xref: /third_party/python/Doc/library/wsgiref.rst

:mod:`wsgiref` --- WSGI Utilities and Reference Implementation
==============================================================

.. module:: wsgiref
   :synopsis: WSGI Utilities and Reference Implementation.

.. moduleauthor:: Phillip J. Eby <pje@telecommunity.com>
.. sectionauthor:: Phillip J. Eby <pje@telecommunity.com>

--------------

The Web Server Gateway Interface (WSGI) is a standard interface between web
server software and web applications written in Python. Having a standard
interface makes it easy to use an application that supports WSGI with a number
of different web servers.

Only authors of web servers and programming frameworks need to know every detail
and corner case of the WSGI design.  You don't need to understand every detail
of WSGI just to install a WSGI application or to write a web application using
an existing framework.

:mod:`wsgiref` is a reference implementation of the WSGI specification that can
be used to add WSGI support to a web server or framework.  It provides utilities
for manipulating WSGI environment variables and response headers, base classes
for implementing WSGI servers, a demo HTTP server that serves WSGI applications,
and a validation tool that checks WSGI servers and applications for conformance
to the WSGI specification (:pep:`3333`).

See `wsgi.readthedocs.io <https://wsgi.readthedocs.io/>`_ for more information about WSGI, and links
to tutorials and other resources.

.. XXX If you're just trying to write a web application...


:mod:`wsgiref.util` -- WSGI environment utilities
-------------------------------------------------

.. module:: wsgiref.util
   :synopsis: WSGI environment utilities.


This module provides a variety of utility functions for working with WSGI
environments.  A WSGI environment is a dictionary containing HTTP request
variables as described in :pep:`3333`.  All of the functions taking an *environ*
parameter expect a WSGI-compliant dictionary to be supplied; please see
:pep:`3333` for a detailed specification.


.. function:: guess_scheme(environ)

   Return a guess for whether ``wsgi.url_scheme`` should be "http" or "https", by
   checking for a ``HTTPS`` environment variable in the *environ* dictionary.  The
   return value is a string.

   This function is useful when creating a gateway that wraps CGI or a CGI-like
   protocol such as FastCGI.  Typically, servers providing such protocols will
   include a ``HTTPS`` variable with a value of "1", "yes", or "on" when a request
   is received via SSL.  So, this function returns "https" if such a value is
   found, and "http" otherwise.


.. function:: request_uri(environ, include_query=True)

   Return the full request URI, optionally including the query string, using the
   algorithm found in the "URL Reconstruction" section of :pep:`3333`.  If
   *include_query* is false, the query string is not included in the resulting URI.


.. function:: application_uri(environ)

   Similar to :func:`request_uri`, except that the ``PATH_INFO`` and
   ``QUERY_STRING`` variables are ignored.  The result is the base URI of the
   application object addressed by the request.


.. function:: shift_path_info(environ)

   Shift a single name from ``PATH_INFO`` to ``SCRIPT_NAME`` and return the name.
   The *environ* dictionary is *modified* in-place; use a copy if you need to keep
   the original ``PATH_INFO`` or ``SCRIPT_NAME`` intact.

   If there are no remaining path segments in ``PATH_INFO``, ``None`` is returned.

   Typically, this routine is used to process each portion of a request URI path,
   for example to treat the path as a series of dictionary keys. This routine
   modifies the passed-in environment to make it suitable for invoking another WSGI
   application that is located at the target URI. For example, if there is a WSGI
   application at ``/foo``, and the request URI path is ``/foo/bar/baz``, and the
   WSGI application at ``/foo`` calls :func:`shift_path_info`, it will receive the
   string "bar", and the environment will be updated to be suitable for passing to
   a WSGI application at ``/foo/bar``.  That is, ``SCRIPT_NAME`` will change from
   ``/foo`` to ``/foo/bar``, and ``PATH_INFO`` will change from ``/bar/baz`` to
   ``/baz``.

   When ``PATH_INFO`` is just a "/", this routine returns an empty string and
   appends a trailing slash to ``SCRIPT_NAME``, even though empty path segments are
   normally ignored, and ``SCRIPT_NAME`` doesn't normally end in a slash.  This is
   intentional behavior, to ensure that an application can tell the difference
   between URIs ending in ``/x`` from ones ending in ``/x/`` when using this
   routine to do object traversal.


