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Eby <pje at telecommunity.com></dd> <dt class="field-even">Discussions-To:</dt> <dd class="field-even"><a class="reference external" href="https://mail.python.org/mailman/listinfo/web-sig">Web-SIG list</a></dd> <dt class="field-odd">Status:</dt> <dd class="field-odd"><abbr title="Accepted and implementation complete, or no longer active">Final</abbr></dd> <dt class="field-even">Type:</dt> <dd class="field-even"><abbr title="Non-normative PEP containing background, guidelines or other information relevant to the Python ecosystem">Informational</abbr></dd> <dt class="field-odd">Created:</dt> <dd class="field-odd">07-Dec-2003</dd> <dt class="field-even">Post-History:</dt> <dd class="field-even">07-Dec-2003, 08-Aug-2004, 20-Aug-2004, 27-Aug-2004, 27-Sep-2010</dd> <dt class="field-odd">Superseded-By:</dt> <dd class="field-odd"><a class="reference external" href="../pep-3333/">3333</a></dd> </dl> <hr class="docutils" /> <section id="contents"> <details><summary>Table of Contents</summary><ul class="simple"> <li><a class="reference internal" href="#preface">Preface</a></li> <li><a class="reference internal" href="#abstract">Abstract</a></li> <li><a class="reference internal" href="#rationale-and-goals">Rationale and Goals</a></li> <li><a class="reference internal" href="#specification-overview">Specification Overview</a><ul> <li><a class="reference internal" href="#the-application-framework-side">The Application/Framework Side</a></li> <li><a class="reference internal" href="#the-server-gateway-side">The Server/Gateway Side</a></li> <li><a class="reference internal" href="#middleware-components-that-play-both-sides">Middleware: Components that Play Both Sides</a></li> </ul> </li> <li><a class="reference internal" href="#specification-details">Specification Details</a><ul> <li><a class="reference internal" href="#environ-variables"><code class="docutils literal notranslate">environ</code> Variables</a><ul> <li><a class="reference internal" href="#input-and-error-streams">Input and Error Streams</a></li> </ul> </li> <li><a class="reference internal" href="#the-start-response-callable">The <code class="docutils literal notranslate">start_response()</code> Callable</a><ul> <li><a class="reference internal" href="#handling-the-content-length-header">Handling the <code class="docutils literal notranslate">Content-Length</code> Header</a></li> </ul> </li> <li><a class="reference internal" href="#buffering-and-streaming">Buffering and Streaming</a><ul> <li><a class="reference internal" href="#middleware-handling-of-block-boundaries">Middleware Handling of Block Boundaries</a></li> <li><a class="reference internal" href="#the-write-callable">The <code class="docutils literal notranslate">write()</code> Callable</a></li> </ul> </li> <li><a class="reference internal" href="#unicode-issues">Unicode Issues</a></li> <li><a class="reference internal" href="#error-handling">Error Handling</a></li> <li><a class="reference internal" href="#http-1-1-expect-continue">HTTP 1.1 Expect/Continue</a></li> <li><a class="reference internal" href="#other-http-features">Other HTTP Features</a></li> <li><a class="reference internal" href="#thread-support">Thread Support</a></li> </ul> </li> <li><a class="reference internal" href="#implementation-application-notes">Implementation/Application Notes</a><ul> <li><a class="reference internal" href="#server-extension-apis">Server Extension APIs</a></li> <li><a class="reference internal" href="#application-configuration">Application Configuration</a></li> <li><a class="reference internal" href="#url-reconstruction">URL Reconstruction</a></li> <li><a class="reference internal" href="#supporting-older-2-2-versions-of-python">Supporting Older (<2.2) Versions of Python</a></li> <li><a class="reference internal" href="#optional-platform-specific-file-handling">Optional Platform-Specific File Handling</a></li> </ul> </li> <li><a class="reference internal" href="#questions-and-answers">Questions and Answers</a></li> <li><a class="reference internal" href="#proposed-under-discussion">Proposed/Under Discussion</a></li> <li><a class="reference internal" href="#acknowledgements">Acknowledgements</a></li> <li><a class="reference internal" href="#references">References</a></li> <li><a class="reference internal" href="#copyright">Copyright</a></li> </ul> </details></section> <section id="preface"> <h2><a class="toc-backref" href="#preface" role="doc-backlink">Preface</a></h2> Note: For an updated version of this spec that supports Python 3.x and includes community errata, addenda, and clarifications, please see <a class="pep reference internal" href="../pep-3333/" title="PEP 3333 – Python Web Server Gateway Interface v1.0.1">PEP 3333</a> instead. </section> <section id="abstract"> <h2><a class="toc-backref" href="#abstract" role="doc-backlink">Abstract</a></h2> This document specifies a proposed standard interface between web servers and Python web applications or frameworks, to promote web application portability across a variety of web servers. </section> <section id="rationale-and-goals"> <h2><a