dispatch generic functions
872      PEP written and implemented by Łukasz Langa.
873
874:func:`~functools.total_ordering` now supports a return value of
875:const:`NotImplemented` from the underlying comparison function.  (Contributed
876by Katie Miller in :issue:`10042`.)
877
878A pure-python version of the :func:`~functools.partial` function is now in the
879stdlib; in CPython it is overridden by the C accelerated version, but it is
880available for other implementations to use.  (Contributed by Brian Thorne in
881:issue:`12428`.)
882
883
884gc
885--
886
887New function :func:`~gc.get_stats` returns a list of three per-generation
888dictionaries containing the collections statistics since interpreter startup.
889(Contributed by Antoine Pitrou in :issue:`16351`.)
890
891
892glob
893----
894
895A new function :func:`~glob.escape` provides a way to escape special characters
896in a filename so that they do not become part of the globbing expansion but are
897instead matched literally.  (Contributed by Serhiy Storchaka in :issue:`8402`.)
898
899
900hashlib
901-------
902
903A new :func:`hashlib.pbkdf2_hmac` function provides
904the `PKCS#5 password-based key derivation function 2
905<https://en.wikipedia.org/wiki/PBKDF2>`_.  (Contributed by Christian
906Heimes in :issue:`18582`.)
907
908The :attr:`~hashlib.hash.name` attribute of :mod:`hashlib` hash objects is now
909a formally supported interface.  It has always existed in CPython's
910:mod:`hashlib` (although it did not return lower case names for all supported
911hashes), but it was not a public interface and so some other Python
912implementations have not previously supported it.  (Contributed by Jason R.
913Coombs in :issue:`18532`.)
914
915
916hmac
917----
918
919:mod:`hmac` now accepts ``bytearray`` as well as ``bytes`` for the *key*
920argument to the :func:`~hmac.new` function, and the *msg* parameter to both the
921:func:`~hmac.new` function and the :meth:`~hmac.HMAC.update` method now
922accepts any type supported by the :mod:`hashlib` module.  (Contributed
923by Jonas Borgström in :issue:`18240`.)
924
925The *digestmod* argument to the :func:`hmac.new` function may now be any hash
926digest name recognized by :mod:`hashlib`.  In addition, the current behavior in
927which the value of *digestmod* defaults to ``MD5`` is deprecated: in a
928future version of Python there will be no default value.  (Contributed by
929Christian Heimes in :issue:`17276`.)
930
931With the addition of :attr:`~hmac.HMAC.block_size` and :attr:`~hmac.HMAC.name`
932attributes (and the formal documentation of the :attr:`~hmac.HMAC.digest_size`
933attribute), the :mod:`hmac` module now conforms fully to the :pep:`247` API.
934(Contributed by Christian Heimes in :issue:`18775`.)
935
936
937html
938----
939
940New function :func:`~html.unescape` function converts HTML5 character references to
941the corresponding Unicode characters.  (Contributed by Ezio Melotti in
942:issue:`2927`.)
943
944:class:`~html.parser.HTMLParser` accepts a new keyword argument
945*convert_charrefs* that, when ``True``, automatically converts all character
946references.  For backward-compatibility, its value defaults to ``False``, but
947it will change to ``True`` in a future version of Python, so you are invited to
948set it explicitly and update your code to use this new feature.  (Contributed
949by Ezio Melotti in :issue:`13633`.)
950
951The *strict* argument of :class:`~html.parser.HTMLParser` is now deprecated.
952(Contributed by Ezio Melotti in :issue:`15114`.)
953
954
955http
956----
957
958:meth:`~http.server.BaseHTTPRequestHandler.send_error` now accepts an
959optional additional *explain* parameter which can be used to provide an
960extended error description, overriding the hardcoded default if there is one.
961This extended error description will be formatted using the
962:attr:`~http.server.HTTP.error_message_format` attribute and sent as the body
963of the error response.  (Contributed by Karl Cow in :issue:`12921`.)
964
965The :mod:`http.server` :ref:`command line interface <http-server-cli>` now has
966a ``-b/--bind`` option that causes the server to listen on a specific address.
967(Contributed by Malte Swart in :issue:`17764`.)
968
969
970idlelib and IDLE
971----------------
972
973Since idlelib implements the IDLE shell and editor and is not intended for
974import by other programs, it gets improvements with every release. See
975:file:`Lib/idlelib/NEWS.txt` for a cumulative list of changes since 3.3.0,
976as well as changes made in future 3.4.x releases. This file is also available
977from the IDLE :menuselection:`Help --> About IDLE` dialog.
978
979
980importlib
981---------
982
983The :class:`~importlib.abc.InspectLoader` ABC defines a new method,
984:meth:`~importlib.abc.InspectLoader.source_to_code` that accepts source
985data and a path and returns a code object.  The default implementation
986is equivalent to ``compile(data, path, 'exec', dont_inherit=True)``.
987(Contributed by Eric Snow and Brett Cannon in :issue:`15627`.)
988
989:class:`~importlib.abc.InspectLoader` also now has a default implementation
990for the :meth:`~importlib.abc.InspectLoader.get_code` method.  However,
991it will normally be desirable to override the default implementation
992for performance reasons.  (Contributed by Brett Cannon in :issue:`18072`.)
993
994The :func:`~importlib.reload` function has been moved from :mod:`imp` to
995:mod:`importlib` as part of the :mod:`imp` module deprecation.  (Contributed by
996Berker Peksag in :issue:`18193`.)
997
998:mod:`importlib.util` now has a :data:`~importlib.util.MAGIC_NUMBER` attribute
999providing access to the bytecode version number.  This replaces the
1000:func:`~imp.get_magic` function in the deprecated :mod:`imp` module.
1001(Contributed by Brett Cannon in :issue:`18192`.)
1002
1003New :mod:`importlib.util` functions :func:`~importlib.util.cache_from_source`
1004and :func:`~importlib.util.source_from_cache` replace the same-named functions
1005in the deprecated :mod:`imp` module.  (Contributed by Brett Cannon in
1006:issue:`18194`.)
1007
1008The :mod:`importlib` bootstrap :class:`.NamespaceLoader` now conforms to
1009the :class:`.InspectLoader` ABC, which means that ``runpy`` and
1010``python -m`` can now be used with namespace packages.  (Contributed
1011by Brett Cannon in :issue:`18058`.)
1012
1013:mod:`importlib.util` has a new function :func:`~importlib.util.decode_source`
1014that decodes source from bytes using universal newline processing.  This is
1015useful for implementing :meth:`.InspectLoader.get_source` methods.
1016
1017:class:`importlib.machinery.ExtensionFileLoader` now has a
1018:meth:`~importlib.machinery.ExtensionFileLoader.get_filename` method.  This was
1019inadvertently omitted in the original implementation.  (Contributed by Eric
1020Snow in :issue:`19152`.)
1021
1022
1023inspect
1024-------
1025
1026The :mod:`inspect` module now offers a basic :ref:`command line interface
1027<inspect-module-cli>` to quickly display source code and other
1028information for modules, classes and functions.  (Contributed by Claudiu Popa
1029and Nick Coghlan in :issue:`18626`.)
1030
1031:func:`~inspect.unwrap` makes it easy to unravel wrapper function chains
1032created by :func:`functools.wraps` (and any other API that sets the
1033``__wrapped__`` attribute on a wrapper function).  (Contributed by
1034Daniel Urban, Aaron Iles and Nick Coghlan in :issue:`13266`.)
1035
1036As part of the implementation of the new :mod:`enum` module, the
1037:mod:`inspect` module now has substantially better support for custom
1038``__dir__`` methods and dynamic class attributes provided through
1039metaclasses.  (Contributed by Ethan Furman in :issue:`18929` and
1040:issue:`19030`.)
1041
1042:func:`~inspect.getfullargspec` and :func:`~inspect.getargspec`
1043now use the :func:`~inspect.signature` API. This allows them to
1044support a much broader range of callables, including those with
1045``__signature__`` attributes, those with metadata provided by argument
1046clinic, :func:`functools.partial` objects and more. Note that, unlike
1047:func:`~inspect.signature`, these functions still ignore ``__wrapped__``
1048attributes, and report the already bound first argument for bound methods,
1049so it is still necessary to update your code to use
1050:func:`~inspect.signature` directly if those features are desired.
1051(Contributed by Yury Selivanov in :issue:`17481`.)
1052
1053:func:`~inspect.signature` now supports duck types of CPython functions,
1054which adds support for functions compiled with Cython.  (Contributed
1055by Stefan Behnel and Yury Selivanov in :issue:`17159`.)
1056
1057
1058ipaddress
1059---------
1060
1061:mod:`ipaddress` was added to the standard library in Python 3.3 as a
1062:term:`provisional API`. With the release of Python 3.4, this qualification
1063has been removed: :mod:`ipaddress` is now considered a stable API, covered
1064by the normal standard library requirements to maintain backwards
1065compatibility.
1066
1067A new :attr:`~ipaddress.IPv4Address.is_global` property is ``True`` if
1068an address is globally routeable.  (Contributed by Peter Moody in
1069:issue:`17400`.)
1070
1071
1072logging
1073-------
1074
1075The :class:`~logging.handlers.TimedRotatingFileHandler` has a new *atTime*
1076parameter that can be used to specify the time of day when rollover should
1077happen.  (Contributed by Ronald Oussoren in :issue:`9556`.)
1078
1079:class:`~logging.handlers.SocketHandler` and
1080:class:`~logging.handlers.DatagramHandler` now support Unix domain sockets (by
1081setting *port* to ``None``).  (Contributed by Vinay Sajip in commit
1082ce46195b56a9.)
1083
1084:func:`~logging.config.fileConfig` now accepts a
1085:class:`configparser.RawConfigParser` subclass instance for the *fname*
1086parameter.  This facilitates using a configuration file when logging
1087configuration is just a part of the overall application configuration, or where
1088the application modifies the configuration before passing it to
1089:func:`~logging.config.fileConfig`.  (Contributed by Vinay Sajip in
1090:issue:`16110`.)
1091
1092Logging configuration data received from a socket via the
1093:func:`logging.config.listen` function can now be validated before being
1094processed by supplying a verification function as the argument to the new
1095*verify* keyword argument.  (Contributed by Vinay Sajip in :issue:`15452`.)