.. function:: setup_testing_defaults(environ)

   Update *environ* with trivial defaults for testing purposes.

   This routine adds various parameters required for WSGI, including ``HTTP_HOST``,
   ``SERVER_NAME``, ``SERVER_PORT``, ``REQUEST_METHOD``, ``SCRIPT_NAME``,
   ``PATH_INFO``, and all of the :pep:`3333`\ -defined ``wsgi.*`` variables.  It
   only supplies default values, and does not replace any existing settings for
   these variables.

   This routine is intended to make it easier for unit tests of WSGI servers and
   applications to set up dummy environments.  It should NOT be used by actual WSGI
   servers or applications, since the data is fake!

   Example usage::

      from wsgiref.util import setup_testing_defaults
      from wsgiref.simple_server import make_server

      # A relatively simple WSGI application. It's going to print out the
      # environment dictionary after being updated by setup_testing_defaults
      def simple_app(environ, start_response):
          setup_testing_defaults(environ)

          status = '200 OK'
          headers = [('Content-type', 'text/plain; charset=utf-8')]

          start_response(status, headers)

          ret = [("%s: %s\n" % (key, value)).encode("utf-8")
                 for key, value in environ.items()]
          return ret

      with make_server('', 8000, simple_app) as httpd:
          print("Serving on port 8000...")
          httpd.serve_forever()


In addition to the environment functions above, the :mod:`wsgiref.util` module
also provides these miscellaneous utilities:


.. function:: is_hop_by_hop(header_name)

   Return ``True`` if 'header_name' is an HTTP/1.1 "Hop-by-Hop" header, as defined by
   :rfc:`2616`.


.. class:: FileWrapper(filelike, blksize=8192)

   A wrapper to convert a file-like object to an :term:`iterator`.  The resulting objects
   support both :meth:`__getitem__` and :meth:`__iter__` iteration styles, for
   compatibility with Python 2.1 and Jython. As the object is iterated over, the
   optional *blksize* parameter will be repeatedly passed to the *filelike*
   object's :meth:`read` method to obtain bytestrings to yield.  When :meth:`read`
   returns an empty bytestring, iteration is ended and is not resumable.

   If *filelike* has a :meth:`close` method, the returned object will also have a
   :meth:`close` method, and it will invoke the *filelike* object's :meth:`close`
   method when called.

   Example usage::

      from io import StringIO
      from wsgiref.util import FileWrapper

      # We're using a StringIO-buffer for as the file-like object
      filelike = StringIO("This is an example file-like object"*10)
      wrapper = FileWrapper(filelike, blksize=5)

      for chunk in wrapper:
          print(chunk)

   .. deprecated:: 3.8
      Support for :meth:`sequence protocol <__getitem__>` is deprecated.


:mod:`wsgiref.headers` -- WSGI response header tools
----------------------------------------------------

.. module:: wsgiref.headers
   :synopsis: WSGI response header tools.


This module provides a single class, :class:`Headers`, for convenient
manipulation of WSGI response headers using a mapping-like interface.


.. class:: Headers([headers])

   Create a mapping-like object wrapping *headers*, which must be a list of header
   name/value tuples as described in :pep:`3333`. The default value of *headers* is
   an empty list.

   :class:`Headers` objects support typical mapping operations including
   :meth:`__getitem__`, :meth:`get`, :meth:`__setitem__`, :meth:`setdefault`,
   :meth:`__delitem__` and :meth:`__contains__`.  For each of
   these methods, the key is the header name (treated case-insensitively), and the
   value is the first value associated with that header name.  Setting a header
   deletes any existing values for that header, then adds a new value at the end of
   the wrapped header list.  Headers' existing order is generally maintained, with
   new headers added to the end of the wrapped list.

   Unlike a dictionary, :class:`Headers` objects do not raise an error when you try
   to get or delete a key that isn't in the wrapped header list. Getting a
   nonexistent header just returns ``None``, and deleting a nonexistent header does
   nothing.

   :class:`Headers` objects also support :meth:`keys`, :meth:`values`, and
   :meth:`items` methods.  The lists returned by :meth:`keys` and :meth:`items` can
   include the same key more than once if there is a multi-valued header.  The
   ``len()`` of a :class:`Headers` object is the same as the length of its
   :meth:`items`, which is the same as the length of the wrapped header list.  In
   fact, the :meth:`items` method just returns a copy of the wrapped header list.

