/Doc/library/wsgiref.rst

  1:mod:`wsgiref` --- WSGI Utilities and Reference Implementation
  2==============================================================
  3
  4.. module:: wsgiref
  5   :synopsis: WSGI Utilities and Reference Implementation.
  6.. moduleauthor:: Phillip J. Eby <pje@telecommunity.com>
  7.. sectionauthor:: Phillip J. Eby <pje@telecommunity.com>
  8
  9
 10.. versionadded:: 2.5
 11
 12The Web Server Gateway Interface (WSGI) is a standard interface between web
 13server software and web applications written in Python. Having a standard
 14interface makes it easy to use an application that supports WSGI with a number
 15of different web servers.
 16
 17Only authors of web servers and programming frameworks need to know every detail
 18and corner case of the WSGI design.  You don't need to understand every detail
 19of WSGI just to install a WSGI application or to write a web application using
 20an existing framework.
 21
 22:mod:`wsgiref` is a reference implementation of the WSGI specification that can
 23be used to add WSGI support to a web server or framework.  It provides utilities
 24for manipulating WSGI environment variables and response headers, base classes
 25for implementing WSGI servers, a demo HTTP server that serves WSGI applications,
 26and a validation tool that checks WSGI servers and applications for conformance
 27to the WSGI specification (:pep:`333`).
 28
 29See http://www.wsgi.org for more information about WSGI, and links to tutorials
 30and other resources.
 31
 32.. XXX If you're just trying to write a web application...
 33
 34
 35:mod:`wsgiref.util` -- WSGI environment utilities
 36-------------------------------------------------
 37
 38.. module:: wsgiref.util
 39   :synopsis: WSGI environment utilities.
 40
 41
 42This module provides a variety of utility functions for working with WSGI
 43environments.  A WSGI environment is a dictionary containing HTTP request
 44variables as described in :pep:`333`.  All of the functions taking an *environ*
 45parameter expect a WSGI-compliant dictionary to be supplied; please see
 46:pep:`333` for a detailed specification.
 47
 48
 49.. function:: guess_scheme(environ)
 50
 51   Return a guess for whether ``wsgi.url_scheme`` should be "http" or "https", by
 52   checking for a ``HTTPS`` environment variable in the *environ* dictionary.  The
 53   return value is a string.
 54
 55   This function is useful when creating a gateway that wraps CGI or a CGI-like
 56   protocol such as FastCGI.  Typically, servers providing such protocols will
 57   include a ``HTTPS`` variable with a value of "1" "yes", or "on" when a request
 58   is received via SSL.  So, this function returns "https" if such a value is
 59   found, and "http" otherwise.
 60
 61
 62.. function:: request_uri(environ [, include_query=1])
 63
 64   Return the full request URI, optionally including the query string, using the
 65   algorithm found in the "URL Reconstruction" section of :pep:`333`.  If
 66   *include_query* is false, the query string is not included in the resulting URI.
 67
 68
 69.. function:: application_uri(environ)
 70
 71   Similar to :func:`request_uri`, except that the ``PATH_INFO`` and
 72   ``QUERY_STRING`` variables are ignored.  The result is the base URI of the
 73   application object addressed by the request.
 74
 75
 76.. function:: shift_path_info(environ)
 77
 78   Shift a single name from ``PATH_INFO`` to ``SCRIPT_NAME`` and return the name.
 79   The *environ* dictionary is *modified* in-place; use a copy if you need to keep
 80   the original ``PATH_INFO`` or ``SCRIPT_NAME`` intact.
 81
 82   If there are no remaining path segments in ``PATH_INFO``, ``None`` is returned.
 83
 84   Typically, this routine is used to process each portion of a request URI path,
 85   for example to treat the path as a series of dictionary keys. This routine
 86   modifies the passed-in environment to make it suitable for invoking another WSGI
 87   application that is located at the target URI. For example, if there is a WSGI
 88   application at ``/foo``, and the request URI path is ``/foo/bar/baz``, and the
 89   WSGI application at ``/foo`` calls :func:`shift_path_info`, it will receive the
 90   string "bar", and the environment will be updated to be suitable for passing to
 91   a WSGI application at ``/foo/bar``.  That is, ``SCRIPT_NAME`` will change from
 92   ``/foo`` to ``/foo/bar``, and ``PATH_INFO`` will change from ``/bar/baz`` to
 93   ``/baz``.