1096
1097
1098.. _whatsnew-marshal-3:
1099
1100marshal
1101-------
1102
1103The default :mod:`marshal` version has been bumped to 3.  The code implementing
1104the new version restores the Python2 behavior of recording only one copy of
1105interned strings and preserving the interning on deserialization, and extends
1106this "one copy" ability to any object type (including handling recursive
1107references).  This reduces both the size of ``.pyc`` files and the amount of
1108memory a module occupies in memory when it is loaded from a ``.pyc`` (or
1109``.pyo``) file.  (Contributed by Kristján Valur Jónsson in :issue:`16475`,
1110with additional speedups by Antoine Pitrou in :issue:`19219`.)
1111
1112
1113mmap
1114----
1115
1116mmap objects can now be :mod:`weakref`\ ed.  (Contributed by Valerie Lambert in
1117:issue:`4885`.)
1118
1119
1120multiprocessing
1121---------------
1122
1123.. _whatsnew-multiprocessing-no-fork:
1124
1125On Unix two new :ref:`start methods <multiprocessing-start-methods>`,
1126``spawn`` and ``forkserver``, have been added for starting processes using
1127:mod:`multiprocessing`.  These make the mixing of processes with threads more
1128robust, and the ``spawn`` method matches the semantics that multiprocessing has
1129always used on Windows.  New function
1130:func:`~multiprocessing.get_all_start_methods` reports all start methods
1131available on the platform, :func:`~multiprocessing.get_start_method` reports
1132the current start method, and :func:`~multiprocessing.set_start_method` sets
1133the start method.  (Contributed by Richard Oudkerk in :issue:`8713`.)
1134
1135:mod:`multiprocessing` also now has the concept of a ``context``, which
1136determines how child processes are created.  New function
1137:func:`~multiprocessing.get_context` returns a context that uses a specified
1138start method.  It has the same API as the :mod:`multiprocessing` module itself,
1139so you can use it to create :class:`~multiprocessing.pool.Pool`\ s and other
1140objects that will operate within that context.  This allows a framework and an
1141application or different parts of the same application to use multiprocessing
1142without interfering with each other.  (Contributed by Richard Oudkerk in
1143:issue:`18999`.)
1144
1145Except when using the old *fork* start method, child processes no longer
1146inherit unneeded handles/file descriptors from their parents (part of
1147:issue:`8713`).
1148
1149:mod:`multiprocessing` now relies on :mod:`runpy` (which implements the
1150``-m`` switch) to initialise ``__main__`` appropriately in child processes
1151when using the ``spawn`` or ``forkserver`` start methods. This resolves some
1152edge cases where combining multiprocessing, the ``-m`` command line switch,
1153and explicit relative imports could cause obscure failures in child
1154processes.  (Contributed by Nick Coghlan in :issue:`19946`.)
1155
1156
1157operator
1158--------
1159
1160New function :func:`~operator.length_hint` provides an implementation of the
1161specification for how the :meth:`~object.__length_hint__` special method should
1162be used, as part of the :pep:`424` formal specification of this language
1163feature.  (Contributed by Armin Ronacher in :issue:`16148`.)
1164
1165There is now a pure-python version of the :mod:`operator` module available for
1166reference and for use by alternate implementations of Python.  (Contributed by
1167Zachary Ware in :issue:`16694`.)
1168
1169
1170os
1171--
1172
1173There are new functions to get and set the :ref:`inheritable flag
1174<fd_inheritance>` of a file descriptor (:func:`os.get_inheritable`,
1175:func:`os.set_inheritable`) or a Windows handle
1176(:func:`os.get_handle_inheritable`, :func:`os.set_handle_inheritable`).
1177
1178New function :func:`~os.cpu_count` reports the number of CPUs available on the
1179platform on which Python is running (or ``None`` if the count can't be
1180determined).  The :func:`multiprocessing.cpu_count` function is now implemented
1181in terms of this function).  (Contributed by Trent Nelson, Yogesh Chaudhari,
1182Victor Stinner, and Charles-François Natali in :issue:`17914`.)
1183
1184:func:`os.path.samestat` is now available on the Windows platform (and the
1185:func:`os.path.samefile` implementation is now shared between Unix and
1186Windows).  (Contributed by Brian Curtin in :issue:`11939`.)
1187
1188:func:`os.path.ismount` now recognizes volumes mounted below a drive
1189root on Windows.  (Contributed by Tim Golden in :issue:`9035`.)
1190
1191:func:`os.open` supports two new flags on platforms that provide them,
1192:data:`~os.O_PATH` (un-opened file descriptor), and :data:`~os.O_TMPFILE`
1193(unnamed temporary file; as of 3.4.0 release available only on Linux systems
1194with a kernel version of 3.11 or newer that have uapi headers).  (Contributed
1195by Christian Heimes in :issue:`18673` and Benjamin Peterson, respectively.)
1196
1197
1198pdb
1199---
1200
1201:mod:`pdb` has been enhanced to handle generators, :keyword:`yield`, and
1202``yield from`` in a more useful fashion.  This is especially helpful when
1203debugging :mod:`asyncio` based programs.  (Contributed by Andrew Svetlov and
1204Xavier de Gaye in :issue:`16596`.)
1205
1206The ``print`` command has been removed from :mod:`pdb`, restoring access to the
1207Python :func:`print` function from the pdb command line.  Python2's ``pdb`` did
1208not have a ``print`` command; instead, entering ``print`` executed the
1209``print`` statement.  In Python3 ``print`` was mistakenly made an alias for the
1210pdb :pdbcmd:`p` command.  ``p``, however, prints the ``repr`` of its argument,
1211not the ``str`` like the Python2 ``print`` command did.  Worse, the Python3
1212``pdb print`` command shadowed the Python3 ``print`` function, making it
1213inaccessible at the ``pdb`` prompt.  (Contributed by Connor Osborn in
1214:issue:`18764`.)
1215
1216
1217.. _whatsnew-protocol-4:
1218
1219pickle
1220------
1221
1222:mod:`pickle` now supports (but does not use by default) a new pickle protocol,
1223protocol 4.  This new protocol addresses a number of issues that were present
1224in previous protocols, such as the serialization of nested classes, very large
1225strings and containers, and classes whose :meth:`__new__` method takes
1226keyword-only arguments.  It also provides some efficiency improvements.
1227
1228.. seealso::
1229
1230   :pep:`3154` -- Pickle protocol 4
1231      PEP written by Antoine Pitrou and implemented by Alexandre Vassalotti.
1232
1233
1234plistlib
1235--------
1236
1237:mod:`plistlib` now has an API that is similar to the standard pattern for
1238stdlib serialization protocols, with new :func:`~plistlib.load`,
1239:func:`~plistlib.dump`, :func:`~plistlib.loads`, and :func:`~plistlib.dumps`
1240functions.  (The older API is now deprecated.)  In addition to the already
1241supported XML plist format (:data:`~plistlib.FMT_XML`), it also now supports
1242the binary plist format (:data:`~plistlib.FMT_BINARY`).  (Contributed by Ronald
1243Oussoren and others in :issue:`14455`.)
1244
1245
1246poplib
1247------
1248
1249Two new methods have been added to :mod:`poplib`: :meth:`~poplib.POP3.capa`,
1250which returns the list of capabilities advertised by the POP server, and
1251:meth:`~poplib.POP3.stls`, which switches a clear-text POP3 session into an
1252encrypted POP3 session if the POP server supports it.  (Contributed by Lorenzo
1253Catucci in :issue:`4473`.)
1254
1255
1256pprint
1257------
1258
1259The :mod:`pprint` module's :class:`~pprint.PrettyPrinter` class and its
1260:func:`~pprint.pformat`, and :func:`~pprint.pprint` functions have a new
1261option, *compact*, that controls how the output is formatted.  Currently
1262setting *compact* to ``True`` means that sequences will be printed with as many
1263sequence elements as will fit within *width* on each (indented) line.
1264(Contributed by Serhiy Storchaka in :issue:`19132`.)
1265
1266Long strings are now wrapped using Python's normal line continuation
1267syntax.  (Contributed by Antoine Pitrou in :issue:`17150`.)
1268
1269
1270pty
1271---
1272
1273:func:`pty.spawn` now returns the status value from :func:`os.waitpid` on
1274the child process, instead of ``None``.  (Contributed by Gregory P. Smith.)
1275
1276
1277pydoc
1278-----
1279
1280The :mod:`pydoc` module is now based directly on the :func:`inspect.signature`
1281introspection API, allowing it to provide signature information for a wider
1282variety of callable objects. This change also means that ``__wrapped__``
1283attributes are now taken into account when displaying help information.
1284(Contributed by Larry Hastings in :issue:`19674`.)
1285
1286The :mod:`pydoc` module no longer displays the ``self`` parameter for
1287already bound methods. Instead, it aims to always display the exact current
1288signature of the supplied callable.  (Contributed by Larry Hastings in
1289:issue:`20710`.)
1290
1291In addition to the changes that have been made to :mod:`pydoc` directly,
1292its handling of custom ``__dir__`` methods and various descriptor
1293behaviours has also been improved substantially by the underlying changes in
1294the :mod:`inspect` module.
1295
1296As the :func:`help` builtin is based on :mod:`pydoc`, the above changes also
1297affect the behaviour of :func:`help`.
1298
1299
1300re
1301--
1302
1303New :func:`~re.fullmatch` function and :meth:`.regex.fullmatch` method anchor
1304the pattern at both ends of the string to match.  This provides a way to be
1305explicit about the goal of the match, which avoids a class of subtle bugs where
1306``$`` characters get lost during code changes or the addition of alternatives
1307to an existing regular expression.  (Contributed by Matthew Barnett in
1308:issue:`16203`.)
1309
1310The repr of :ref:`regex objects <re-objects>` now includes the pattern
1311and the flags; the repr of :ref:`match objects <match-objects>` now
1312includes the start, end, and the part of the string that matched.  (Contributed
1313by Hugo Lopes Tavares and Serhiy Storchaka in :issue:`13592` and
1314:issue:`17087`.)