   Calling ``bytes()`` on a :class:`Headers` object returns a formatted bytestring
   suitable for transmission as HTTP response headers.  Each header is placed on a
   line with its value, separated by a colon and a space. Each line is terminated
   by a carriage return and line feed, and the bytestring is terminated with a
   blank line.

   In addition to their mapping interface and formatting features, :class:`Headers`
   objects also have the following methods for querying and adding multi-valued
   headers, and for adding headers with MIME parameters:


   .. method:: Headers.get_all(name)

      Return a list of all the values for the named header.

      The returned list will be sorted in the order they appeared in the original
      header list or were added to this instance, and may contain duplicates.  Any
      fields deleted and re-inserted are always appended to the header list.  If no
      fields exist with the given name, returns an empty list.


   .. method:: Headers.add_header(name, value, **_params)

      Add a (possibly multi-valued) header, with optional MIME parameters specified
      via keyword arguments.

      *name* is the header field to add.  Keyword arguments can be used to set MIME
      parameters for the header field.  Each parameter must be a string or ``None``.
      Underscores in parameter names are converted to dashes, since dashes are illegal
      in Python identifiers, but many MIME parameter names include dashes.  If the
      parameter value is a string, it is added to the header value parameters in the
      form ``name="value"``. If it is ``None``, only the parameter name is added.
      (This is used for MIME parameters without a value.)  Example usage::

         h.add_header('content-disposition', 'attachment', filename='bud.gif')

      The above will add a header that looks like this::

         Content-Disposition: attachment; filename="bud.gif"


   .. versionchanged:: 3.5
      *headers* parameter is optional.


:mod:`wsgiref.simple_server` -- a simple WSGI HTTP server
---------------------------------------------------------

.. module:: wsgiref.simple_server
   :synopsis: A simple WSGI HTTP server.


This module implements a simple HTTP server (based on :mod:`http.server`)
that serves WSGI applications.  Each server instance serves a single WSGI
application on a given host and port.  If you want to serve multiple
applications on a single host and port, you should create a WSGI application
that parses ``PATH_INFO`` to select which application to invoke for each
request.  (E.g., using the :func:`shift_path_info` function from
:mod:`wsgiref.util`.)


.. function:: make_server(host, port, app, server_class=WSGIServer, handler_class=WSGIRequestHandler)

   Create a new WSGI server listening on *host* and *port*, accepting connections
   for *app*.  The return value is an instance of the supplied *server_class*, and
   will process requests using the specified *handler_class*.  *app* must be a WSGI
   application object, as defined by :pep:`3333`.

   Example usage::

      from wsgiref.simple_server import make_server, demo_app

      with make_server('', 8000, demo_app) as httpd:
          print("Serving HTTP on port 8000...")

          # Respond to requests until process is killed
          httpd.serve_forever()

          # Alternative: serve one request, then exit
          httpd.handle_request()


.. function:: demo_app(environ, start_response)

   This function is a small but complete WSGI application that returns a text page
   containing the message "Hello world!" and a list of the key/value pairs provided
   in the *environ* parameter.  It's useful for verifying that a WSGI server (such
   as :mod:`wsgiref.simple_server`) is able to run a simple WSGI application
   correctly.


.. class:: WSGIServer(server_address, RequestHandlerClass)

   Create a :class:`WSGIServer` instance.  *server_address* should be a
   ``(host,port)`` tuple, and *RequestHandlerClass* should be the subclass of
   :class:`http.server.BaseHTTPRequestHandler` that will be used to process
   requests.

   You do not normally need to call this constructor, as the :func:`make_server`
   function can handle all the details for you.

   :class:`WSGIServer` is a subclass of :class:`http.server.HTTPServer`, so all
   of its methods (such as :meth:`serve_forever` and :meth:`handle_request`) are
   available. :class:`WSGIServer` also provides these WSGI-specific methods:


   .. method:: WSGIServer.set_app(application)

      Sets the callable *application* as the WSGI application that will receive
      requests.


   .. method:: WSGIServer.get_app()

      Returns the currently-set application callable.

