1:mod:`!shelve` --- Python object persistence 2============================================ 3 4.. module:: shelve 5 :synopsis: Python object persistence. 6 7**Source code:** :source:`Lib/shelve.py` 8 9.. index:: pair: module; pickle 10 11-------------- 12 13A "shelf" is a persistent, dictionary-like object. The difference with "dbm" 14databases is that the values (not the keys!) in a shelf can be essentially 15arbitrary Python objects --- anything that the :mod:`pickle` module can handle. 16This includes most class instances, recursive data types, and objects containing 17lots of shared sub-objects. The keys are ordinary strings. 18 19 20.. function:: open(filename, flag='c', protocol=None, writeback=False) 21 22 Open a persistent dictionary. The filename specified is the base filename for 23 the underlying database. As a side-effect, an extension may be added to the 24 filename and more than one file may be created. By default, the underlying 25 database file is opened for reading and writing. The optional *flag* parameter 26 has the same interpretation as the *flag* parameter of :func:`dbm.open`. 27 28 By default, pickles created with :const:`pickle.DEFAULT_PROTOCOL` are used 29 to serialize values. The version of the pickle protocol can be specified 30 with the *protocol* parameter. 31 32 Because of Python semantics, a shelf cannot know when a mutable 33 persistent-dictionary entry is modified. By default modified objects are 34 written *only* when assigned to the shelf (see :ref:`shelve-example`). If the 35 optional *writeback* parameter is set to ``True``, all entries accessed are also 36 cached in memory, and written back on :meth:`~Shelf.sync` and 37 :meth:`~Shelf.close`; this can make it handier to mutate mutable entries in 38 the persistent dictionary, but, if many entries are accessed, it can consume 39 vast amounts of memory for the cache, and it can make the close operation 40 very slow since all accessed entries are written back (there is no way to 41 determine which accessed entries are mutable, nor which ones were actually 42 mutated). 43 44 .. versionchanged:: 3.10 45 :const:`pickle.DEFAULT_PROTOCOL` is now used as the default pickle 46 protocol. 47 48 .. versionchanged:: 3.11 49 Accepts :term:`path-like object` for filename. 50 51 .. note:: 52 53 Do not rely on the shelf being closed automatically; always call 54 :meth:`~Shelf.close` explicitly when you don't need it any more, or 55 use :func:`shelve.open` as a context manager:: 56 57 with shelve.open('spam') as db: 58 db['eggs'] = 'eggs' 59 60.. _shelve-security: 61 62.. warning:: 63 64 Because the :mod:`shelve` module is backed by :mod:`pickle`, it is insecure 65 to load a shelf from an untrusted source. Like with pickle, loading a shelf 66 can execute arbitrary code. 67 68Shelf objects support most of methods and operations supported by dictionaries 69(except copying, constructors and operators ``|`` and ``|=``). This eases the 70transition from dictionary based scripts to those requiring persistent storage. 71 72Two additional methods are supported: 73 74.. method:: Shelf.sync() 75 76 Write back all entries in the cache if the shelf was opened with *writeback* 77 set to :const:`True`. Also empty the cache and synchronize the persistent 78 dictionary on disk, if feasible. This is called automatically when the shelf 79 is closed with :meth:`close`. 80 81.. method:: Shelf.close() 82 83 Synchronize and close the persistent *dict* object. Operations on a closed 84 shelf will fail with a :exc:`ValueError`. 85 86 87.. seealso:: 88 89 `Persistent dictionary recipe <https://code.activestate.com/recipes/576642-persistent-dict-with-multiple-standard-file-format/>`_ 90 with widely supported storage formats and having the speed of native 91 dictionaries. 92 93 94Restrictions 95------------ 96 97.. index:: 98 pair: module; dbm.ndbm 99 pair: module; dbm.gnu 100 101* The choice of which database package will be used (such as :mod:`dbm.ndbm` or 102 :mod:`dbm.gnu`) depends on which interface is available. Therefore it is not 103 safe to open the database directly using :mod:`dbm`. The database is also 104 (unfortunately) subject to the limitations of :mod:`dbm`, if it is used --- 105 this means that (the pickled representation of) the objects stored in the 106 database should be fairly small, and in rare cases key collisions may cause 107 the database to refuse updates. 