1315
1316
1317resource
1318--------
1319
1320New :func:`~resource.prlimit` function, available on Linux platforms with a
1321kernel version of 2.6.36 or later and glibc of 2.13 or later, provides the
1322ability to query or set the resource limits for processes other than the one
1323making the call.  (Contributed by Christian Heimes in :issue:`16595`.)
1324
1325On Linux kernel version 2.6.36 or later, there are also some new
1326Linux specific constants: :attr:`~resource.RLIMIT_MSGQUEUE`,
1327:attr:`~resource.RLIMIT_NICE`, :attr:`~resource.RLIMIT_RTPRIO`,
1328:attr:`~resource.RLIMIT_RTTIME`, and :attr:`~resource.RLIMIT_SIGPENDING`.
1329(Contributed by Christian Heimes in :issue:`19324`.)
1330
1331On FreeBSD version 9 and later, there some new FreeBSD specific constants:
1332:attr:`~resource.RLIMIT_SBSIZE`, :attr:`~resource.RLIMIT_SWAP`, and
1333:attr:`~resource.RLIMIT_NPTS`.  (Contributed by Claudiu Popa in
1334:issue:`19343`.)
1335
1336
1337select
1338------
1339
1340:class:`~select.epoll` objects now support the context management protocol.
1341When used in a :keyword:`with` statement, the :meth:`~select.epoll.close`
1342method will be called automatically at the end of the block.  (Contributed
1343by Serhiy Storchaka in :issue:`16488`.)
1344
1345:class:`~select.devpoll` objects now have :meth:`~select.devpoll.fileno` and
1346:meth:`~select.devpoll.close` methods, as well as a new attribute
1347:attr:`~select.devpoll.closed`.  (Contributed by Victor Stinner in
1348:issue:`18794`.)
1349
1350
1351shelve
1352------
1353
1354:class:`~shelve.Shelf` instances may now be used in :keyword:`with` statements,
1355and will be automatically closed at the end of the :keyword:`!with` block.
1356(Contributed by Filip Gruszczyński in :issue:`13896`.)
1357
1358
1359shutil
1360------
1361
1362:func:`~shutil.copyfile` now raises a specific :exc:`~shutil.Error` subclass,
1363:exc:`~shutil.SameFileError`, when the source and destination are the same
1364file, which allows an application to take appropriate action on this specific
1365error.  (Contributed by Atsuo Ishimoto and Hynek Schlawack in
1366:issue:`1492704`.)
1367
1368
1369smtpd
1370-----
1371
1372The :class:`~smtpd.SMTPServer` and :class:`~smtpd.SMTPChannel` classes now
1373accept a *map* keyword argument which, if specified, is passed in to
1374:class:`asynchat.async_chat` as its *map* argument.  This allows an application
1375to avoid affecting the global socket map.  (Contributed by Vinay Sajip in
1376:issue:`11959`.)
1377
1378
1379smtplib
1380-------
1381
1382:exc:`~smtplib.SMTPException` is now a subclass of :exc:`OSError`, which allows
1383both socket level errors and SMTP protocol level errors to be caught in one
1384try/except statement by code that only cares whether or not an error occurred.
1385(Contributed by Ned Jackson Lovely in :issue:`2118`.)
1386
1387
1388socket
1389------
1390
1391The socket module now supports the :data:`~socket.CAN_BCM` protocol on
1392platforms that support it.  (Contributed by Brian Thorne in :issue:`15359`.)
1393
1394Socket objects have new methods to get or set their :ref:`inheritable flag
1395<fd_inheritance>`, :meth:`~socket.socket.get_inheritable` and
1396:meth:`~socket.socket.set_inheritable`.
1397
1398The ``socket.AF_*`` and ``socket.SOCK_*`` constants are now enumeration values
1399using the new :mod:`enum` module.  This allows meaningful names to be printed
1400during debugging, instead of integer "magic numbers".
1401
1402The :data:`~socket.AF_LINK` constant is now available on BSD and OSX.
1403
1404:func:`~socket.inet_pton` and :func:`~socket.inet_ntop` are now supported
1405on Windows.  (Contributed by Atsuo Ishimoto in :issue:`7171`.)
1406
1407
1408sqlite3
1409-------
1410
1411A new boolean parameter to the :func:`~sqlite3.connect` function, *uri*, can be
1412used to indicate that the *database* parameter is a ``uri`` (see the `SQLite
1413URI documentation <https://www.sqlite.org/uri.html>`_).  (Contributed by poq in
1414:issue:`13773`.)
1415
1416
1417ssl
1418---
1419
1420.. _whatsnew-tls-11-12:
1421
1422:data:`~ssl.PROTOCOL_TLSv1_1` and :data:`~ssl.PROTOCOL_TLSv1_2` (TLSv1.1 and
1423TLSv1.2 support) have been added; support for these protocols is only available if
1424Python is linked with OpenSSL 1.0.1 or later.  (Contributed by Michele Orrù and
1425Antoine Pitrou in :issue:`16692`.)
1426
1427.. _whatsnew34-sslcontext:
1428
1429New function :func:`~ssl.create_default_context` provides a standard way to
1430obtain an :class:`~ssl.SSLContext` whose settings are intended to be a
1431reasonable balance between compatibility and security.  These settings are
1432more stringent than the defaults provided by the :class:`~ssl.SSLContext`
1433constructor, and may be adjusted in the future, without prior deprecation, if
1434best-practice security requirements change.  The new recommended best
1435practice for using stdlib libraries that support SSL is to use
1436:func:`~ssl.create_default_context` to obtain an :class:`~ssl.SSLContext`
1437object, modify it if needed, and then pass it as the *context* argument
1438of the appropriate stdlib API.  (Contributed by Christian Heimes
1439in :issue:`19689`.)
1440
1441:class:`~ssl.SSLContext` method :meth:`~ssl.SSLContext.load_verify_locations`
1442accepts a new optional argument *cadata*, which can be used to provide PEM or
1443DER encoded certificates directly via strings or bytes, respectively.
1444(Contributed by Christian Heimes in :issue:`18138`.)
1445
1446New function :func:`~ssl.get_default_verify_paths` returns
1447a named tuple of the paths and environment variables that the
1448:meth:`~ssl.SSLContext.set_default_verify_paths` method uses to set
1449OpenSSL's default ``cafile`` and ``capath``.  This can be an aid in
1450debugging default verification issues.  (Contributed by Christian Heimes
1451in :issue:`18143`.)
1452
1453:class:`~ssl.SSLContext` has a new method,
1454:meth:`~ssl.SSLContext.cert_store_stats`, that reports the number of loaded
1455``X.509`` certs, ``X.509 CA`` certs, and certificate revocation lists
1456(``crl``\ s), as well as a :meth:`~ssl.SSLContext.get_ca_certs` method that
1457returns a list of the loaded ``CA`` certificates.  (Contributed by Christian
1458Heimes in :issue:`18147`.)
1459
1460If OpenSSL 0.9.8 or later is available, :class:`~ssl.SSLContext` has a new
1461attribute :attr:`~ssl.SSLContext.verify_flags` that can be used to control the
1462certificate verification process by setting it to some combination of the new
1463constants :data:`~ssl.VERIFY_DEFAULT`, :data:`~ssl.VERIFY_CRL_CHECK_LEAF`,
1464:data:`~ssl.VERIFY_CRL_CHECK_CHAIN`, or :data:`~ssl.VERIFY_X509_STRICT`.
1465OpenSSL does not do any CRL verification by default.  (Contributed by
1466Christien Heimes in :issue:`8813`.)
1467
1468New :class:`~ssl.SSLContext` method :meth:`~ssl.SSLContext.load_default_certs`
1469loads a set of default "certificate authority" (CA) certificates from default
1470locations, which vary according to the platform.  It can be used to load both
1471TLS web server authentication certificates
1472(``purpose=``:data:`~ssl.Purpose.SERVER_AUTH`) for a client to use to verify a
1473server, and certificates for a server to use in verifying client certificates
1474(``purpose=``:data:`~ssl.Purpose.CLIENT_AUTH`).  (Contributed by Christian
1475Heimes in :issue:`19292`.)
1476
1477.. _whatsnew34-win-cert-store:
1478
1479Two new windows-only functions, :func:`~ssl.enum_certificates` and
1480:func:`~ssl.enum_crls` provide the ability to retrieve certificates,
1481certificate information, and CRLs from the Windows cert store.  (Contributed
1482by Christian Heimes in :issue:`17134`.)
1483
1484.. _whatsnew34-sni:
1485
1486Support for server-side SNI (Server Name Indication) using the new
1487:meth:`ssl.SSLContext.set_servername_callback` method.
1488(Contributed by Daniel Black in :issue:`8109`.)
1489
1490The dictionary returned by :meth:`.SSLSocket.getpeercert` contains additional
1491``X509v3`` extension items: ``crlDistributionPoints``, ``calIssuers``, and
1492``OCSP`` URIs.  (Contributed by Christian Heimes in :issue:`18379`.)
1493
1494
1495stat
1496----
1497
1498The :mod:`stat` module is now backed by a C implementation in :mod:`_stat`. A C
1499implementation is required as most of the values aren't standardized and
1500are platform-dependent.  (Contributed by Christian Heimes in :issue:`11016`.)
1501
1502The module supports new :mod:`~stat.ST_MODE` flags, :mod:`~stat.S_IFDOOR`,
1503:attr:`~stat.S_IFPORT`, and :attr:`~stat.S_IFWHT`.  (Contributed by
1504Christian Hiemes in :issue:`11016`.)
1505
1506
1507struct
1508------
1509
1510New function :mod:`~struct.iter_unpack` and a new
1511:meth:`struct.Struct.iter_unpack` method on compiled formats provide streamed
1512unpacking of a buffer containing repeated instances of a given format of data.
1513(Contributed by Antoine Pitrou in :issue:`17804`.)
1514
1515
1516subprocess
1517----------
1518
1519:func:`~subprocess.check_output` now accepts an *input* argument that can
1520be used to provide the contents of ``stdin`` for the command that is run.
1521(Contributed by Zack Weinberg in :issue:`16624`.)
1522
1523:func:`~subprocess.getstatus` and :func:`~subprocess.getstatusoutput` now
1524work on Windows.  This change was actually inadvertently made in 3.3.4.