 94
 95   When ``PATH_INFO`` is just a "/", this routine returns an empty string and
 96   appends a trailing slash to ``SCRIPT_NAME``, even though empty path segments are
 97   normally ignored, and ``SCRIPT_NAME`` doesn't normally end in a slash.  This is
 98   intentional behavior, to ensure that an application can tell the difference
 99   between URIs ending in ``/x`` from ones ending in ``/x/`` when using this
100   routine to do object traversal.
101
102
103.. function:: setup_testing_defaults(environ)
104
105   Update *environ* with trivial defaults for testing purposes.
106
107   This routine adds various parameters required for WSGI, including ``HTTP_HOST``,
108   ``SERVER_NAME``, ``SERVER_PORT``, ``REQUEST_METHOD``, ``SCRIPT_NAME``,
109   ``PATH_INFO``, and all of the :pep:`333`\ -defined ``wsgi.*`` variables.  It
110   only supplies default values, and does not replace any existing settings for
111   these variables.
112
113   This routine is intended to make it easier for unit tests of WSGI servers and
114   applications to set up dummy environments.  It should NOT be used by actual WSGI
115   servers or applications, since the data is fake!
116
117   Example usage::
118
119      from wsgiref.util import setup_testing_defaults
120      from wsgiref.simple_server import make_server
121
122      # A relatively simple WSGI application. It's going to print out the
123      # environment dictionary after being updated by setup_testing_defaults
124      def simple_app(environ, start_response):
125          setup_testing_defaults(environ)
126
127          status = '200 OK'
128          headers = [('Content-type', 'text/plain')]
129
130          start_response(status, headers)
131
132          ret = ["%s: %s\n" % (key, value)
133                 for key, value in environ.iteritems()]
134          return ret
135
136      httpd = make_server('', 8000, simple_app)
137      print "Serving on port 8000..."
138      httpd.serve_forever()
139
140
141In addition to the environment functions above, the :mod:`wsgiref.util` module
142also provides these miscellaneous utilities:
143
144
145.. function:: is_hop_by_hop(header_name)
146
147   Return true if 'header_name' is an HTTP/1.1 "Hop-by-Hop" header, as defined by
148   :rfc:`2616`.
149
150
151.. class:: FileWrapper(filelike [, blksize=8192])
152
153   A wrapper to convert a file-like object to an :term:`iterator`.  The resulting objects
154   support both :meth:`__getitem__` and :meth:`__iter__` iteration styles, for
155   compatibility with Python 2.1 and Jython. As the object is iterated over, the
156   optional *blksize* parameter will be repeatedly passed to the *filelike*
157   object's :meth:`read` method to obtain strings to yield.  When :meth:`read`
158   returns an empty string, iteration is ended and is not resumable.
159
160   If *filelike* has a :meth:`close` method, the returned object will also have a
161   :meth:`close` method, and it will invoke the *filelike* object's :meth:`close`
162   method when called.
163
164   Example usage::
165
166      from StringIO import StringIO
167      from wsgiref.util import FileWrapper
168
169      # We're using a StringIO-buffer for as the file-like object
170      filelike = StringIO("This is an example file-like object"*10)
171      wrapper = FileWrapper(filelike, blksize=5)
172
173      for chunk in wrapper:
174          print chunk
175
176
177
178:mod:`wsgiref.headers` -- WSGI response header tools
179----------------------------------------------------
180
181.. module:: wsgiref.headers
182   :synopsis: WSGI response header tools.
183
184
185This module provides a single class, :class:`Headers`, for convenient
186manipulation of WSGI response headers using a mapping-like interface.
187
188
189.. class:: Headers(headers)
190
191   Create a mapping-like object wrapping *headers*, which must be a list of header
192   name/value tuples as described in :pep:`333`.  Any changes made to the new
193   :class:`Headers` object will directly update the *headers* list it was created
194   with.