class="toc-backref" href="#rationale-and-goals" role="doc-backlink">Rationale and Goals</a></h2> Python currently boasts a wide variety of web application frameworks, such as Zope, Quixote, Webware, SkunkWeb, PSO, and Twisted Web – to name just a few <a class="footnote-reference brackets" href="#id4" id="id1">[1]</a>. This wide variety of choices can be a problem for new Python users, because generally speaking, their choice of web framework will limit their choice of usable web servers, and vice versa. By contrast, although Java has just as many web application frameworks available, Java’s “servlet” API makes it possible for applications written with any Java web application framework to run in any web server that supports the servlet API. The availability and widespread use of such an API in web servers for Python – whether those servers are written in Python (e.g. Medusa), embed Python (e.g. mod_python), or invoke Python via a gateway protocol (e.g. CGI, FastCGI, etc.) – would separate choice of framework from choice of web server, freeing users to choose a pairing that suits them, while freeing framework and server developers to focus on their preferred area of specialization. This PEP, therefore, proposes a simple and universal interface between web servers and web applications or frameworks: the Python Web Server Gateway Interface (WSGI). But the mere existence of a WSGI spec does nothing to address the existing state of servers and frameworks for Python web applications. Server and framework authors and maintainers must actually implement WSGI for there to be any effect. However, since no existing servers or frameworks support WSGI, there is little immediate reward for an author who implements WSGI support. Thus, WSGI must be easy to implement, so that an author’s initial investment in the interface can be reasonably low. Thus, simplicity of implementation on both the server and framework sides of the interface is absolutely critical to the utility of the WSGI interface, and is therefore the principal criterion for any design decisions. Note, however, that simplicity of implementation for a framework author is not the same thing as ease of use for a web application author. WSGI presents an absolutely “no frills” interface to the framework author, because bells and whistles like response objects and cookie handling would just get in the way of existing frameworks’ handling of these issues. Again, the goal of WSGI is to facilitate easy interconnection of existing servers and applications or frameworks, not to create a new web framework. Note also that this goal precludes WSGI from requiring anything that is not already available in deployed versions of Python. Therefore, new standard library modules are not proposed or required by this specification, and nothing in WSGI requires a Python version greater than 2.2.2. (It would be a good idea, however, for future versions of Python to include support for this interface in web servers provided by the standard library.) In addition to ease of implementation for existing and future frameworks and servers, it should also be easy to create request preprocessors, response postprocessors, and other WSGI-based “middleware” components that look like an application to their containing server, while acting as a server for their contained applications. If middleware can be both simple and robust, and WSGI is widely available in servers and frameworks, it allows for the possibility of an entirely new kind of Python web application framework: one consisting of loosely-coupled WSGI middleware components. Indeed, existing framework authors may even choose to refactor their frameworks’ existing services to be provided in this way, becoming more like libraries used with WSGI, and less like monolithic frameworks. This would then allow application developers to choose “best-of-breed” components for specific functionality, rather than having to commit to all the pros and cons of a single framework. Of course, as of this writing, that day is doubtless quite far off. In the meantime, it is a sufficient short-term goal for WSGI to enable the use of any framework with any server. Finally, it should be mentioned that the current version of WSGI does not prescribe any particular mechanism for “deploying” an application for use with a web server or server gateway. At the present time, this is necessarily implementation-defined by the server or gateway. After a sufficient number of servers and frameworks have implemented WSGI to provide field experience with varying deployment requirements, it may make sense to create another PEP, describing a deployment standard for WSGI servers and application frameworks. </section> <section id="specification-overview"> <h2><a class="toc-backref" href="#specification-overview" role="doc-backlink">Specification Overview</a></h2> The WSGI interface has two sides: the “server” or “gateway” side, and the “application” or “framework” side. The server side invokes a callable object that is provided by the application side. The specifics of how that object is provided are up to the server or gateway. It