1525(Contributed by Tim Golden in :issue:`10197`.)
1526
1527
1528sunau
1529-----
1530
1531The :meth:`~sunau.getparams` method now returns a namedtuple rather than a
1532plain tuple.  (Contributed by Claudiu Popa in :issue:`18901`.)
1533
1534:meth:`sunau.open` now supports the context management protocol: when used in a
1535:keyword:`with` block, the ``close`` method of the returned object will be
1536called automatically at the end of the block.  (Contributed by Serhiy Storchaka
1537in :issue:`18878`.)
1538
1539:meth:`.AU_write.setsampwidth` now supports 24 bit samples, thus adding
1540support for writing 24 sample using the module.  (Contributed by
1541Serhiy Storchaka in :issue:`19261`.)
1542
1543The :meth:`~sunau.AU_write.writeframesraw` and
1544:meth:`~sunau.AU_write.writeframes` methods now accept any :term:`bytes-like
1545object`.  (Contributed by Serhiy Storchaka in :issue:`8311`.)
1546
1547
1548sys
1549---
1550
1551New function :func:`sys.getallocatedblocks` returns the current number of
1552blocks allocated by the interpreter.  (In CPython with the default
1553``--with-pymalloc`` setting, this is allocations made through the
1554:c:func:`PyObject_Malloc` API.)  This can be useful for tracking memory leaks,
1555especially if automated via a test suite.  (Contributed by Antoine Pitrou
1556in :issue:`13390`.)
1557
1558When the Python interpreter starts in :ref:`interactive mode
1559<tut-interactive>`, it checks for an :data:`~sys.__interactivehook__` attribute
1560on the :mod:`sys` module.  If the attribute exists, its value is called with no
1561arguments just before interactive mode is started.  The check is made after the
1562:envvar:`PYTHONSTARTUP` file is read, so it can be set there.  The :mod:`site`
1563module :ref:`sets it <rlcompleter-config>` to a function that enables tab
1564completion and history saving (in :file:`~/.python-history`) if the platform
1565supports :mod:`readline`.  If you do not want this (new) behavior, you can
1566override it in :envvar:`PYTHONSTARTUP`, :mod:`sitecustomize`, or
1567:mod:`usercustomize` by deleting this attribute from :mod:`sys` (or setting it
1568to some other callable).  (Contributed by Éric Araujo and Antoine Pitrou in
1569:issue:`5845`.)
1570
1571
1572tarfile
1573-------
1574
1575The :mod:`tarfile` module now supports a simple :ref:`tarfile-commandline` when
1576called as a script directly or via ``-m``.  This can be used to create and
1577extract tarfile archives.  (Contributed by Berker Peksag in :issue:`13477`.)
1578
1579
1580textwrap
1581--------
1582
1583The :class:`~textwrap.TextWrapper` class has two new attributes/constructor
1584arguments: :attr:`~textwrap.TextWrapper.max_lines`, which limits the number of
1585lines in the output, and :attr:`~textwrap.TextWrapper.placeholder`, which is a
1586string that will appear at the end of the output if it has been truncated
1587because of *max_lines*.  Building on these capabilities, a new convenience
1588function :func:`~textwrap.shorten` collapses all of the whitespace in the input
1589to single spaces and produces a single line of a given *width* that ends with
1590the *placeholder* (by default, ``[...]``).  (Contributed by Antoine Pitrou and
1591Serhiy Storchaka in :issue:`18585` and :issue:`18725`.)
1592
1593
1594threading
1595---------
1596
1597The :class:`~threading.Thread` object representing the main thread can be
1598obtained from the new :func:`~threading.main_thread` function.  In normal
1599conditions this will be the thread from which the Python interpreter was
1600started.  (Contributed by Andrew Svetlov in :issue:`18882`.)
1601
1602
1603traceback
1604---------
1605
1606A new :func:`traceback.clear_frames` function takes a traceback object
1607and clears the local variables in all of the frames it references,
1608reducing the amount of memory consumed.  (Contributed by Andrew Kuchling in
1609:issue:`1565525`.)
1610
1611
1612types
1613-----
1614
1615A new :func:`~types.DynamicClassAttribute` descriptor provides a way to define
1616an attribute that acts normally when looked up through an instance object, but
1617which is routed to the *class* ``__getattr__`` when looked up through the
1618class.  This allows one to have properties active on a class, and have virtual
1619attributes on the class with the same name (see :mod:`Enum` for an example).
1620(Contributed by Ethan Furman in :issue:`19030`.)
1621
1622
1623urllib
1624------
1625
1626:mod:`urllib.request` now supports ``data:`` URLs via the
1627:class:`~urllib.request.DataHandler` class.  (Contributed by Mathias Panzenböck
1628in :issue:`16423`.)
1629
1630The http method that will be used by a :class:`~urllib.request.Request` class
1631can now be specified by setting a :class:`~urllib.request.Request.method`
1632class attribute on the subclass.  (Contributed by Jason R Coombs in
1633:issue:`18978`.)
1634
1635:class:`~urllib.request.Request` objects are now reusable: if the
1636:attr:`~urllib.request.Request.full_url` or :attr:`~urllib.request.Request.data`
1637attributes are modified, all relevant internal properties are updated.  This
1638means, for example, that it is now possible to use the same
1639:class:`~urllib.request.Request` object in more than one
1640:meth:`.OpenerDirector.open` call with different *data* arguments, or to
1641modify a :class:`~urllib.request.Request`\ 's ``url`` rather than recomputing it
1642from scratch.  There is also a new
1643:meth:`~urllib.request.Request.remove_header` method that can be used to remove
1644headers from a :class:`~urllib.request.Request`.  (Contributed by Alexey
1645Kachayev in :issue:`16464`, Daniel Wozniak in :issue:`17485`, and Damien Brecht
1646and Senthil Kumaran in :issue:`17272`.)
1647
1648:class:`~urllib.error.HTTPError` objects now have a
1649:attr:`~urllib.error.HTTPError.headers` attribute that provides access to the
1650HTTP response headers associated with the error.  (Contributed by
1651Berker Peksag in :issue:`15701`.)
1652
1653
1654unittest
1655--------
1656
1657The :class:`~unittest.TestCase` class has a new method,
1658:meth:`~unittest.TestCase.subTest`, that produces a context manager whose
1659:keyword:`with` block becomes a "sub-test".  This context manager allows a test
1660method to dynamically generate subtests  by, say, calling the ``subTest``
1661context manager inside a loop.  A single test method can thereby produce an
1662indefinite number of separately-identified and separately-counted tests, all of
1663which will run even if one or more of them fail.  For example::
1664
1665    class NumbersTest(unittest.TestCase):
1666        def test_even(self):
1667            for i in range(6):
1668                with self.subTest(i=i):
1669                    self.assertEqual(i % 2, 0)
1670
1671will result in six subtests, each identified in the unittest verbose output
1672with a label consisting of the variable name ``i`` and a particular value for
1673that variable (``i=0``, ``i=1``, etc).  See :ref:`subtests` for the full
1674version of this example.  (Contributed by Antoine Pitrou in :issue:`16997`.)
1675
1676:func:`unittest.main` now accepts an iterable of test names for
1677*defaultTest*, where previously it only accepted a single test name as a
1678string.  (Contributed by Jyrki Pulliainen in :issue:`15132`.)
1679
1680If :class:`~unittest.SkipTest` is raised during test discovery (that is, at the
1681module level in the test file), it is now reported as a skip instead of an
1682error.  (Contributed by Zach Ware in :issue:`16935`.)
1683
1684:meth:`~unittest.TestLoader.discover` now sorts the discovered files to provide
1685consistent test ordering.  (Contributed by Martin Melin and Jeff Ramnani in
1686:issue:`16709`.)
1687
1688:class:`~unittest.TestSuite` now drops references to tests as soon as the test
1689has been run, if the test is successful.  On Python interpreters that do
1690garbage collection, this allows the tests to be garbage collected if nothing
1691else is holding a reference to the test.  It is possible to override this
1692behavior by creating a :class:`~unittest.TestSuite` subclass that defines a
1693custom ``_removeTestAtIndex`` method.  (Contributed by Tom Wardill, Matt
1694McClure, and Andrew Svetlov in :issue:`11798`.)
1695
1696A new test assertion context-manager, :meth:`~unittest.TestCase.assertLogs`,
1697will ensure that a given block of code emits a log message using the
1698:mod:`logging` module.  By default the message can come from any logger and
1699have a priority of ``INFO`` or higher, but both the logger name and an
1700alternative minimum logging level may be specified.  The object returned by the
1701context manager can be queried for the :class:`~logging.LogRecord`\ s and/or
1702formatted messages that were logged.  (Contributed by Antoine Pitrou in
1703:issue:`18937`.)
1704
1705Test discovery now works with namespace packages (Contributed by Claudiu Popa
1706in :issue:`17457`.)
1707
1708:mod:`unittest.mock` objects now inspect their specification signatures when
1709matching calls, which means an argument can now be matched by either position
1710or name, instead of only by position.  (Contributed by Antoine Pitrou in
1711:issue:`17015`.)
1712
1713:func:`~mock.mock_open` objects now have ``readline`` and ``readlines``
1714methods.  (Contributed by Toshio Kuratomi in :issue:`17467`.)
1715
1716
1717venv
1718----
1719
1720:mod:`venv` now includes activation scripts for the ``csh`` and ``fish``
1721shells.  (Contributed by Andrew Svetlov in :issue:`15417`.)
1722
1723:class:`~venv.EnvBuilder` and the :func:`~venv.create` convenience function
1724take a new keyword argument *with_pip*, which defaults to ``False``, that
1725controls whether or not :class:`~venv.EnvBuilder` ensures that ``pip`` is
1726installed in the virtual environment.  (Contributed by Nick Coghlan in
1727:issue:`19552` as part of the :pep:`453` implementation.)
1728
1729
1730wave
1731----
1732
1733The :meth:`~wave.getparams` method now returns a namedtuple rather than a
1734plain tuple.  (Contributed by Claudiu Popa in :issue:`17487`.)