195
196   :class:`Headers` objects support typical mapping operations including
197   :meth:`__getitem__`, :meth:`get`, :meth:`__setitem__`, :meth:`setdefault`,
198   :meth:`__delitem__`, :meth:`__contains__` and :meth:`has_key`.  For each of
199   these methods, the key is the header name (treated case-insensitively), and the
200   value is the first value associated with that header name.  Setting a header
201   deletes any existing values for that header, then adds a new value at the end of
202   the wrapped header list.  Headers' existing order is generally maintained, with
203   new headers added to the end of the wrapped list.
204
205   Unlike a dictionary, :class:`Headers` objects do not raise an error when you try
206   to get or delete a key that isn't in the wrapped header list. Getting a
207   nonexistent header just returns ``None``, and deleting a nonexistent header does
208   nothing.
209
210   :class:`Headers` objects also support :meth:`keys`, :meth:`values`, and
211   :meth:`items` methods.  The lists returned by :meth:`keys` and :meth:`items` can
212   include the same key more than once if there is a multi-valued header.  The
213   ``len()`` of a :class:`Headers` object is the same as the length of its
214   :meth:`items`, which is the same as the length of the wrapped header list.  In
215   fact, the :meth:`items` method just returns a copy of the wrapped header list.
216
217   Calling ``str()`` on a :class:`Headers` object returns a formatted string
218   suitable for transmission as HTTP response headers.  Each header is placed on a
219   line with its value, separated by a colon and a space. Each line is terminated
220   by a carriage return and line feed, and the string is terminated with a blank
221   line.
222
223   In addition to their mapping interface and formatting features, :class:`Headers`
224   objects also have the following methods for querying and adding multi-valued
225   headers, and for adding headers with MIME parameters:
226
227
228   .. method:: Headers.get_all(name)
229
230      Return a list of all the values for the named header.
231
232      The returned list will be sorted in the order they appeared in the original
233      header list or were added to this instance, and may contain duplicates.  Any
234      fields deleted and re-inserted are always appended to the header list.  If no
235      fields exist with the given name, returns an empty list.
236
237
238   .. method:: Headers.add_header(name, value, **_params)
239
240      Add a (possibly multi-valued) header, with optional MIME parameters specified
241      via keyword arguments.
242
243      *name* is the header field to add.  Keyword arguments can be used to set MIME
244      parameters for the header field.  Each parameter must be a string or ``None``.
245      Underscores in parameter names are converted to dashes, since dashes are illegal
246      in Python identifiers, but many MIME parameter names include dashes.  If the
247      parameter value is a string, it is added to the header value parameters in the
248      form ``name="value"``. If it is ``None``, only the parameter name is added.
249      (This is used for MIME parameters without a value.)  Example usage::
250
251         h.add_header('content-disposition', 'attachment', filename='bud.gif')
252
253      The above will add a header that looks like this::
254
255         Content-Disposition: attachment; filename="bud.gif"
256
257
258:mod:`wsgiref.simple_server` -- a simple WSGI HTTP server
259---------------------------------------------------------
260
261.. module:: wsgiref.simple_server
262   :synopsis: A simple WSGI HTTP server.
263
264
265This module implements a simple HTTP server (based on :mod:`BaseHTTPServer`)
266that serves WSGI applications.  Each server instance serves a single WSGI
267application on a given host and port.  If you want to serve multiple
268applications on a single host and port, you should create a WSGI application
269that parses ``PATH_INFO`` to select which application to invoke for each
270request.  (E.g., using the :func:`shift_path_info` function from
271:mod:`wsgiref.util`.)
272
273
274.. function:: make_server(host, port, app [, server_class=WSGIServer [, handler_class=WSGIRequestHandler]])
275
276   Create a new WSGI server listening on *host* and *port*, accepting connections
277   for *app*.  The return value is an instance of the supplied *server_class*, and
278   will process requests using the specified *handler_class*.  *app* must be a WSGI
279   application object, as defined by :pep:`333`.
280
281   Example usage::
282
283      from wsgiref.simple_server import make_server, demo_app
284
285      httpd = make_server('', 8000, demo_app)
286      print "Serving HTTP on port 8000..."
287
288      # Respond to requests until process is killed
289      httpd.serve_forever()
290
291      # Alternative: serve one request, then exit
292      httpd.handle_request()
293
294
295.. function:: demo_app(environ, start_response)
296
297   This function is a small but complete WSGI application that returns a text page
298   containing the message "Hello world!" and a list of the key/value pairs provided
299   in the *environ* parameter.  It's useful for verifying that a WSGI server (such
300   as :mod:`wsgiref.simple_server`) is able to run a simple WSGI application
301   correctly.
