Python 3.6.5 Documentation >  "wsgiref" — WSGI Utilities and Reference Implementation

"wsgiref" — WSGI Utilities and Reference Implementation
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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.

"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 https://wsgi.readthedocs.org/ for more information about WSGI, and
links to tutorials and other resources.

"wsgiref.util" – 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.

wsgiref.util.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.

wsgiref.util.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.

wsgiref.util.application_uri(environ)

Similar to "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.

wsgiref.util.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 "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.

wsgiref.util.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 "wsgiref.util"
module also provides these miscellaneous utilities:

wsgiref.util.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 wsgiref.util.FileWrapper(filelike, blksize=8192)

A wrapper to convert a file-like object to an *iterator*. The
resulting objects support both "__getitem__()" and "__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 "read()" method to
obtain bytestrings to yield. When "read()" returns an empty
bytestring, iteration is ended and is not resumable.

If *filelike* has a "close()" method, the returned object will also
have a "close()" method, and it will invoke the *filelike* object’s
"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)

"wsgiref.headers" – WSGI response header tools
==============================================

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

class wsgiref.headers.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.

"Headers" objects support typical mapping operations including
"__getitem__()", "get()", "__setitem__()", "setdefault()",
"__delitem__()" and "__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, "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.

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

Calling "bytes()" on a "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,
"Headers" objects also have the following methods for querying and
adding multi-valued headers, and for adding headers with MIME
parameters:

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.

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"

Changed in version 3.5: *headers* parameter is optional.

"wsgiref.simple_server" – a simple WSGI HTTP server
===================================================

This module implements a simple HTTP server (based on "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 "shift_path_info()"
function from "wsgiref.util".)

wsgiref.simple_server.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()

wsgiref.simple_server.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 "wsgiref.simple_server")
is able to run a simple WSGI application correctly.

class wsgiref.simple_server.WSGIServer(server_address, RequestHandlerClass)

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

You do not normally need to call this constructor, as the
"make_server()" function can handle all the details for you.

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

set_app(application)

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

get_app()

Returns the currently-set application callable.

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

class wsgiref.simple_server.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*
("WSGIServer" instance).

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

get_environ()

Returns a dictionary containing the WSGI environment for a
request. The default implementation copies the contents of the
"WSGIServer" object’s "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**.

get_stderr()

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

handle()

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

"wsgiref.validate" — 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 "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 "paste.lint" module from Ian Bicking’s
“Python Paste” library.

wsgiref.validate.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 "AssertionError" being
raised; note, however, that how these errors are handled is server-
dependent. For example, "wsgiref.simple_server" and other servers
based on "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 "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 "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()

"wsgiref.handlers" – 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 wsgiref.handlers.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 "BaseCGIHandler" that sets
"wsgi.run_once" to true, "wsgi.multithread" to false, and
"wsgi.multiprocess" to true, and always uses "sys" and "os" to
obtain the necessary CGI streams and environment.

class wsgiref.handlers.IISCGIHandler

A specialized alternative to "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 "CGIHandler", i.e., by calling "IISCGIHandler().run(app)", where
"app" is the WSGI application object you wish to invoke.

New in version 3.2.

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

Similar to "CGIHandler", but instead of using the "sys" and "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 "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 "SimpleHandler".

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

Similar to "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 "BaseCGIHandler".

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

The "write()" method of *stdout* should write each chunk in full,
like "io.BufferedIOBase".

class wsgiref.handlers.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.

"BaseHandler" instances have only one method intended for external
use:

run(app)

Run the specified WSGI application, *app*.

All of the other "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:

_write(data)

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

_flush()

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

get_stdin()

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

get_stderr()

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

add_cgi_vars()

Insert CGI variables for the current request into the "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 "BaseHandler" subclass.

Attributes and methods for customizing the WSGI environment:

wsgi_multithread

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

wsgi_multiprocess

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

wsgi_run_once

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

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 "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.

server_software

If the "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 "BaseCGIHandler"
and "CGIHandler") that are not HTTP origin servers.

Changed in version 3.3: The term “Python” is replaced with
implementation specific term like “CPython”, “Jython” etc.

get_scheme()

Return the URL scheme being used for the current request. The
default implementation uses the "guess_scheme()" function from
"wsgiref.util" to guess whether the scheme should be “http” or
“https”, based on the current request’s "environ" variables.

setup_environ()

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

Methods and attributes for customizing exception handling:

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.

traceback_limit

The maximum number of frames to include in tracebacks output by
the default "log_exception()" method. If "None", all frames are
included.

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 "error_status",
"error_headers", and "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.

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.

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".

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:

wsgi_file_wrapper

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

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 "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:

origin_server

This attribute should be set to a true value if the handler’s
"_write()" and "_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 "BaseHandler", but
false in "BaseCGIHandler" and "CGIHandler".

http_version

If "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"".

wsgiref.handlers.read_environ()

Transcode CGI variables from "os.environ" to PEP 3333 “bytes in
unicode” strings, returning a new dictionary. This function is
used by "CGIHandler" and "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.

New in version 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 (see PEP 333).
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()