1735
1736:meth:`wave.open` now supports the context management protocol.  (Contributed
1737by Claudiu Popa in :issue:`17616`.)
1738
1739:mod:`wave` can now :ref:`write output to unseekable files
1740<wave-write-objects>`.  (Contributed by David Jones, Guilherme Polo, and Serhiy
1741Storchaka in :issue:`5202`.)
1742
1743The :meth:`~wave.Wave_write.writeframesraw` and
1744:meth:`~wave.Wave_write.writeframes` methods now accept any :term:`bytes-like
1745object`.  (Contributed by Serhiy Storchaka in :issue:`8311`.)
1746
1747
1748weakref
1749-------
1750
1751New :class:`~weakref.WeakMethod` class simulates weak references to bound
1752methods.  (Contributed by Antoine Pitrou in :issue:`14631`.)
1753
1754New :class:`~weakref.finalize` class makes it possible to register a callback
1755to be invoked when an object is garbage collected, without needing to
1756carefully manage the lifecycle of the weak reference itself.  (Contributed by
1757Richard Oudkerk in :issue:`15528`.)
1758
1759The callback, if any, associated with a :class:`~weakref.ref` is now
1760exposed via the :attr:`~weakref.ref.__callback__` attribute.  (Contributed
1761by Mark Dickinson in :issue:`17643`.)
1762
1763
1764xml.etree
1765---------
1766
1767A new parser, :class:`~xml.etree.ElementTree.XMLPullParser`, allows a
1768non-blocking applications to parse XML documents.  An example can be
1769seen at :ref:`elementtree-pull-parsing`.  (Contributed by Antoine
1770Pitrou in :issue:`17741`.)
1771
1772The :mod:`xml.etree.ElementTree` :func:`~xml.etree.ElementTree.tostring` and
1773:func:`~xml.etree.ElementTree.tostringlist` functions, and the
1774:class:`~xml.etree.ElementTree.ElementTree`
1775:meth:`~xml.etree.ElementTree.ElementTree.write` method, now have a
1776*short_empty_elements* :ref:`keyword-only parameter <keyword-only_parameter>`
1777providing control over whether elements with no content are written in
1778abbreviated (``<tag />``) or expanded (``<tag></tag>``) form.  (Contributed by
1779Ariel Poliak and Serhiy Storchaka in :issue:`14377`.)
1780
1781
1782zipfile
1783-------
1784
1785The :meth:`~zipfile.PyZipFile.writepy` method of the
1786:class:`~zipfile.PyZipFile` class has a new *filterfunc* option that can be
1787used to control which directories and files are added to the archive.  For
1788example, this could be used to exclude test files from the archive.
1789(Contributed by Christian Tismer in :issue:`19274`.)
1790
1791The *allowZip64* parameter to :class:`~zipfile.ZipFile` and
1792:class:`~zipfile.PyZipfile` is now ``True`` by default.  (Contributed by
1793William Mallard in :issue:`17201`.)
1794
1795
1796
1797CPython Implementation Changes
1798==============================
1799
1800
1801.. _whatsnew-pep-445:
1802
1803PEP 445: Customization of CPython Memory Allocators
1804---------------------------------------------------
1805
1806:pep:`445` adds new C level interfaces to customize memory allocation in
1807the CPython interpreter.
1808
1809.. seealso::
1810
1811   :pep:`445` -- Add new APIs to customize Python memory allocators
1812      PEP written and implemented by Victor Stinner.
1813
1814
1815.. _whatsnew-pep-442:
1816
1817PEP 442: Safe Object Finalization
1818---------------------------------
1819
1820:pep:`442` removes the current limitations and quirks of object finalization
1821in CPython. With it, objects with :meth:`__del__` methods, as well as
1822generators with :keyword:`finally` clauses, can be finalized when they are
1823part of a reference cycle.
1824
1825As part of this change, module globals are no longer forcibly set to
1826:const:`None` during interpreter shutdown in most cases, instead relying
1827on the normal operation of the cyclic garbage collector.  This avoids a
1828whole class of interpreter-shutdown-time errors, usually involving
1829``__del__`` methods, that have plagued Python since the cyclic GC
1830was first introduced.
1831
1832.. seealso::
1833
1834   :pep:`442` -- Safe object finalization
1835      PEP written and implemented by Antoine Pitrou.
1836
1837
1838.. _whatsnew-pep-456:
1839
1840PEP 456: Secure and Interchangeable Hash Algorithm
1841--------------------------------------------------
1842
1843:pep:`456` follows up on earlier security fix work done on Python's hash
1844algorithm to address certain DOS attacks to which public facing APIs backed by
1845dictionary lookups may be subject.  (See :issue:`14621` for the start of the
1846current round of improvements.)  The PEP unifies CPython's hash code to make it
1847easier for a packager to substitute a different hash algorithm, and switches
1848Python's default implementation to a SipHash implementation on platforms that
1849have a 64 bit data type.  Any performance differences in comparison with the
1850older FNV algorithm are trivial.
1851
1852The PEP adds additional fields to the :attr:`sys.hash_info` struct sequence to
1853describe the hash algorithm in use by the currently executing binary.  Otherwise,
1854the PEP does not alter any existing CPython APIs.
1855
1856
1857.. _whatsnew-pep-436:
1858
1859PEP 436: Argument Clinic
1860------------------------
1861
1862"Argument Clinic" (:pep:`436`) is now part of the CPython build process
1863and can be used to simplify the process of defining and maintaining
1864accurate signatures for builtins and standard library extension modules
1865implemented in C.
1866
1867Some standard library extension modules have been converted to use Argument
1868Clinic in Python 3.4, and :mod:`pydoc` and :mod:`inspect` have been updated
1869accordingly.
1870
1871It is expected that signature metadata for programmatic introspection will
1872be added to additional callables implemented in C as part of Python 3.4
1873maintenance releases.
1874
1875.. note::
1876   The Argument Clinic PEP is not fully up to date with the state of the
1877   implementation. This has been deemed acceptable by the release manager
1878   and core development team in this case, as Argument Clinic will not
1879   be made available as a public API for third party use in Python 3.4.
1880
1881.. seealso::
1882
1883   :pep:`436` -- The Argument Clinic DSL
1884       PEP written and implemented by Larry Hastings.
1885
1886
1887Other Build and C API Changes
1888-----------------------------
1889
1890* The new :c:func:`PyType_GetSlot` function has been added to the stable ABI,
1891  allowing retrieval of function pointers from named type slots when using
1892  the limited API.  (Contributed by Martin von Löwis in :issue:`17162`.)
1893
1894* The new :c:func:`Py_SetStandardStreamEncoding` pre-initialization API
1895  allows applications embedding the CPython interpreter to reliably force
1896  a particular encoding and error handler for the standard streams.
1897  (Contributed by Bastien Montagne and Nick Coghlan in :issue:`16129`.)
1898
1899* Most Python C APIs that don't mutate string arguments are now correctly
1900  marked as accepting ``const char *`` rather than ``char *``.  (Contributed
1901  by Serhiy Storchaka in :issue:`1772673`.)
1902
1903* A new shell version of ``python-config`` can be used even when a python
1904  interpreter is not available (for example, in cross compilation scenarios).
1905
1906* :c:func:`PyUnicode_FromFormat` now supports width and precision
1907  specifications for ``%s``, ``%A``, ``%U``, ``%V``, ``%S``, and ``%R``.
1908  (Contributed by Ysj Ray and Victor Stinner in :issue:`7330`.)
1909
1910* New function :c:func:`PyStructSequence_InitType2` supplements the
1911  existing :c:func:`PyStructSequence_InitType` function.  The difference
1912  is that it returns ``0`` on success and ``-1`` on failure.
1913
1914* The CPython source can now be compiled using the address sanity checking
1915  features of recent versions of GCC and clang:  the false alarms in the small
1916  object allocator have been silenced.  (Contributed by Dhiru Kholia in
1917  :issue:`18596`.)
1918
1919* The Windows build now uses `Address Space Layout Randomization
1920  <https://en.wikipedia.org/wiki/Address_space_layout_randomization>`_ and `Data Execution Prevention
1921  <https://en.wikipedia.org/wiki/Data_Execution_Prevention>`_.  (Contributed by
1922  Christian Heimes in :issue:`16632`.)
1923
1924* New function :c:func:`PyObject_LengthHint` is the C API equivalent
1925  of :func:`operator.length_hint`.  (Contributed by Armin Ronacher in
1926  :issue:`16148`.)
1927
1928
1929.. _other-improvements-3.4:
1930
1931Other Improvements
1932------------------
1933
1934.. _whatsnew-isolated-mode:
1935
1936* The :ref:`python <using-on-cmdline>` command has a new :ref:`option
1937  <using-on-misc-options>`, ``-I``, which causes it to run in "isolated mode",
1938  which means that :data:`sys.path` contains neither the script's directory nor
1939  the user's ``site-packages`` directory, and all :envvar:`PYTHON*` environment
1940  variables are ignored (it implies both ``-s`` and ``-E``).  Other
1941  restrictions may also be applied in the future, with the goal being to
1942  isolate the execution of a script from the user's environment.  This is
1943  appropriate, for example, when Python is used to run a system script.  On
1944  most POSIX systems it can and should be used in the ``#!`` line of system
1945  scripts.  (Contributed by Christian Heimes in :issue:`16499`.)
1946
1947* Tab-completion is now enabled by default in the interactive interpreter
1948  on systems that support :mod:`readline`.  History is also enabled by default,
1949  and is written to (and read from) the file :file:`~/.python-history`.
1950  (Contributed by Antoine Pitrou and Éric Araujo in :issue:`5845`.)
1951
1952* Invoking the Python interpreter with ``--version`` now outputs the version to
1953  standard output instead of standard error (:issue:`18338`). Similar changes
1954  were made to :mod:`argparse` (:issue:`18920`) and other modules that have
1955  script-like invocation capabilities (:issue:`18922`).
1956
1957* The CPython Windows installer now adds ``.py`` to the :envvar:`PATHEXT`
1958  variable when extensions are registered, allowing users to run a python
1959  script at the windows command prompt by just typing its name without the
1960  ``.py`` extension.  (Contributed by Paul Moore in :issue:`18569`.)
