21.4. `wsgiref` — Outils et implémentation de référence de WSGI¶

WSGI (Web Server Gateway Interface) est une interface standard entre le serveur web et une application web écrite en Python. Avoir une interface standardisée permet de faciliter l’usage de ces applications avec un certain nombre de serveurs web différents.

Seules les personnes programmant des serveurs web et des cadriciels ont besoin de connaître les détails d’implémentation et les cas particuliers de l’architecture de WSGI. En tant qu’utilisateur WSGI vous avez uniquement besoin d’installer WSGI ou d’utiliser un cadriciel existant.

wsgiref est une implémentation de référence de la spécification WSGI qui peut être utilisée pour ajouter la prise en charge de WSGI par un serveur web ou par un cadriciel. Elle fournit des outils pour manipuler les variables d’environnement WSGI, les en-têtes de réponse, les classes de base pour implémenter des serveurs WSGI, un serveur de démonstration d’application WSGI et un outil de validation qui vérifie que les serveurs et les applications WSGI sont conformes à la spécification WSGI (PEP 3333).

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

21.4.1. `wsgiref.util` — outils pour les environnements WSGI¶

Ce module fournit un certain nombre de fonctions pour manipuler des environnements WSGI. Un environnement WSGI est un dictionnaire contenant les variables de la requête HTTP comme décrit dans la PEP 3333. Toutes les fonctions ayant comme argument environ s’attendent à ce qu’un dictionnaire compatible WSGI soit fourni ; voir la PEP 3333 pour la spécification détaillée.

wsgiref.util.guess_scheme(environ)¶

Tente de déterminer s’il faut assigner « http » ou « https » à wsgi.url_sheme, en vérifiant si une variable d’environnement HTTPS est dans le dictionnaire environ. La valeur renvoyée est une chaîne de caractères.

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)¶

Met à jour environ avec des valeurs par défaut pour des cas de tests.

Cette fonction ajoute des paramètres requis pour WSGI, en particulier HTTP_HOST, SERVER_NAME, SERVER_PORT, REQUEST_METHOD, SCRIPT_NAME, PATH_INFO et toutes les autres variables WSGI définies dans la PEP 3333. Elle ne fournit que des valeurs par défaut sans toucher aux valeurs déjà définies de ces variables.

Cette fonction a pour but de faciliter les tests unitaires des serveurs et des applications WSGI dans des environnements factices. Elle ne devrait pas être utilisée dans une application ou un serveur WSGI, étant donné que les données sont factices !

Exemple d’utilisation :

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.

Exemple d’utilisation :

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)

21.4.2. `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)¶

Renvoie une liste de toutes les valeurs pour l’en-tête name.

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')

Le code ci-dessus ajoute un en-tête qui ressemble à ceci :

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

Modifié dans la version 3.5: Le paramètre headers est optionnel.

21.4.3. `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.

Exemple d’utilisation :

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.

21.4.4. `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).

Exemple d’utilisation :

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()

21.4.5. `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.

Nouveau dans la 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.

Modifié dans la 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.

Nouveau dans la version 3.2.

21.4.6. Exemples¶

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()

21.4. `wsgiref` — Outils et implémentation de référence de WSGI¶

21.4.1. `wsgiref.util` — outils pour les environnements WSGI¶

21.4.2. `wsgiref.headers` – WSGI response header tools¶

21.4.3. `wsgiref.simple_server` – a simple WSGI HTTP server¶

21.4.4. `wsgiref.validate` — WSGI conformance checker¶

21.4.5. `wsgiref.handlers` – server/gateway base classes¶

21.4.6. Exemples¶

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21.4. wsgiref — Outils et implémentation de référence de WSGI¶

21.4.1. wsgiref.util — outils pour les environnements WSGI¶

21.4.2. wsgiref.headers – WSGI response header tools¶

21.4.3. wsgiref.simple_server – a simple WSGI HTTP server¶

21.4.4. wsgiref.validate — WSGI conformance checker¶

21.4.5. wsgiref.handlers – server/gateway base classes¶

21.4.6. Exemples¶

21.4. `wsgiref` — Outils et implémentation de référence de WSGI¶

21.4.1. `wsgiref.util` — outils pour les environnements WSGI¶

21.4.2. `wsgiref.headers` – WSGI response header tools¶

21.4.3. `wsgiref.simple_server` – a simple WSGI HTTP server¶

21.4.4. `wsgiref.validate` — WSGI conformance checker¶

21.4.5. `wsgiref.handlers` – server/gateway base classes¶