is assumed that some servers or gateways will require an application’s deployer to write a short script to create an instance of the server or gateway, and supply it with the application object. Other servers and gateways may use configuration files or other mechanisms to specify where an application object should be imported from, or otherwise obtained. In addition to “pure” servers/gateways and applications/frameworks, it is also possible to create “middleware” components that implement both sides of this specification. Such components act as an application to their containing server, and as a server to a contained application, and can be used to provide extended APIs, content transformation, navigation, and other useful functions. Throughout this specification, we will use the term “a callable” to mean “a function, method, class, or an instance with a <code class="docutils literal notranslate">__call__</code> method”. It is up to the server, gateway, or application implementing the callable to choose the appropriate implementation technique for their needs. Conversely, a server, gateway, or application that is invoking a callable must not have any dependency on what kind of callable was provided to it. Callables are only to be called, not introspected upon. <section id="the-application-framework-side"> <h3><a class="toc-backref" href="#the-application-framework-side" role="doc-backlink">The Application/Framework Side</a></h3> The application object is simply a callable object that accepts two arguments. The term “object” should not be misconstrued as requiring an actual object instance: a function, method, class, or instance with a <code class="docutils literal notranslate">__call__</code> method are all acceptable for use as an application object. Application objects must be able to be invoked more than once, as virtually all servers/gateways (other than CGI) will make such repeated requests. (Note: although we refer to it as an “application” object, this should not be construed to mean that application developers will use WSGI as a web programming API! It is assumed that application developers will continue to use existing, high-level framework services to develop their applications. WSGI is a tool for framework and server developers, and is not intended to directly support application developers.) Here are two example application objects; one is a function, and the other is a class: <div class="highlight-default notranslate"><div class="highlight"><pre>def simple_app(environ, start_response): """Simplest possible application object""" status = '200 OK' response_headers = [('Content-type', 'text/plain')] start_response(status, response_headers) return ['Hello world!\n'] class AppClass: """Produce the same output, but using a class (Note: 'AppClass' is the "application" here, so calling it returns an instance of 'AppClass', which is then the iterable return value of the "application callable" as required by the spec. If we wanted to use *instances* of 'AppClass' as application objects instead, we would have to implement a '__call__' method, which would be invoked to execute the application, and we would need to create an instance for use by the server or gateway. """ def __init__(self, environ, start_response): self.environ = environ self.start = start_response def __iter__(self): status = '200 OK' response_headers = [('Content-type', 'text/plain')] self.start(status, response_headers) yield "Hello world!\n" </pre></div> </div> </section> <section id="the-server-gateway-side"> <h3><a class="toc-backref" href="#the-server-gateway-side" role="doc-backlink">The Server/Gateway Side</a></h3> The server or gateway invokes the application callable once for each request it receives from an HTTP client, that is directed at the application. To illustrate, here is a simple CGI gateway, implemented as a function taking an application object. Note that this simple example has limited error handling, because by default an uncaught exception will be dumped to <code class="docutils literal notranslate">sys.stderr</code> and logged by the web server. <div class="highlight-default notranslate"><div class="highlight"><pre>import os, sys def run_with_cgi(application): environ = dict(os.environ.items()) environ['wsgi.input'] = sys.stdin environ['wsgi.errors'] = sys.stderr environ['wsgi.version'] = (1, 0) environ['wsgi.multithread'] = False environ['wsgi.multiprocess'] = True environ['wsgi.run_once'] = True if environ.get('HTTPS', 'off') in ('on', '1'): environ['wsgi.url_scheme'] = 'https' else: environ['wsgi.url_scheme'] = 'http' headers_set = [] headers_sent = [] def write(data): if not headers_set: raise AssertionError("write() before start_response()") elif not headers_sent: # Before the first output, send the stored headers status, response_headers = headers_sent[:] = headers_set sys.stdout.write('Status: %s\r\n' % status) for header in response_headers: sys.stdout.write('%s: %s\r\n' % header) sys.stdout.write('\r\n') sys.stdout.write(data) sys.stdout.flush() def start_response(status, response_headers, exc_info=None): if exc_info: try: if headers_sent: # Re-raise original exception if headers sent raise exc_info[0], exc_info[1], exc_info[2] finally: exc_info = None # avoid dangling circular ref elif headers_set: raise AssertionError("Headers already set!") headers_set[:] = [status, response_headers] return write result = application(environ, start_response) try: for data in result: if data: # don't send headers until body appears write(data) if not headers_sent: write('') # send headers now if body was empty finally: if hasattr(result, 'close'): result.close() </pre></div> </div> </section> <section id="middleware-components-that-play-both-sides"> <h3><a class="toc-backref" href="#middleware-components-that-play-both-sides" role="doc-backlink">Middleware: Components that Play Both Sides</a></h3> Note that a single object may play the role of a server with respect to some application(s), while also acting as an application with respect to some server(s). Such “middleware” components can perform such functions as: <ul class="simple"> <li>Routing a request to different application objects based on the target URL, after rewriting the <code class="docutils literal notranslate">environ</code> accordingly.</li> <li>Allowing multiple applications or frameworks to run side by side in the same process</li> <li>Load balancing and remote processing, by forwarding requests and responses over a network</li> <li>Perform content postprocessing, such as applying XSL stylesheets</li> </ul> The presence of middleware in general is transparent to both the “server/gateway” and the “application/framework” sides of the interface, and should require no special support. A user who desires to incorporate middleware into an application simply provides the middleware component to the server, as if it were an application, and configures the middleware component to invoke the application, as if the middleware component were a server. Of course, the “application” that the middleware wraps may in fact be another middleware component wrapping another application, and so on, creating what is referred to as a “middleware stack”. For the most part, middleware must conform to the restrictions and requirements of both the server and application sides of WSGI. In some cases, however, requirements for middleware are more stringent than for a “pure” server or application, and these points will be noted in the specification. Here is a (tongue-in-cheek) example of a middleware component that converts <code class="docutils literal notranslate">text/plain</code> responses to pig Latin, using Joe Strout’s <code class="docutils literal notranslate">piglatin.py</code>. (Note: a “real” middleware component would probably use a more robust way of checking the content type, and should also check for a content encoding. Also, this simple example ignores the possibility that a word might be split across a block boundary.) <div class="highlight-default notranslate"><div class="highlight"><pre>from piglatin import piglatin class LatinIter: """Transform iterated output to piglatin, if it's okay to do so Note that the "okayness" can change until the application yields its first non-empty string, so 'transform_ok' has to be a mutable truth value. """ def __init__(self, result, transform_ok): if hasattr(result, 'close'): self.close = result.close self._next = iter(result).next self.transform_ok = transform_ok def __iter__(self): return self def next(self): if self.transform_ok: return piglatin(self._next()) else: return self._next() class Latinator: # by default, don't transform output transform = False def __init__(self, application): self.application = application def __call__(self, environ, start_response): transform_ok = [] def start_latin(status, response_headers, exc_info=None): # Reset ok flag, in case this is a repeat call del transform_ok[:] for name, value in response_headers: if name.lower() == 'content-type' and value == 'text/plain': transform_ok.append(True) # Strip content-length if present, else it'll be wrong response_headers = [(name, value) for name, value in response_headers if name.lower() != 'content-length' ] break write = start_response(status, response_headers, exc_info) if transform_ok: def write_latin(data): write(piglatin(data)) return write_latin else: return write return LatinIter(self.application(environ, start_latin), transform_ok) # Run foo_app under a Latinator's control, using the example CGI gateway from foo_app import foo_app run_with_cgi(Latinator(foo_app)) </pre></div> </div> </section> </section> <section id="specification-details"> <h2><a class="toc-backref" href="#specification-details" role="doc-backlink">Specification Details</a></h2> The application object must accept two positional arguments. For the sake of illustration, we have named them <code class="docutils literal notranslate">environ</code> and <code class="docutils literal notranslate">start_response</code>, but they are not required to have these names. A server or gateway must invoke the application object using positional (not keyword) arguments. (E.g. by calling <code class="docutils literal notranslate">result = application(environ, start_response)</code> as