1961
1962* A new ``make`` target `coverage-report
1963  <https://devguide.python.org/coverage/#measuring-coverage-of-c-code-with-gcov-and-lcov>`_
1964  will build python, run the test suite, and generate an HTML coverage report
1965  for the C codebase using ``gcov`` and `lcov
1966  <http://ltp.sourceforge.net/coverage/lcov.php>`_.
1967
1968* The ``-R`` option to the :ref:`python regression test suite <regrtest>` now
1969  also checks for memory allocation leaks, using
1970  :func:`sys.getallocatedblocks()`.  (Contributed by Antoine Pitrou in
1971  :issue:`13390`.)
1972
1973* ``python -m`` now works with namespace packages.
1974
1975* The :mod:`stat` module is now implemented in C, which means it gets the
1976  values for its constants from the C header files, instead of having the
1977  values hard-coded in the python module as was previously the case.
1978
1979* Loading multiple python modules from a single OS module (``.so``, ``.dll``)
1980  now works correctly (previously it silently returned the first python
1981  module in the file).  (Contributed by Václav Šmilauer in :issue:`16421`.)
1982
1983* A new opcode, :opcode:`LOAD_CLASSDEREF`, has been added to fix a bug in the
1984  loading of free variables in class bodies that could be triggered by certain
1985  uses of :ref:`__prepare__ <prepare>`.  (Contributed by Benjamin Peterson in
1986  :issue:`17853`.)
1987
1988* A number of MemoryError-related crashes were identified and fixed by Victor
1989  Stinner using his :pep:`445`-based ``pyfailmalloc`` tool (:issue:`18408`,
1990  :issue:`18520`).
1991
1992* The ``pyvenv`` command now accepts a ``--copies`` option
1993  to use copies rather than symlinks even on systems where symlinks are the
1994  default.  (Contributed by Vinay Sajip in :issue:`18807`.)
1995
1996* The ``pyvenv`` command also accepts a ``--without-pip``
1997  option to suppress the otherwise-automatic bootstrapping of pip into
1998  the virtual environment.  (Contributed by Nick Coghlan in :issue:`19552`
1999  as part of the :pep:`453` implementation.)
2000
2001* The encoding name is now optional in the value set for the
2002  :envvar:`PYTHONIOENCODING` environment variable.  This makes it possible to
2003  set just the error handler, without changing the default encoding.
2004  (Contributed by Serhiy Storchaka in :issue:`18818`.)
2005
2006* The :mod:`bz2`, :mod:`lzma`, and :mod:`gzip` module ``open`` functions now
2007  support ``x`` (exclusive creation) mode.  (Contributed by Tim Heaney and
2008  Vajrasky Kok in :issue:`19201`, :issue:`19222`, and :issue:`19223`.)
2009
2010
2011Significant Optimizations
2012-------------------------
2013
2014* The UTF-32 decoder is now 3x to 4x faster.  (Contributed by Serhiy Storchaka
2015  in :issue:`14625`.)
2016
2017* The cost of hash collisions for sets is now reduced.  Each hash table
2018  probe now checks a series of consecutive, adjacent key/hash pairs before
2019  continuing to make random probes through the hash table.  This exploits
2020  cache locality to make collision resolution less expensive.
2021  The collision resolution scheme can be described as a hybrid of linear
2022  probing and open addressing.  The number of additional linear probes
2023  defaults to nine.  This can be changed at compile-time by defining
2024  LINEAR_PROBES to be any value.  Set LINEAR_PROBES=0 to turn-off
2025  linear probing entirely.  (Contributed by Raymond Hettinger in
2026  :issue:`18771`.)
2027
2028* The interpreter starts about 30% faster. A couple of measures lead to the
2029  speedup. The interpreter loads fewer modules on startup, e.g. the :mod:`re`,
2030  :mod:`collections` and :mod:`locale` modules and their dependencies are no
2031  longer imported by default. The marshal module has been improved to load
2032  compiled Python code faster.  (Contributed by Antoine Pitrou, Christian
2033  Heimes and Victor Stinner in :issue:`19219`, :issue:`19218`, :issue:`19209`,
2034  :issue:`19205` and :issue:`9548`.)
2035
2036* :class:`bz2.BZ2File` is now as fast or faster than the Python2 version for
2037  most cases.  :class:`lzma.LZMAFile` has also been optimized.  (Contributed by
2038  Serhiy Storchaka and Nadeem Vawda in :issue:`16034`.)
2039
2040* :func:`random.getrandbits` is 20%-40% faster for small integers (the most
2041  common use case).  (Contributed by Serhiy Storchaka in :issue:`16674`.)
2042
2043* By taking advantage of the new storage format for strings, pickling of
2044  strings is now significantly faster.  (Contributed by Victor Stinner and
2045  Antoine Pitrou in :issue:`15596`.)
2046
2047* A performance issue in :meth:`io.FileIO.readall` has been solved.  This
2048  particularly affects Windows, and significantly speeds up the case of piping
2049  significant amounts of data through :mod:`subprocess`.  (Contributed
2050  by Richard Oudkerk in :issue:`15758`.)
2051
2052* :func:`html.escape` is now 10x faster.  (Contributed by Matt Bryant in
2053  :issue:`18020`.)
2054
2055* On Windows, the native ``VirtualAlloc`` is now used instead of the CRT
2056  ``malloc`` in ``obmalloc``.  Artificial benchmarks show about a 3% memory
2057  savings.
2058
2059* :func:`os.urandom` now uses a lazily-opened persistent file descriptor
2060  so as to avoid using many file descriptors when run in parallel from
2061  multiple threads.  (Contributed by Antoine Pitrou in :issue:`18756`.)
2062
2063
2064.. _deprecated-3.4:
2065
2066Deprecated
2067==========
2068
2069This section covers various APIs and other features that have been deprecated
2070in Python 3.4, and will be removed in Python 3.5 or later.  In most (but not
2071all) cases, using the deprecated APIs will produce a :exc:`DeprecationWarning`
2072when the interpreter is run with deprecation warnings enabled (for example, by
2073using ``-Wd``).
2074
2075
2076Deprecations in the Python API
2077------------------------------
2078
2079* As mentioned in :ref:`whatsnew-pep-451`, a number of :mod:`importlib`
2080  methods and functions are deprecated: :meth:`importlib.find_loader` is
2081  replaced by :func:`importlib.util.find_spec`;
2082  :meth:`importlib.machinery.PathFinder.find_module` is replaced by
2083  :meth:`importlib.machinery.PathFinder.find_spec`;
2084  :meth:`importlib.abc.MetaPathFinder.find_module` is replaced by
2085  :meth:`importlib.abc.MetaPathFinder.find_spec`;
2086  :meth:`importlib.abc.PathEntryFinder.find_loader` and
2087  :meth:`~importlib.abc.PathEntryFinder.find_module` are replaced by
2088  :meth:`importlib.abc.PathEntryFinder.find_spec`; all of the ``xxxLoader`` ABC
2089  ``load_module`` methods (:meth:`importlib.abc.Loader.load_module`,
2090  :meth:`importlib.abc.InspectLoader.load_module`,
2091  :meth:`importlib.abc.FileLoader.load_module`,
2092  :meth:`importlib.abc.SourceLoader.load_module`) should no longer be
2093  implemented, instead loaders should implement an
2094  ``exec_module`` method
2095  (:meth:`importlib.abc.Loader.exec_module`,
2096  :meth:`importlib.abc.InspectLoader.exec_module`
2097  :meth:`importlib.abc.SourceLoader.exec_module`) and let the import system
2098  take care of the rest; and
2099  :meth:`importlib.abc.Loader.module_repr`,
2100  :meth:`importlib.util.module_for_loader`, :meth:`importlib.util.set_loader`,
2101  and :meth:`importlib.util.set_package` are no longer needed because their
2102  functions are now handled automatically by the import system.
2103
2104* The :mod:`imp` module is pending deprecation. To keep compatibility with
2105  Python 2/3 code bases, the module's removal is currently not scheduled.
2106
2107* The :mod:`formatter` module is pending deprecation and is slated for removal
2108  in Python 3.6.
2109
2110* ``MD5`` as the default *digestmod* for the :func:`hmac.new` function is
2111  deprecated.  Python 3.6 will require an explicit digest name or constructor as
2112  *digestmod* argument.
2113
2114* The internal ``Netrc`` class in the :mod:`ftplib` module has been documented
2115  as deprecated in its docstring for quite some time.  It now emits a
2116  :exc:`DeprecationWarning` and will be removed completely in Python 3.5.
2117
2118* The undocumented *endtime* argument to :meth:`subprocess.Popen.wait` should
2119  not have been exposed and is hopefully not in use; it is deprecated and
2120  will mostly likely be removed in Python 3.5.
2121
2122* The *strict* argument of :class:`~html.parser.HTMLParser` is deprecated.
2123
2124* The :mod:`plistlib` :func:`~plistlib.readPlist`,
2125  :func:`~plistlib.writePlist`, :func:`~plistlib.readPlistFromBytes`, and
2126  :func:`~plistlib.writePlistToBytes` functions are deprecated in favor of the
2127  corresponding new functions :func:`~plistlib.load`, :func:`~plistlib.dump`,
2128  :func:`~plistlib.loads`, and :func:`~plistlib.dumps`.  :func:`~plistlib.Data`
2129  is deprecated in favor of just using the :class:`bytes` constructor.
2130
2131* The :mod:`sysconfig` key ``SO`` is deprecated, it has been replaced by
2132  ``EXT_SUFFIX``.
2133
2134* The ``U`` mode accepted by various ``open`` functions is deprecated.
2135  In Python3 it does not do anything useful, and should be replaced by
2136  appropriate uses of :class:`io.TextIOWrapper` (if needed) and its *newline*
2137  argument.
2138
2139* The *parser* argument of :func:`xml.etree.ElementTree.iterparse` has
2140  been deprecated, as has the *html* argument of
2141  :func:`~xml.etree.ElementTree.XMLParser`.  To prepare for the removal of the
2142  latter, all arguments to ``XMLParser`` should be passed by keyword.
2143
2144
2145Deprecated Features
2146-------------------
2147
2148* Running :ref:`idle` with the ``-n`` flag (no subprocess) is deprecated.