   Normally, however, you do not need to use these additional methods, as
   :meth:`set_app` is normally called by :func:`make_server`, and the
   :meth:`get_app` exists mainly for the benefit of request handler instances.


.. class:: WSGIRequestHandler(request, client_address, server)

   Create an HTTP handler for the given *request* (i.e. a socket), *client_address*
   (a ``(host,port)`` tuple), and *server* (:class:`WSGIServer` instance).

   You do not need to create instances of this class directly; they are
   automatically created as needed by :class:`WSGIServer` objects.  You can,
   however, subclass this class and supply it as a *handler_class* to the
   :func:`make_server` function.  Some possibly relevant methods for overriding in
   subclasses:


   .. method:: WSGIRequestHandler.get_environ()

      Returns a dictionary containing the WSGI environment for a request.  The default
      implementation copies the contents of the :class:`WSGIServer` object's
      :attr:`base_environ` dictionary attribute and then adds various headers derived
      from the HTTP request.  Each call to this method should return a new dictionary
      containing all of the relevant CGI environment variables as specified in
      :pep:`3333`.


   .. method:: WSGIRequestHandler.get_stderr()

      Return the object that should be used as the ``wsgi.errors`` stream. The default
      implementation just returns ``sys.stderr``.


   .. method:: WSGIRequestHandler.handle()

      Process the HTTP request.  The default implementation creates a handler instance
      using a :mod:`wsgiref.handlers` class to implement the actual WSGI application
      interface.


:mod:`wsgiref.validate` --- WSGI conformance checker
----------------------------------------------------

.. module:: wsgiref.validate
   :synopsis: WSGI conformance checker.


When creating new WSGI application objects, frameworks, servers, or middleware,
it can be useful to validate the new code's conformance using
:mod:`wsgiref.validate`.  This module provides a function that creates WSGI
application objects that validate communications between a WSGI server or
gateway and a WSGI application object, to check both sides for protocol
conformance.

Note that this utility does not guarantee complete :pep:`3333` compliance; an
absence of errors from this module does not necessarily mean that errors do not
exist.  However, if this module does produce an error, then it is virtually
certain that either the server or application is not 100% compliant.

This module is based on the :mod:`paste.lint` module from Ian Bicking's "Python
Paste" library.


.. function:: validator(application)

   Wrap *application* and return a new WSGI application object.  The returned
   application will forward all requests to the original *application*, and will
   check that both the *application* and the server invoking it are conforming to
   the WSGI specification and to :rfc:`2616`.

   Any detected nonconformance results in an :exc:`AssertionError` being raised;
   note, however, that how these errors are handled is server-dependent.  For
   example, :mod:`wsgiref.simple_server` and other servers based on
   :mod:`wsgiref.handlers` (that don't override the error handling methods to do
   something else) will simply output a message that an error has occurred, and
   dump the traceback to ``sys.stderr`` or some other error stream.

   This wrapper may also generate output using the :mod:`warnings` module to
   indicate behaviors that are questionable but which may not actually be
   prohibited by :pep:`3333`.  Unless they are suppressed using Python command-line
   options or the :mod:`warnings` API, any such warnings will be written to
   ``sys.stderr`` (*not* ``wsgi.errors``, unless they happen to be the same
   object).

   Example usage::

      from wsgiref.validate import validator
      from wsgiref.simple_server import make_server

      # Our callable object which is intentionally not compliant to the
      # standard, so the validator is going to break
      def simple_app(environ, start_response):
          status = '200 OK'  # HTTP Status
          headers = [('Content-type', 'text/plain')]  # HTTP Headers
          start_response(status, headers)

          # This is going to break because we need to return a list, and
          # the validator is going to inform us
          return b"Hello World"

      # This is the application wrapped in a validator
      validator_app = validator(simple_app)

      with make_server('', 8000, validator_app) as httpd:
          print("Listening on port 8000....")
          httpd.serve_forever()


:mod:`wsgiref.handlers` -- server/gateway base classes
------------------------------------------------------

.. module:: wsgiref.handlers
   :synopsis: WSGI server/gateway base classes.


This module provides base handler classes for implementing WSGI servers and
gateways.  These base classes handle most of the work of communicating with a
WSGI application, as long as they are given a CGI-like environment, along with
input, output, and error streams.