302
303
304.. class:: WSGIServer(server_address, RequestHandlerClass)
305
306   Create a :class:`WSGIServer` instance.  *server_address* should be a
307   ``(host,port)`` tuple, and *RequestHandlerClass* should be the subclass of
308   :class:`BaseHTTPServer.BaseHTTPRequestHandler` that will be used to process
309   requests.
310
311   You do not normally need to call this constructor, as the :func:`make_server`
312   function can handle all the details for you.
313
314   :class:`WSGIServer` is a subclass of :class:`BaseHTTPServer.HTTPServer`, so all
315   of its methods (such as :meth:`serve_forever` and :meth:`handle_request`) are
316   available. :class:`WSGIServer` also provides these WSGI-specific methods:
317
318
319   .. method:: WSGIServer.set_app(application)
320
321      Sets the callable *application* as the WSGI application that will receive
322      requests.
323
324
325   .. method:: WSGIServer.get_app()
326
327      Returns the currently-set application callable.
328
329   Normally, however, you do not need to use these additional methods, as
330   :meth:`set_app` is normally called by :func:`make_server`, and the
331   :meth:`get_app` exists mainly for the benefit of request handler instances.
332
333
334.. class:: WSGIRequestHandler(request, client_address, server)
335
336   Create an HTTP handler for the given *request* (i.e. a socket), *client_address*
337   (a ``(host,port)`` tuple), and *server* (:class:`WSGIServer` instance).
338
339   You do not need to create instances of this class directly; they are
340   automatically created as needed by :class:`WSGIServer` objects.  You can,
341   however, subclass this class and supply it as a *handler_class* to the
342   :func:`make_server` function.  Some possibly relevant methods for overriding in
343   subclasses:
344
345
346   .. method:: WSGIRequestHandler.get_environ()
347
348      Returns a dictionary containing the WSGI environment for a request.  The default
349      implementation copies the contents of the :class:`WSGIServer` object's
350      :attr:`base_environ` dictionary attribute and then adds various headers derived
351      from the HTTP request.  Each call to this method should return a new dictionary
352      containing all of the relevant CGI environment variables as specified in
353      :pep:`333`.
354
355
356   .. method:: WSGIRequestHandler.get_stderr()
357
358      Return the object that should be used as the ``wsgi.errors`` stream. The default
359      implementation just returns ``sys.stderr``.
360
361
362   .. method:: WSGIRequestHandler.handle()
363
364      Process the HTTP request.  The default implementation creates a handler instance
365      using a :mod:`wsgiref.handlers` class to implement the actual WSGI application
366      interface.
367
368
369:mod:`wsgiref.validate` --- WSGI conformance checker
370----------------------------------------------------
371
372.. module:: wsgiref.validate
373   :synopsis: WSGI conformance checker.
374
375
376When creating new WSGI application objects, frameworks, servers, or middleware,
377it can be useful to validate the new code's conformance using
378:mod:`wsgiref.validate`.  This module provides a function that creates WSGI
379application objects that validate communications between a WSGI server or
380gateway and a WSGI application object, to check both sides for protocol
381conformance.
382
383Note that this utility does not guarantee complete :pep:`333` compliance; an
384absence of errors from this module does not necessarily mean that errors do not
385exist.  However, if this module does produce an error, then it is virtually
386certain that either the server or application is not 100% compliant.
387
388This module is based on the :mod:`paste.lint` module from Ian Bicking's "Python
389Paste" library.
390
391
392.. function:: validator(application)
393
394   Wrap *application* and return a new WSGI application object.  The returned
395   application will forward all requests to the original *application*, and will
396   check that both the *application* and the server invoking it are conforming to
397   the WSGI specification and to RFC 2616.
398
399   Any detected nonconformance results in an :exc:`AssertionError` being raised;
400   note, however, that how these errors are handled is server-dependent.  For
401   example, :mod:`wsgiref.simple_server` and other servers based on
402   :mod:`wsgiref.handlers` (that don't override the error handling methods to do
403   something else) will simply output a message that an error has occurred, and
404   dump the traceback to ``sys.stderr`` or some other error stream.