2149  However, the feature will not be removed until :issue:`18823` is resolved.
2150
2151* The site module adding a "site-python" directory to sys.path, if it
2152  exists, is deprecated (:issue:`19375`).
2153
2154
2155
2156Removed
2157=======
2158
2159
2160Operating Systems No Longer Supported
2161-------------------------------------
2162
2163Support for the following operating systems has been removed from the source
2164and build tools:
2165
2166* OS/2 (:issue:`16135`).
2167* Windows 2000 (changeset e52df05b496a).
2168* Windows systems where ``COMSPEC`` points to ``command.com`` (:issue:`14470`).
2169* VMS (:issue:`16136`).
2170
2171
2172API and Feature Removals
2173------------------------
2174
2175The following obsolete and previously deprecated APIs and features have been
2176removed:
2177
2178* The unmaintained ``Misc/TextMate`` and ``Misc/vim`` directories have been
2179  removed (see the `devguide <https://devguide.python.org>`_
2180  for suggestions on what to use instead).
2181
2182* The ``SO`` makefile macro is removed (it was replaced by the
2183  ``SHLIB_SUFFIX`` and ``EXT_SUFFIX`` macros) (:issue:`16754`).
2184
2185* The ``PyThreadState.tick_counter`` field has been removed; its value has
2186  been meaningless since Python 3.2, when the "new GIL" was introduced
2187  (:issue:`19199`).
2188
2189* ``PyLoader`` and ``PyPycLoader`` have been removed from :mod:`importlib`.
2190  (Contributed by Taras Lyapun in :issue:`15641`.)
2191
2192* The *strict* argument to :class:`~http.client.HTTPConnection` and
2193  :class:`~http.client.HTTPSConnection` has been removed.  HTTP 0.9-style
2194  "Simple Responses" are no longer supported.
2195
2196* The deprecated :mod:`urllib.request.Request` getter and setter methods
2197  ``add_data``, ``has_data``, ``get_data``, ``get_type``, ``get_host``,
2198  ``get_selector``, ``set_proxy``, ``get_origin_req_host``, and
2199  ``is_unverifiable`` have been removed (use direct attribute access instead).
2200
2201* Support for loading the deprecated ``TYPE_INT64`` has been removed from
2202  :mod:`marshal`.  (Contributed by Dan Riti in :issue:`15480`.)
2203
2204* :class:`inspect.Signature`: positional-only parameters are now required
2205  to have a valid name.
2206
2207* :meth:`object.__format__` no longer accepts non-empty format strings, it now
2208  raises a :exc:`TypeError` instead.  Using a non-empty string has been
2209  deprecated since Python 3.2.  This change has been made to prevent a
2210  situation where previously working (but incorrect) code would start failing
2211  if an object gained a __format__ method, which means that your code may now
2212  raise a :exc:`TypeError` if you are using an ``'s'`` format code with objects
2213  that do not have a __format__ method that handles it.  See :issue:`7994` for
2214  background.
2215
2216* :meth:`difflib.SequenceMatcher.isbjunk` and
2217  :meth:`difflib.SequenceMatcher.isbpopular` were deprecated in 3.2, and have
2218  now been removed: use ``x in sm.bjunk`` and
2219  ``x in sm.bpopular``, where *sm* is a :class:`~difflib.SequenceMatcher` object
2220  (:issue:`13248`).
2221
2222
2223Code Cleanups
2224-------------
2225
2226* The unused and undocumented internal ``Scanner`` class has been removed from
2227  the :mod:`pydoc` module.
2228
2229* The private and effectively unused ``_gestalt`` module has been removed,
2230  along with the private :mod:`platform` functions ``_mac_ver_lookup``,
2231  ``_mac_ver_gstalt``, and ``_bcd2str``, which would only have ever been called
2232  on badly broken OSX systems (see :issue:`18393`).
2233
2234* The hardcoded copies of certain :mod:`stat` constants that were included in
2235  the :mod:`tarfile` module namespace have been removed.
2236
2237
2238
2239Porting to Python 3.4
2240=====================
2241
2242This section lists previously described changes and other bugfixes
2243that may require changes to your code.
2244
2245
2246Changes in 'python' Command Behavior
2247------------------------------------
2248
2249* In a posix shell, setting the :envvar:`PATH` environment variable to
2250  an empty value is equivalent to not setting it at all.  However, setting
2251  :envvar:`PYTHONPATH` to an empty value was *not* equivalent to not setting it
2252  at all: setting :envvar:`PYTHONPATH` to an empty value was equivalent to
2253  setting it to ``.``, which leads to confusion when reasoning by analogy to
2254  how :envvar:`PATH` works.  The behavior now conforms to the posix convention
2255  for :envvar:`PATH`.
2256
2257* The [X refs, Y blocks] output of a debug (``--with-pydebug``) build of the
2258  CPython interpreter is now off by default.  It can be re-enabled using the
2259  ``-X showrefcount`` option.  (Contributed by Ezio Melotti in :issue:`17323`.)
2260
2261* The python command and most stdlib scripts (as well as :mod:`argparse`) now
2262  output ``--version`` information to ``stdout`` instead of ``stderr`` (for
2263  issue list see :ref:`other-improvements-3.4` above).
2264
2265
2266Changes in the Python API
2267-------------------------
2268
2269* The ABCs defined in :mod:`importlib.abc` now either raise the appropriate
2270  exception or return a default value instead of raising
2271  :exc:`NotImplementedError` blindly. This will only affect code calling
2272  :func:`super` and falling through all the way to the ABCs. For compatibility,
2273  catch both :exc:`NotImplementedError` or the appropriate exception as needed.
2274
2275* The module type now initializes the :attr:`__package__` and :attr:`__loader__`
2276  attributes to ``None`` by default. To determine if these attributes were set
2277  in a backwards-compatible fashion, use e.g.
2278  ``getattr(module, '__loader__', None) is not None``.  (:issue:`17115`.)
2279
2280* :meth:`importlib.util.module_for_loader` now sets ``__loader__`` and
2281  ``__package__`` unconditionally to properly support reloading. If this is not
2282  desired then you will need to set these attributes manually. You can use
2283  :func:`importlib.util.module_to_load` for module management.
2284
2285* Import now resets relevant attributes (e.g. ``__name__``, ``__loader__``,
2286  ``__package__``, ``__file__``, ``__cached__``) unconditionally when reloading.
2287  Note that this restores a pre-3.3 behavior in that it means a module is
2288  re-found when re-loaded (:issue:`19413`).
2289
2290* Frozen packages no longer set ``__path__`` to a list containing the package
2291  name, they now set it to an empty list.  The previous behavior could cause
2292  the import system to do the wrong thing on submodule imports if there was
2293  also a directory with the same name as the frozen package.  The correct way
2294  to determine if a module is a package or not is to use ``hasattr(module,
2295  '__path__')`` (:issue:`18065`).
2296
2297* Frozen modules no longer define a ``__file__`` attribute. It's semantically
2298  incorrect for frozen modules to set the attribute as they are not loaded from
2299  any explicit location. If you must know that a module comes from frozen code
2300  then you can see if the module's ``__spec__.location`` is set to ``'frozen'``,
2301  check if the loader is a subclass of
2302  :class:`importlib.machinery.FrozenImporter`,
2303  or if Python 2 compatibility is necessary you can use :func:`imp.is_frozen`.
2304
2305* :func:`py_compile.compile` now raises :exc:`FileExistsError` if the file path
2306  it would write to is a symlink or a non-regular file. This is to act as a
2307  warning that import will overwrite those files with a regular file regardless
2308  of what type of file path they were originally.
2309
2310* :meth:`importlib.abc.SourceLoader.get_source` no longer raises
2311  :exc:`ImportError` when the source code being loaded triggers a
2312  :exc:`SyntaxError` or :exc:`UnicodeDecodeError`. As :exc:`ImportError` is
2313  meant to be raised only when source code cannot be found but it should, it was
2314  felt to be over-reaching/overloading of that meaning when the source code is
2315  found but improperly structured. If you were catching ImportError before and
2316  wish to continue to ignore syntax or decoding issues, catch all three
2317  exceptions now.
2318
2319* :func:`functools.update_wrapper` and :func:`functools.wraps` now correctly
2320  set the ``__wrapped__`` attribute to the function being wrapped, even if
2321  that function also had its ``__wrapped__`` attribute set. This means
2322  ``__wrapped__`` attributes now correctly link a stack of decorated
2323  functions rather than every ``__wrapped__`` attribute in the chain
2324  referring to the innermost function. Introspection libraries that
2325  assumed the previous behaviour was intentional can use
2326  :func:`inspect.unwrap` to access the first function in the chain that has
2327  no ``__wrapped__`` attribute.
2328
2329* :func:`inspect.getfullargspec` has been reimplemented on top of
2330  :func:`inspect.signature` and hence handles a much wider variety of callable
2331  objects than it did in the past. It is expected that additional builtin and
2332  extension module callables will gain signature metadata over the course of
2333  the Python 3.4 series. Code that assumes that
2334  :func:`inspect.getfullargspec` will fail on non-Python callables may need
2335  to be adjusted accordingly.
2336
2337* :class:`importlib.machinery.PathFinder` now passes on the current working
2338  directory to objects in :data:`sys.path_hooks` for the empty string. This
2339  results in :data:`sys.path_importer_cache` never containing ``''``, thus
2340  iterating through :data:`sys.path_importer_cache` based on :data:`sys.path`
2341  will not find all keys. A module's ``__file__`` when imported in the current
2342  working directory will also now have an absolute path, including when using
2343  ``-m`` with the interpreter (except for ``__main__.__file__`` when a script
2344  has been executed directly using a relative path) (Contributed by Brett
2345  Cannon in :issue:`18416`).  is specified on the command-line)
2346  (:issue:`18416`).
2347
2348* The removal of the *strict* argument to :class:`~http.client.HTTPConnection`
2349  and :class:`~http.client.HTTPSConnection` changes the meaning of the
2350  remaining arguments if you are specifying them positionally rather than by
2351  keyword.  If you've been paying attention to deprecation warnings your code
2352  should already be specifying any additional arguments via keywords.