shown above.) The <code class="docutils literal notranslate">environ</code> parameter is a dictionary object, containing CGI-style environment variables. This object must be a builtin Python dictionary (not a subclass, <code class="docutils literal notranslate">UserDict</code> or other dictionary emulation), and the application is allowed to modify the dictionary in any way it desires. The dictionary must also include certain WSGI-required variables (described in a later section), and may also include server-specific extension variables, named according to a convention that will be described below. The <code class="docutils literal notranslate">start_response</code> parameter is a callable accepting two required positional arguments, and one optional argument. For the sake of illustration, we have named these arguments <code class="docutils literal notranslate">status</code>, <code class="docutils literal notranslate">response_headers</code>, and <code class="docutils literal notranslate">exc_info</code>, but they are not required to have these names, and the application must invoke the <code class="docutils literal notranslate">start_response</code> callable using positional arguments (e.g. <code class="docutils literal notranslate">start_response(status, response_headers)</code>). The <code class="docutils literal notranslate">status</code> parameter is a status string of the form <code class="docutils literal notranslate">"999 Message here"</code>, and <code class="docutils literal notranslate">response_headers</code> is a list of <code class="docutils literal notranslate">(header_name, header_value)</code> tuples describing the HTTP response header. The optional <code class="docutils literal notranslate">exc_info</code> parameter is described below in the sections on <a class="reference internal" href="#the-start-response-callable">The start_response() Callable</a> and <a class="reference internal" href="#error-handling">Error Handling</a>. It is used only when the application has trapped an error and is attempting to display an error message to the browser. The <code class="docutils literal notranslate">start_response</code> callable must return a <code class="docutils literal notranslate">write(body_data)</code> callable that takes one positional parameter: a string to be written as part of the HTTP response body. (Note: the <code class="docutils literal notranslate">write()</code> callable is provided only to support certain existing frameworks’ imperative output APIs; it should not be used by new applications or frameworks if it can be avoided. See the <a class="reference internal" href="#buffering-and-streaming">Buffering and Streaming</a> section for more details.) When called by the server, the application object must return an iterable yielding zero or more strings. This can be accomplished in a variety of ways, such as by returning a list of strings, or by the application being a generator function that yields strings, or by the application being a class whose instances are iterable. Regardless of how it is accomplished, the application object must always return an iterable yielding zero or more strings. The server or gateway must transmit the yielded strings to the client in an unbuffered fashion, completing the transmission of each string before requesting another one. (In other words, applications should perform their own buffering. See the <a class="reference internal" href="#buffering-and-streaming">Buffering and Streaming</a> section below for more on how application output must be handled.) The server or gateway should treat the yielded strings as binary byte sequences: in particular, it should ensure that line endings are not altered. The application is responsible for ensuring that the string(s) to be written are in a format suitable for the client. (The server or gateway may apply HTTP transfer encodings, or perform other transformations for the purpose of implementing HTTP features such as byte-range transmission. See <a class="reference internal" href="#other-http-features">Other HTTP Features</a>, below, for more details.) If a call to <code class="docutils literal notranslate">len(iterable)</code> succeeds, the server must be able to rely on the result being accurate. That is, if the iterable returned by the application provides a working <code class="docutils literal notranslate">__len__()</code> method, it must return an accurate result. (See the <a class="reference internal" href="#handling-the-content-length-header">Handling the Content-Length Header</a> section for information on how this would normally be used.) If the iterable returned by the application has a <code class="docutils literal notranslate">close()</code> method, the server or gateway must call that method upon completion of the current request, whether the request was completed normally, or terminated early due to an error (this is to support resource release by the application). This protocol is intended to complement <a class="pep reference internal" href="../pep-0325/" title="PEP 325 – Resource-Release Support for Generators">PEP 