405
406   This wrapper may also generate output using the :mod:`warnings` module to
407   indicate behaviors that are questionable but which may not actually be
408   prohibited by :pep:`333`.  Unless they are suppressed using Python command-line
409   options or the :mod:`warnings` API, any such warnings will be written to
410   ``sys.stderr`` (*not* ``wsgi.errors``, unless they happen to be the same
411   object).
412
413   Example usage::
414
415      from wsgiref.validate import validator
416      from wsgiref.simple_server import make_server
417
418      # Our callable object which is intentionally not compliant to the
419      # standard, so the validator is going to break
420      def simple_app(environ, start_response):
421          status = '200 OK' # HTTP Status
422          headers = [('Content-type', 'text/plain')] # HTTP Headers
423          start_response(status, headers)
424
425          # This is going to break because we need to return a list, and
426          # the validator is going to inform us
427          return "Hello World"
428
429      # This is the application wrapped in a validator
430      validator_app = validator(simple_app)
431
432      httpd = make_server('', 8000, validator_app)
433      print "Listening on port 8000...."
434      httpd.serve_forever()
435
436
437:mod:`wsgiref.handlers` -- server/gateway base classes
438------------------------------------------------------
439
440.. module:: wsgiref.handlers
441   :synopsis: WSGI server/gateway base classes.
442
443
444This module provides base handler classes for implementing WSGI servers and
445gateways.  These base classes handle most of the work of communicating with a
446WSGI application, as long as they are given a CGI-like environment, along with
447input, output, and error streams.
448
449
450.. class:: CGIHandler()
451
452   CGI-based invocation via ``sys.stdin``, ``sys.stdout``, ``sys.stderr`` and
453   ``os.environ``.  This is useful when you have a WSGI application and want to run
454   it as a CGI script.  Simply invoke ``CGIHandler().run(app)``, where ``app`` is
455   the WSGI application object you wish to invoke.
456
457   This class is a subclass of :class:`BaseCGIHandler` that sets ``wsgi.run_once``
458   to true, ``wsgi.multithread`` to false, and ``wsgi.multiprocess`` to true, and
459   always uses :mod:`sys` and :mod:`os` to obtain the necessary CGI streams and
460   environment.
461
462
463.. class:: BaseCGIHandler(stdin, stdout, stderr, environ [, multithread=True [, multiprocess=False]])
464
465   Similar to :class:`CGIHandler`, but instead of using the :mod:`sys` and
466   :mod:`os` modules, the CGI environment and I/O streams are specified explicitly.
467   The *multithread* and *multiprocess* values are used to set the
468   ``wsgi.multithread`` and ``wsgi.multiprocess`` flags for any applications run by
469   the handler instance.
470
471   This class is a subclass of :class:`SimpleHandler` intended for use with
472   software other than HTTP "origin servers".  If you are writing a gateway
473   protocol implementation (such as CGI, FastCGI, SCGI, etc.) that uses a
474   ``Status:`` header to send an HTTP status, you probably want to subclass this
475   instead of :class:`SimpleHandler`.
476
477
478.. class:: SimpleHandler(stdin, stdout, stderr, environ [,multithread=True [, multiprocess=False]])
479
480   Similar to :class:`BaseCGIHandler`, but designed for use with HTTP origin
481   servers.  If you are writing an HTTP server implementation, you will probably
482   want to subclass this instead of :class:`BaseCGIHandler`
483
484   This class is a subclass of :class:`BaseHandler`.  It overrides the
485   :meth:`__init__`, :meth:`get_stdin`, :meth:`get_stderr`, :meth:`add_cgi_vars`,
486   :meth:`_write`, and :meth:`_flush` methods to support explicitly setting the
487   environment and streams via the constructor.  The supplied environment and
488   streams are stored in the :attr:`stdin`, :attr:`stdout`, :attr:`stderr`, and
489   :attr:`environ` attributes.
490
491
492.. class:: BaseHandler()
493
494   This is an abstract base class for running WSGI applications.  Each instance
495   will handle a single HTTP request, although in principle you could create a
496   subclass that was reusable for multiple requests.
497
498   :class:`BaseHandler` instances have only one method intended for external use:
499
500
501   .. method:: BaseHandler.run(app)
502
503      Run the specified WSGI application, *app*.
504
505   All of the other :class:`BaseHandler` methods are invoked by this method in the
506   process of running the application, and thus exist primarily to allow
507   customizing the process.