2353
2354* Strings between ``from __future__ import ...`` statements now *always* raise
2355  a :exc:`SyntaxError`.  Previously if there was no leading docstring, an
2356  interstitial string would sometimes be ignored.  This brings CPython into
2357  compliance with the language spec; Jython and PyPy already were.
2358  (:issue:`17434`).
2359
2360* :meth:`ssl.SSLSocket.getpeercert` and :meth:`ssl.SSLSocket.do_handshake`
2361  now raise an :exc:`OSError` with ``ENOTCONN`` when the ``SSLSocket`` is not
2362  connected, instead of the previous behavior of raising an
2363  :exc:`AttributeError`.  In addition, :meth:`~ssl.SSLSocket.getpeercert`
2364  will raise a :exc:`ValueError` if the handshake has not yet been done.
2365
2366* :func:`base64.b32decode` now raises a :exc:`binascii.Error` when the
2367  input string contains non-b32-alphabet characters, instead of a
2368  :exc:`TypeError`.  This particular :exc:`TypeError` was missed when the other
2369  :exc:`TypeError`\ s were converted.  (Contributed by Serhiy Storchaka in
2370  :issue:`18011`.)  Note: this change was also inadvertently applied in Python
2371  3.3.3.
2372
2373* The :attr:`~cgi.FieldStorage.file` attribute is now automatically closed when
2374  the creating :class:`cgi.FieldStorage` instance is garbage collected. If you
2375  were pulling the file object out separately from the :class:`cgi.FieldStorage`
2376  instance and not keeping the instance alive, then you should either store the
2377  entire :class:`cgi.FieldStorage` instance or read the contents of the file
2378  before the :class:`cgi.FieldStorage` instance is garbage collected.
2379
2380* Calling ``read`` or ``write`` on a closed SSL socket now raises an
2381  informative :exc:`ValueError` rather than the previous more mysterious
2382  :exc:`AttributeError` (:issue:`9177`).
2383
2384* :meth:`slice.indices` no longer produces an :exc:`OverflowError` for huge
2385  values.  As a consequence of this fix, :meth:`slice.indices` now raises a
2386  :exc:`ValueError` if given a negative length; previously it returned nonsense
2387  values (:issue:`14794`).
2388
2389* The :class:`complex` constructor, unlike the :mod:`cmath` functions, was
2390  incorrectly accepting :class:`float` values if an object's ``__complex__``
2391  special method returned one.  This now raises a :exc:`TypeError`.
2392  (:issue:`16290`.)
2393
2394* The :class:`int` constructor in 3.2 and 3.3 erroneously accepts :class:`float`
2395  values for the *base* parameter.  It is unlikely anyone was doing this, but
2396  if so, it will now raise a :exc:`TypeError` (:issue:`16772`).
2397
2398* Defaults for keyword-only arguments are now evaluated *after* defaults for
2399  regular keyword arguments, instead of before.  Hopefully no one wrote any
2400  code that depends on the previous buggy behavior (:issue:`16967`).
2401
2402* Stale thread states are now cleared after :func:`~os.fork`.  This may cause
2403  some system resources to be released that previously were incorrectly kept
2404  perpetually alive (for example, database connections kept in thread-local
2405  storage).  (:issue:`17094`.)
2406
2407* Parameter names in ``__annotations__`` dicts are now mangled properly,
2408  similarly to ``__kwdefaults__``.  (Contributed by Yury Selivanov in
2409  :issue:`20625`.)
2410
2411* :attr:`hashlib.hash.name` now always returns the identifier in lower case.
2412  Previously some builtin hashes had uppercase names, but now that it is a
2413  formal public interface the naming has been made consistent (:issue:`18532`).
2414
2415* Because :mod:`unittest.TestSuite` now drops references to tests after they
2416  are run, test harnesses that re-use a :class:`~unittest.TestSuite` to re-run
2417  a set of tests may fail.   Test suites should not be re-used in this fashion
2418  since it means state is retained between test runs, breaking the test
2419  isolation that :mod:`unittest` is designed to provide.  However, if the lack
2420  of isolation is considered acceptable, the old behavior can be restored by
2421  creating a :mod:`~unittest.TestSuite` subclass that defines a
2422  ``_removeTestAtIndex`` method that does nothing (see
2423  :meth:`.TestSuite.__iter__`) (:issue:`11798`).
2424
2425* :mod:`unittest` now uses :mod:`argparse` for command line parsing.  There are
2426  certain invalid command forms that used to work that are no longer allowed;
2427  in theory this should not cause backward compatibility issues since the
2428  disallowed command forms didn't make any sense and are unlikely to be in use.
2429
2430* The :func:`re.split`, :func:`re.findall`, and :func:`re.sub` functions, and
2431  the :meth:`~re.match.group` and :meth:`~re.match.groups` methods of
2432  ``match`` objects now always return a *bytes* object when the string
2433  to be matched is a :term:`bytes-like object`.  Previously the return type
2434  matched the input type, so if your code was depending on the return value
2435  being, say, a ``bytearray``, you will need to change your code.
2436
2437* :mod:`audioop` functions now raise an error immediately if passed string
2438  input, instead of failing randomly later on (:issue:`16685`).
2439
2440* The new *convert_charrefs* argument to :class:`~html.parser.HTMLParser`
2441  currently defaults to ``False`` for backward compatibility, but will
2442  eventually be changed to default to ``True``.  It is recommended that you add
2443  this keyword, with the appropriate value, to any
2444  :class:`~html.parser.HTMLParser` calls in your code (:issue:`13633`).
2445
2446* Since the *digestmod* argument to the :func:`hmac.new` function will in the
2447  future have no default, all calls to :func:`hmac.new` should be changed to
2448  explicitly specify a *digestmod* (:issue:`17276`).
2449
2450* Calling :func:`sysconfig.get_config_var` with the ``SO`` key, or looking
2451  ``SO`` up in the results of a call to :func:`sysconfig.get_config_vars`
2452  is deprecated.  This key should be replaced by ``EXT_SUFFIX`` or
2453  ``SHLIB_SUFFIX``, depending on the context (:issue:`19555`).
2454
2455* Any calls to ``open`` functions that specify ``U`` should be modified.
2456  ``U`` is ineffective in Python3 and will eventually raise an error if used.
2457  Depending on the function, the equivalent of its old Python2 behavior can be
2458  achieved using either a *newline* argument, or if necessary by wrapping the
2459  stream in :mod:`~io.TextIOWrapper` to use its *newline* argument
2460  (:issue:`15204`).
2461
2462* If you use ``pyvenv`` in a script and desire that pip
2463  *not* be installed, you must add ``--without-pip`` to your command
2464  invocation.
2465
2466* The default behavior of :func:`json.dump` and :func:`json.dumps` when
2467  an indent is specified has changed: it no longer produces trailing
2468  spaces after the item separating commas at the ends of lines.  This
2469  will matter only if you have tests that are doing white-space-sensitive
2470  comparisons of such output (:issue:`16333`).
2471
2472* :mod:`doctest` now looks for doctests in extension module ``__doc__``
2473  strings, so if your doctest test discovery includes extension modules that
2474  have things that look like doctests in them you may see test failures you've
2475  never seen before when running your tests (:issue:`3158`).
2476
2477* The :mod:`collections.abc` module has been slightly refactored as
2478  part of the Python startup improvements.  As a consequence of this, it is no
2479  longer the case that importing :mod:`collections` automatically imports
2480  :mod:`collections.abc`.  If your program depended on the (undocumented)
2481  implicit import, you will need to add an explicit ``import collections.abc``
2482  (:issue:`20784`).
2483
2484
2485Changes in the C API
2486--------------------
2487
2488* :c:func:`PyEval_EvalFrameEx`, :c:func:`PyObject_Repr`, and
2489  :c:func:`PyObject_Str`, along with some other internal C APIs, now include
2490  a debugging assertion that ensures they are not used in situations where
2491  they may silently discard a currently active exception. In cases where
2492  discarding the active exception is expected and desired (for example,
2493  because it has already been saved locally with :c:func:`PyErr_Fetch` or
2494  is being deliberately replaced with a different exception), an explicit
2495  :c:func:`PyErr_Clear` call will be needed to avoid triggering the
2496  assertion when invoking these operations (directly or indirectly) and
2497  running against a version of Python that is compiled with assertions
2498  enabled.
2499
2500* :c:func:`PyErr_SetImportError` now sets :exc:`TypeError` when its **msg**
2501  argument is not set. Previously only ``NULL`` was returned with no exception
2502  set.
2503
2504* The result of the :c:data:`PyOS_ReadlineFunctionPointer` callback must
2505  now be a string allocated by :c:func:`PyMem_RawMalloc` or
2506  :c:func:`PyMem_RawRealloc`, or *NULL* if an error occurred, instead of a
2507  string allocated by :c:func:`PyMem_Malloc` or :c:func:`PyMem_Realloc`
2508  (:issue:`16742`)
2509
2510* :c:func:`PyThread_set_key_value` now always set the value. In Python
2511  3.3, the function did nothing if the key already exists (if the current
2512  value is a non-NULL pointer).
2513
2514* The ``f_tstate`` (thread state) field of the :c:type:`PyFrameObject`
2515  structure has been removed to fix a bug: see :issue:`14432` for the
2516  rationale.
2517
2518Changed in 3.4.3
2519================
2520
2521.. _pep-476:
2522
2523PEP 476: Enabling certificate verification by default for stdlib http clients
2524-----------------------------------------------------------------------------
2525
2526:mod:`http.client` and modules which use it, such as :mod:`urllib.request` and
2527:mod:`xmlrpc.client`, will now verify that the server presents a certificate
2528which is signed by a CA in the platform trust store and whose hostname matches
2529the hostname being requested by default, significantly improving security for
2530many applications.
2531
2532For applications which require the old previous behavior, they can pass an
2533alternate context::
2534
2535    import urllib.request
2536    import ssl
2537
2538    # This disables all verification
2539    context = ssl._create_unverified_context()
2540
2541    # This allows using a specific certificate for the host, which doesn't need
2542    # to be in the trust store
2543    context = ssl.create_default_context(cafile="/path/to/file.crt")
2544
2545    urllib.request.urlopen("https://invalid-cert", context=context)
2546