325</a>’s generator support, and other common iterables with <code class="docutils literal notranslate">close()</code> methods. (Note: the application must invoke the <code class="docutils literal notranslate">start_response()</code> callable before the iterable yields its first body string, so that the server can send the headers before any body content. However, this invocation may be performed by the iterable’s first iteration, so servers must not assume that <code class="docutils literal notranslate">start_response()</code> has been called before they begin iterating over the iterable.) Finally, servers and gateways must not directly use any other attributes of the iterable returned by the application, unless it is an instance of a type specific to that server or gateway, such as a “file wrapper” returned by <code class="docutils literal notranslate">wsgi.file_wrapper</code> (see <a class="reference internal" href="#optional-platform-specific-file-handling">Optional Platform-Specific File Handling</a>). In the general case, only attributes specified here, or accessed via e.g. the <a class="pep reference internal" href="../pep-0234/" title="PEP 234 – Iterators">PEP 234</a> iteration APIs are acceptable. <section id="environ-variables"> <h3><a class="toc-backref" href="#environ-variables" role="doc-backlink"><code class="docutils literal notranslate">environ</code> Variables</a></h3> The <code class="docutils literal notranslate">environ</code> dictionary is required to contain these CGI environment variables, as defined by the Common Gateway Interface specification <a class="footnote-reference brackets" href="#id5" id="id2">[2]</a>. The following variables must be present, unless their value would be an empty string, in which case they may be omitted, except as otherwise noted below. <dl class="simple"> <dt><code class="docutils literal notranslate">REQUEST_METHOD</code></dt><dd>The HTTP request method, such as <code class="docutils literal notranslate">"GET"</code> or <code class="docutils literal notranslate">"POST"</code>. This cannot ever be an empty string, and so is always required.</dd> <dt><code class="docutils literal notranslate">SCRIPT_NAME</code></dt><dd>The initial portion of the request URL’s “path” that corresponds to the application object, so that the application knows its virtual “location”. This may be an empty string, if the application corresponds to the “root” of the server.</dd> <dt><code class="docutils literal notranslate">PATH_INFO</code></dt><dd>The remainder of the request URL’s “path”, designating the virtual “location” of the request’s target within the application. This may be an empty string, if the request URL targets the application root and does not have a trailing slash.</dd> <dt><code class="docutils literal notranslate">QUERY_STRING</code></dt><dd>The portion of the request URL that follows the <code class="docutils literal notranslate">"?"</code>, if any. May be empty or absent.</dd> <dt><code class="docutils literal notranslate">CONTENT_TYPE</code></dt><dd>The contents of any <code class="docutils literal notranslate">Content-Type</code> fields in the HTTP request. May be empty or absent.</dd> <dt><code class="docutils literal notranslate">CONTENT_LENGTH</code></dt><dd>The contents of any <code class="docutils literal notranslate">Content-Length</code> fields in the HTTP request. May be empty or absent.</dd> <dt><code class="docutils literal notranslate">SERVER_NAME</code>, <code class="docutils literal notranslate">SERVER_PORT</code></dt><dd>When combined with <code class="docutils literal notranslate">SCRIPT_NAME</code> and <code class="docutils literal notranslate">PATH_INFO</code>, these variables can be used to complete the URL. Note, however, that <code class="docutils literal notranslate">HTTP_HOST</code>, if present, should be used in preference to <code class="docutils literal notranslate">SERVER_NAME</code> for reconstructing the request URL. See the <a class="reference internal" href="#url-reconstruction">URL Reconstruction</a> section below for more detail. <code class="docutils literal notranslate">SERVER_NAME</code> and <code class="docutils literal notranslate">SERVER_PORT</code> can never be empty strings, and so are always required.</dd> <dt><code class="docutils literal notranslate">SERVER_PROTOCOL</code></dt><dd>The version of the protocol the client used to send the request. Typically this will be something like <code class="docutils literal notranslate">"HTTP/1.0"</code> or <code class="docutils literal notranslate">"HTTP/1.1"</code> and may be used by the application to determine how to treat any HTTP request headers. (This variable should probably be called <code class="docutils literal notranslate">REQUEST_PROTOCOL</code>, since it denotes the protocol used in the request, and is not necessarily the protocol that will be used in the server’s response. However, for compatibility with CGI we have to keep the existing name.)