.. class:: CGIHandler()

   CGI-based invocation via ``sys.stdin``, ``sys.stdout``, ``sys.stderr`` and
   ``os.environ``.  This is useful when you have a WSGI application and want to run
   it as a CGI script.  Simply invoke ``CGIHandler().run(app)``, where ``app`` is
   the WSGI application object you wish to invoke.

   This class is a subclass of :class:`BaseCGIHandler` that sets ``wsgi.run_once``
   to true, ``wsgi.multithread`` to false, and ``wsgi.multiprocess`` to true, and
   always uses :mod:`sys` and :mod:`os` to obtain the necessary CGI streams and
   environment.


.. class:: IISCGIHandler()

   A specialized alternative to :class:`CGIHandler`, for use when deploying on
   Microsoft's IIS web server, without having set the config allowPathInfo
   option (IIS>=7) or metabase allowPathInfoForScriptMappings (IIS<7).

   By default, IIS gives a ``PATH_INFO`` that duplicates the ``SCRIPT_NAME`` at
   the front, causing problems for WSGI applications that wish to implement
   routing. This handler strips any such duplicated path.

   IIS can be configured to pass the correct ``PATH_INFO``, but this causes
   another bug where ``PATH_TRANSLATED`` is wrong. Luckily this variable is
   rarely used and is not guaranteed by WSGI. On IIS<7, though, the
   setting can only be made on a vhost level, affecting all other script
   mappings, many of which break when exposed to the ``PATH_TRANSLATED`` bug.
   For this reason IIS<7 is almost never deployed with the fix (Even IIS7
   rarely uses it because there is still no UI for it.).

   There is no way for CGI code to tell whether the option was set, so a
   separate handler class is provided.  It is used in the same way as
   :class:`CGIHandler`, i.e., by calling ``IISCGIHandler().run(app)``, where
   ``app`` is the WSGI application object you wish to invoke.

   .. versionadded:: 3.2


.. class:: BaseCGIHandler(stdin, stdout, stderr, environ, multithread=True, multiprocess=False)

   Similar to :class:`CGIHandler`, but instead of using the :mod:`sys` and
   :mod:`os` modules, the CGI environment and I/O streams are specified explicitly.
   The *multithread* and *multiprocess* values are used to set the
   ``wsgi.multithread`` and ``wsgi.multiprocess`` flags for any applications run by
   the handler instance.

   This class is a subclass of :class:`SimpleHandler` intended for use with
   software other than HTTP "origin servers".  If you are writing a gateway
   protocol implementation (such as CGI, FastCGI, SCGI, etc.) that uses a
   ``Status:`` header to send an HTTP status, you probably want to subclass this
   instead of :class:`SimpleHandler`.


.. class:: SimpleHandler(stdin, stdout, stderr, environ, multithread=True, multiprocess=False)

   Similar to :class:`BaseCGIHandler`, but designed for use with HTTP origin
   servers.  If you are writing an HTTP server implementation, you will probably
   want to subclass this instead of :class:`BaseCGIHandler`.

   This class is a subclass of :class:`BaseHandler`.  It overrides the
   :meth:`__init__`, :meth:`get_stdin`, :meth:`get_stderr`, :meth:`add_cgi_vars`,
   :meth:`_write`, and :meth:`_flush` methods to support explicitly setting the
   environment and streams via the constructor.  The supplied environment and
   streams are stored in the :attr:`stdin`, :attr:`stdout`, :attr:`stderr`, and
   :attr:`environ` attributes.

   The :meth:`~io.BufferedIOBase.write` method of *stdout* should write
   each chunk in full, like :class:`io.BufferedIOBase`.


.. class:: BaseHandler()

   This is an abstract base class for running WSGI applications.  Each instance
   will handle a single HTTP request, although in principle you could create a
   subclass that was reusable for multiple requests.

   :class:`BaseHandler` instances have only one method intended for external use:


   .. method:: BaseHandler.run(app)

      Run the specified WSGI application, *app*.

   All of the other :class:`BaseHandler` methods are invoked by this method in the
   process of running the application, and thus exist primarily to allow
   customizing the process.