108 109* The :mod:`shelve` module does not support *concurrent* read/write access to 110 shelved objects. (Multiple simultaneous read accesses are safe.) When a 111 program has a shelf open for writing, no other program should have it open for 112 reading or writing. Unix file locking can be used to solve this, but this 113 differs across Unix versions and requires knowledge about the database 114 implementation used. 115 116* On macOS :mod:`dbm.ndbm` can silently corrupt the database file on updates, 117 which can cause hard crashes when trying to read from the database. 118 119 120.. class:: Shelf(dict, protocol=None, writeback=False, keyencoding='utf-8') 121 122 A subclass of :class:`collections.abc.MutableMapping` which stores pickled 123 values in the *dict* object. 124 125 By default, pickles created with :const:`pickle.DEFAULT_PROTOCOL` are used 126 to serialize values. The version of the pickle protocol can be specified 127 with the *protocol* parameter. See the :mod:`pickle` documentation for a 128 discussion of the pickle protocols. 129 130 If the *writeback* parameter is ``True``, the object will hold a cache of all 131 entries accessed and write them back to the *dict* at sync and close times. 132 This allows natural operations on mutable entries, but can consume much more 133 memory and make sync and close take a long time. 134 135 The *keyencoding* parameter is the encoding used to encode keys before they 136 are used with the underlying dict. 137 138 A :class:`Shelf` object can also be used as a context manager, in which 139 case it will be automatically closed when the :keyword:`with` block ends. 140 141 .. versionchanged:: 3.2 142 Added the *keyencoding* parameter; previously, keys were always encoded in 143 UTF-8. 144 145 .. versionchanged:: 3.4 146 Added context manager support. 147 148 .. versionchanged:: 3.10 149 :const:`pickle.DEFAULT_PROTOCOL` is now used as the default pickle 150 protocol. 151 152 153.. class:: BsdDbShelf(dict, protocol=None, writeback=False, keyencoding='utf-8') 154 155 A subclass of :class:`Shelf` which exposes :meth:`!first`, :meth:`!next`, 156 :meth:`!previous`, :meth:`!last` and :meth:`!set_location` methods. 157 These are available 158 in the third-party :mod:`!bsddb` module from `pybsddb 159 <https://www.jcea.es/programacion/pybsddb.htm>`_ but not in other database 160 modules. The *dict* object passed to the constructor must support those 161 methods. This is generally accomplished by calling one of 162 :func:`!bsddb.hashopen`, :func:`!bsddb.btopen` or :func:`!bsddb.rnopen`. The 163 optional *protocol*, *writeback*, and *keyencoding* parameters have the same 164 interpretation as for the :class:`Shelf` class. 165 166 167.. class:: DbfilenameShelf(filename, flag='c', protocol=None, writeback=False) 168 169 A subclass of :class:`Shelf` which accepts a *filename* instead of a dict-like 170 object. The underlying file will be opened using :func:`dbm.open`. By 171 default, the file will be created and opened for both read and write. The 172 optional *flag* parameter has the same interpretation as for the :func:`.open` 173 function. The optional *protocol* and *writeback* parameters have the same 174 interpretation as for the :class:`Shelf` class. 175 176 177.. _shelve-example: 178 179Example 180------- 181 182To summarize the interface (``key`` is a string, ``data`` is an arbitrary 183object):: 184 185 import shelve 186 187 d = shelve.open(filename) # open -- file may get suffix added by low-level 188 # library 189 190 d[key] = data # store data at key (overwrites old data if 191 # using an existing key) 192 data = d[key] # retrieve a COPY of data at key (raise KeyError 193 # if no such key) 194 del d[key] # delete data stored at key (raises KeyError 195 # if no such key) 196 197 flag = key in d # true if the key exists 198 klist = list(d.keys()) # a list of all existing keys (slow!) 199 200 # as d was opened WITHOUT writeback=True, beware: 201 d['xx'] = [0, 1, 2] # this works as expected, but... 202 d['xx'].append(3) # *this doesn't!* -- d['xx'] is STILL [0, 1, 2]! 203 204 # having opened d without writeback=True, you need to code carefully: 205 temp = d['xx'] # extracts the copy 206 temp.append(5) # mutates the copy 207 d['xx'] = temp # stores the copy right back, to persist it 208 209 # or, d=shelve.open(filename,writeback=True) would let you just code 210 # d['xx'].append(5) and have it work as expected, BUT it would also 211 # consume more memory and make the d.close() operation slower. 212 213 d.close() # close it 214 215 216.. seealso:: 217 218 Module :mod:`dbm` 219 Generic interface to ``dbm``-style databases. 220 221 Module :mod:`pickle` 222 Object serialization used by :mod:`shelve`. 223 224