508
509   The following methods MUST be overridden in a subclass:
510
511
512   .. method:: BaseHandler._write(data)
513
514      Buffer the string *data* for transmission to the client.  It's okay if this
515      method actually transmits the data; :class:`BaseHandler` just separates write
516      and flush operations for greater efficiency when the underlying system actually
517      has such a distinction.
518
519
520   .. method:: BaseHandler._flush()
521
522      Force buffered data to be transmitted to the client.  It's okay if this method
523      is a no-op (i.e., if :meth:`_write` actually sends the data).
524
525
526   .. method:: BaseHandler.get_stdin()
527
528      Return an input stream object suitable for use as the ``wsgi.input`` of the
529      request currently being processed.
530
531
532   .. method:: BaseHandler.get_stderr()
533
534      Return an output stream object suitable for use as the ``wsgi.errors`` of the
535      request currently being processed.
536
537
538   .. method:: BaseHandler.add_cgi_vars()
539
540      Insert CGI variables for the current request into the :attr:`environ` attribute.
541
542   Here are some other methods and attributes you may wish to override. This list
543   is only a summary, however, and does not include every method that can be
544   overridden.  You should consult the docstrings and source code for additional
545   information before attempting to create a customized :class:`BaseHandler`
546   subclass.
547
548   Attributes and methods for customizing the WSGI environment:
549
550
551   .. attribute:: BaseHandler.wsgi_multithread
552
553      The value to be used for the ``wsgi.multithread`` environment variable.  It
554      defaults to true in :class:`BaseHandler`, but may have a different default (or
555      be set by the constructor) in the other subclasses.
556
557
558   .. attribute:: BaseHandler.wsgi_multiprocess
559
560      The value to be used for the ``wsgi.multiprocess`` environment variable.  It
561      defaults to true in :class:`BaseHandler`, but may have a different default (or
562      be set by the constructor) in the other subclasses.
563
564
565   .. attribute:: BaseHandler.wsgi_run_once
566
567      The value to be used for the ``wsgi.run_once`` environment variable.  It
568      defaults to false in :class:`BaseHandler`, but :class:`CGIHandler` sets it to
569      true by default.
570
571
572   .. attribute:: BaseHandler.os_environ
573
574      The default environment variables to be included in every request's WSGI
575      environment.  By default, this is a copy of ``os.environ`` at the time that
576      :mod:`wsgiref.handlers` was imported, but subclasses can either create their own
577      at the class or instance level.  Note that the dictionary should be considered
578      read-only, since the default value is shared between multiple classes and
579      instances.
580
581
582   .. attribute:: BaseHandler.server_software
583
584      If the :attr:`origin_server` attribute is set, this attribute's value is used to
585      set the default ``SERVER_SOFTWARE`` WSGI environment variable, and also to set a
586      default ``Server:`` header in HTTP responses.  It is ignored for handlers (such
587      as :class:`BaseCGIHandler` and :class:`CGIHandler`) that are not HTTP origin
588      servers.
589
590
591   .. method:: BaseHandler.get_scheme()
592
593      Return the URL scheme being used for the current request.  The default
594      implementation uses the :func:`guess_scheme` function from :mod:`wsgiref.util`
595      to guess whether the scheme should be "http" or "https", based on the current
596      request's :attr:`environ` variables.
597
598
599   .. method:: BaseHandler.setup_environ()
600
601      Set the :attr:`environ` attribute to a fully-populated WSGI environment.  The
602      default implementation uses all of the above methods and attributes, plus the
603      :meth:`get_stdin`, :meth:`get_stderr`, and :meth:`add_cgi_vars` methods and the
604      :attr:`wsgi_file_wrapper` attribute.  It also inserts a ``SERVER_SOFTWARE`` key
605      if not present, as long as the :attr:`origin_server` attribute is a true value
606      and the :attr:`server_software` attribute is set.
607
608   Methods and attributes for customizing exception handling:
609
610
611   .. method:: BaseHandler.log_exception(exc_info)
612
613      Log the *exc_info* tuple in the server log.  *exc_info* is a ``(type, value,
614      traceback)`` tuple.  The default implementation simply writes the traceback to
615      the request's ``wsgi.errors`` stream and flushes it.  Subclasses can override
616      this method to change the format or retarget the output, mail the traceback to
617      an administrator, or whatever other action may be deemed suitable.