   The following methods MUST be overridden in a subclass:


   .. method:: BaseHandler._write(data)

      Buffer the bytes *data* for transmission to the client.  It's okay if this
      method actually transmits the data; :class:`BaseHandler` just separates write
      and flush operations for greater efficiency when the underlying system actually
      has such a distinction.


   .. method:: BaseHandler._flush()

      Force buffered data to be transmitted to the client.  It's okay if this method
      is a no-op (i.e., if :meth:`_write` actually sends the data).


   .. method:: BaseHandler.get_stdin()

      Return an input stream object suitable for use as the ``wsgi.input`` of the
      request currently being processed.


   .. method:: BaseHandler.get_stderr()

      Return an output stream object suitable for use as the ``wsgi.errors`` of the
      request currently being processed.


   .. method:: BaseHandler.add_cgi_vars()

      Insert CGI variables for the current request into the :attr:`environ` attribute.

   Here are some other methods and attributes you may wish to override. This list
   is only a summary, however, and does not include every method that can be
   overridden.  You should consult the docstrings and source code for additional
   information before attempting to create a customized :class:`BaseHandler`
   subclass.

   Attributes and methods for customizing the WSGI environment:


   .. attribute:: BaseHandler.wsgi_multithread

      The value to be used for the ``wsgi.multithread`` environment variable.  It
      defaults to true in :class:`BaseHandler`, but may have a different default (or
      be set by the constructor) in the other subclasses.


   .. attribute:: BaseHandler.wsgi_multiprocess

      The value to be used for the ``wsgi.multiprocess`` environment variable.  It
      defaults to true in :class:`BaseHandler`, but may have a different default (or
      be set by the constructor) in the other subclasses.


   .. attribute:: BaseHandler.wsgi_run_once

      The value to be used for the ``wsgi.run_once`` environment variable.  It
      defaults to false in :class:`BaseHandler`, but :class:`CGIHandler` sets it to
      true by default.


   .. attribute:: BaseHandler.os_environ

      The default environment variables to be included in every request's WSGI
      environment.  By default, this is a copy of ``os.environ`` at the time that
      :mod:`wsgiref.handlers` was imported, but subclasses can either create their own
      at the class or instance level.  Note that the dictionary should be considered
      read-only, since the default value is shared between multiple classes and
      instances.


   .. attribute:: BaseHandler.server_software

      If the :attr:`origin_server` attribute is set, this attribute's value is used to
      set the default ``SERVER_SOFTWARE`` WSGI environment variable, and also to set a
      default ``Server:`` header in HTTP responses.  It is ignored for handlers (such
      as :class:`BaseCGIHandler` and :class:`CGIHandler`) that are not HTTP origin
      servers.

      .. versionchanged:: 3.3
         The term "Python" is replaced with implementation specific term like
         "CPython", "Jython" etc.

   .. method:: BaseHandler.get_scheme()

      Return the URL scheme being used for the current request.  The default
      implementation uses the :func:`guess_scheme` function from :mod:`wsgiref.util`
      to guess whether the scheme should be "http" or "https", based on the current
      request's :attr:`environ` variables.


   .. method:: BaseHandler.setup_environ()

      Set the :attr:`environ` attribute to a fully-populated WSGI environment.  The
      default implementation uses all of the above methods and attributes, plus the
      :meth:`get_stdin`, :meth:`get_stderr`, and :meth:`add_cgi_vars` methods and the
      :attr:`wsgi_file_wrapper` attribute.  It also inserts a ``SERVER_SOFTWARE`` key
      if not present, as long as the :attr:`origin_server` attribute is a true value
      and the :attr:`server_software` attribute is set.

   Methods and attributes for customizing exception handling:


   .. method:: BaseHandler.log_exception(exc_info)

      Log the *exc_info* tuple in the server log.  *exc_info* is a ``(type, value,
      traceback)`` tuple.  The default implementation simply writes the traceback to
      the request's ``wsgi.errors`` stream and flushes it.  Subclasses can override
      this method to change the format or retarget the output, mail the traceback to
      an administrator, or whatever other action may be deemed suitable.


   .. attribute:: BaseHandler.traceback_limit

      The maximum number of frames to include in tracebacks output by the default
      :meth:`log_exception` method.  If ``None``, all frames are included.


   .. method:: BaseHandler.error_output(environ, start_response)

      This method is a WSGI application to generate an error page for the user.  It is
      only invoked if an error occurs before headers are sent to the client.

      This method can access the current error information using ``sys.exc_info()``,
      and should pass that information to *start_response* when calling it (as
      described in the "Error Handling" section of :pep:`3333`).

      The default implementation just uses the :attr:`error_status`,
      :attr:`error_headers`, and :attr:`error_body` attributes to generate an output
      page.  Subclasses can override this to produce more dynamic error output.

      Note, however, that it's not recommended from a security perspective to spit out
      diagnostics to any old user; ideally, you should have to do something special to
      enable diagnostic output, which is why the default implementation doesn't
      include any.


   .. attribute:: BaseHandler.error_status

      The HTTP status used for error responses.  This should be a status string as
      defined in :pep:`3333`; it defaults to a 500 code and message.


   .. attribute:: BaseHandler.error_headers

      The HTTP headers used for error responses.  This should be a list of WSGI
      response headers (``(name, value)`` tuples), as described in :pep:`3333`.  The
      default list just sets the content type to ``text/plain``.


   .. attribute:: BaseHandler.error_body

      The error response body.  This should be an HTTP response body bytestring. It
      defaults to the plain text, "A server error occurred.  Please contact the
      administrator."

   Methods and attributes for :pep:`3333`'s "Optional Platform-Specific File
   Handling" feature:


   .. attribute:: BaseHandler.wsgi_file_wrapper

      A ``wsgi.file_wrapper`` factory, or ``None``.  The default value of this
      attribute is the :class:`wsgiref.util.FileWrapper` class.


   .. method:: BaseHandler.sendfile()

      Override to implement platform-specific file transmission.  This method is
      called only if the application's return value is an instance of the class
      specified by the :attr:`wsgi_file_wrapper` attribute.  It should return a true
      value if it was able to successfully transmit the file, so that the default
      transmission code will not be executed. The default implementation of this
      method just returns a false value.

   Miscellaneous methods and attributes:


   .. attribute:: BaseHandler.origin_server

      This attribute should be set to a true value if the handler's :meth:`_write` and
      :meth:`_flush` are being used to communicate directly to the client, rather than
      via a CGI-like gateway protocol that wants the HTTP status in a special
      ``Status:`` header.

      This attribute's default value is true in :class:`BaseHandler`, but false in
      :class:`BaseCGIHandler` and :class:`CGIHandler`.


   .. attribute:: BaseHandler.http_version

      If :attr:`origin_server` is true, this string attribute is used to set the HTTP
      version of the response set to the client.  It defaults to ``"1.0"``.


.. function:: read_environ()

   Transcode CGI variables from ``os.environ`` to :pep:`3333` "bytes in unicode"
   strings, returning a new dictionary.  This function is used by
   :class:`CGIHandler` and :class:`IISCGIHandler` in place of directly using
   ``os.environ``, which is not necessarily WSGI-compliant on all platforms
   and web servers using Python 3 -- specifically, ones where the OS's
   actual environment is Unicode (i.e. Windows), or ones where the environment
   is bytes, but the system encoding used by Python to decode it is anything
   other than ISO-8859-1 (e.g. Unix systems using UTF-8).

   If you are implementing a CGI-based handler of your own, you probably want
   to use this routine instead of just copying values out of ``os.environ``
   directly.

   .. versionadded:: 3.2


Examples
--------

This is a working "Hello World" WSGI application::

   from wsgiref.simple_server import make_server

   # Every WSGI application must have an application object - a callable
   # object that accepts two arguments. For that purpose, we're going to
   # use a function (note that you're not limited to a function, you can
   # use a class for example). The first argument passed to the function
   # is a dictionary containing CGI-style environment variables and the
   # second variable is the callable object.
   def hello_world_app(environ, start_response):
       status = '200 OK'  # HTTP Status
       headers = [('Content-type', 'text/plain; charset=utf-8')]  # HTTP Headers
       start_response(status, headers)

       # The returned object is going to be printed
       return [b"Hello World"]

   with make_server('', 8000, hello_world_app) as httpd:
       print("Serving on port 8000...")

       # Serve until process is killed
       httpd.serve_forever()


Example of a WSGI application serving the current directory, accept optional
directory and port number (default: 8000) on the command line:

.. literalinclude:: ../../Tools/scripts/serve.py