618
619
620   .. attribute:: BaseHandler.traceback_limit
621
622      The maximum number of frames to include in tracebacks output by the default
623      :meth:`log_exception` method.  If ``None``, all frames are included.
624
625
626   .. method:: BaseHandler.error_output(environ, start_response)
627
628      This method is a WSGI application to generate an error page for the user.  It is
629      only invoked if an error occurs before headers are sent to the client.
630
631      This method can access the current error information using ``sys.exc_info()``,
632      and should pass that information to *start_response* when calling it (as
633      described in the "Error Handling" section of :pep:`333`).
634
635      The default implementation just uses the :attr:`error_status`,
636      :attr:`error_headers`, and :attr:`error_body` attributes to generate an output
637      page.  Subclasses can override this to produce more dynamic error output.
638
639      Note, however, that it's not recommended from a security perspective to spit out
640      diagnostics to any old user; ideally, you should have to do something special to
641      enable diagnostic output, which is why the default implementation doesn't
642      include any.
643
644
645   .. attribute:: BaseHandler.error_status
646
647      The HTTP status used for error responses.  This should be a status string as
648      defined in :pep:`333`; it defaults to a 500 code and message.
649
650
651   .. attribute:: BaseHandler.error_headers
652
653      The HTTP headers used for error responses.  This should be a list of WSGI
654      response headers (``(name, value)`` tuples), as described in :pep:`333`.  The
655      default list just sets the content type to ``text/plain``.
656
657
658   .. attribute:: BaseHandler.error_body
659
660      The error response body.  This should be an HTTP response body string. It
661      defaults to the plain text, "A server error occurred.  Please contact the
662      administrator."
663
664   Methods and attributes for :pep:`333`'s "Optional Platform-Specific File
665   Handling" feature:
666
667
668   .. attribute:: BaseHandler.wsgi_file_wrapper
669
670      A ``wsgi.file_wrapper`` factory, or ``None``.  The default value of this
671      attribute is the :class:`FileWrapper` class from :mod:`wsgiref.util`.
672
673
674   .. method:: BaseHandler.sendfile()
675
676      Override to implement platform-specific file transmission.  This method is
677      called only if the application's return value is an instance of the class
678      specified by the :attr:`wsgi_file_wrapper` attribute.  It should return a true
679      value if it was able to successfully transmit the file, so that the default
680      transmission code will not be executed. The default implementation of this
681      method just returns a false value.
682
683   Miscellaneous methods and attributes:
684
685
686   .. attribute:: BaseHandler.origin_server
687
688      This attribute should be set to a true value if the handler's :meth:`_write` and
689      :meth:`_flush` are being used to communicate directly to the client, rather than
690      via a CGI-like gateway protocol that wants the HTTP status in a special
691      ``Status:`` header.
692
693      This attribute's default value is true in :class:`BaseHandler`, but false in
694      :class:`BaseCGIHandler` and :class:`CGIHandler`.
695
696
697   .. attribute:: BaseHandler.http_version
698
699      If :attr:`origin_server` is true, this string attribute is used to set the HTTP
700      version of the response set to the client.  It defaults to ``"1.0"``.
701
702
703Examples
704--------
705
706This is a working "Hello World" WSGI application::
707
708   from wsgiref.simple_server import make_server
709
710   # Every WSGI application must have an application object - a callable
711   # object that accepts two arguments. For that purpose, we're going to
712   # use a function (note that you're not limited to a function, you can
713   # use a class for example). The first argument passed to the function
714   # is a dictionary containing CGI-style envrironment variables and the
715   # second variable is the callable object (see PEP333)
716   def hello_world_app(environ, start_response):
717       status = '200 OK' # HTTP Status
718       headers = [('Content-type', 'text/plain')] # HTTP Headers
719       start_response(status, headers)
720
721       # The returned object is going to be printed
722       return ["Hello World"]
723
724   httpd = make_server('', 8000, hello_world_app)
725   print "Serving on port 8000..."
726
727   # Serve until process is killed
728   httpd.serve_forever()