</dd> <dt><code class="docutils literal notranslate">HTTP_</code> Variables</dt><dd>Variables corresponding to the client-supplied HTTP request headers (i.e., variables whose names begin with <code class="docutils literal notranslate">"HTTP_"</code>). The presence or absence of these variables should correspond with the presence or absence of the appropriate HTTP header in the request.</dd> </dl> A server or gateway should attempt to provide as many other CGI variables as are applicable. In addition, if SSL is in use, the server or gateway should also provide as many of the Apache SSL environment variables <a class="footnote-reference brackets" href="#id6" id="id3">[3]</a> as are applicable, such as <code class="docutils literal notranslate">HTTPS=on</code> and <code class="docutils literal notranslate">SSL_PROTOCOL</code>. Note, however, that an application that uses any CGI variables other than the ones listed above are necessarily non-portable to web servers that do not support the relevant extensions. (For example, web servers that do not publish files will not be able to provide a meaningful <code class="docutils literal notranslate">DOCUMENT_ROOT</code> or <code class="docutils literal notranslate">PATH_TRANSLATED</code>.) A WSGI-compliant server or gateway should document what variables it provides, along with their definitions as appropriate. Applications should check for the presence of any variables they require, and have a fallback plan in the event such a variable is absent. Note: missing variables (such as <code class="docutils literal notranslate">REMOTE_USER</code> when no authentication has occurred) should be left out of the <code class="docutils literal notranslate">environ</code> dictionary. Also note that CGI-defined variables must be strings, if they are present at all. It is a violation of this specification for a CGI variable’s value to be of any type other than <code class="docutils literal notranslate">str</code>. In addition to the CGI-defined variables, the <code class="docutils literal notranslate">environ</code> dictionary may also contain arbitrary operating-system “environment variables”, and must contain the following WSGI-defined variables: <table class="docutils align-default"> <thead> <tr class="row-odd"><th class="head">Variable</th> <th class="head">Value</th> </tr> </thead> <tbody> <tr class="row-even"><td><code class="docutils literal notranslate">wsgi.version</code></td> <td>The tuple <code class="docutils literal notranslate">(1, 0)</code>, representing WSGI version 1.0.</td> </tr> <tr class="row-odd"><td><code class="docutils literal notranslate">wsgi.url_scheme</code></td> <td>A string representing the “scheme” portion of the URL at which the application is being invoked. Normally, this will have the value <code class="docutils literal notranslate">"http"</code> or <code class="docutils literal notranslate">"https"</code>, as appropriate.</td> </tr> <tr class="row-even"><td><code class="docutils literal notranslate">wsgi.input</code></td> <td>An input stream (file-like object) from which the HTTP request body can be read. (The server or gateway may perform reads on-demand as requested by the application, or it may pre-read the client’s request body and buffer it in-memory or on disk, or use any other technique for providing such an input stream, according to its preference.)</td> </tr> <tr class="row-odd"><td><code class="docutils literal notranslate">wsgi.errors</code></td> <td>An output stream (file-like object) to which error output can be written, for the purpose of recording program or other errors in a standardized and possibly centralized location. This should be a “text mode” stream; i.e., applications should use <code class="docutils literal notranslate">"\n"</code> as a line ending, and assume that it will be converted to the correct line ending by the server/gateway.For many servers, <code class="docutils literal notranslate">wsgi.errors</code> will be the server’s main error log. Alternatively, this may be <code class="docutils literal notranslate">sys.stderr</code>, or a log file of some sort. The server’s documentation should include an explanation of how to configure this or where to find the recorded output. A server or gateway may supply different error streams to different applications, if this is desired. </td> </tr> <tr class="row-even"><td><code class="docutils literal notranslate">wsgi.multithread</code></td> <td>This value should evaluate true if the application object may be simultaneously invoked by another thread in the same process, and should evaluate false otherwise.</td> </tr> <tr class="row-odd"><td><code class="docutils literal notranslate">wsgi.multiprocess</code></td> <td>This value should evaluate true if an equivalent application object may be simultaneously invoked by another process, and should evaluate false otherwise.</td> </tr> <tr class="row-even"><td><code class="docutils literal notranslate">wsgi.run_once</code></td> <td>This value should evaluate true if the server or gateway expects (but does not guarantee!) that the application will only be invoked this one time during the life of its containing process. Normally, this will only be true for a gateway based on CGI (or something similar).</td> </tr> </tbody> </table> Finally, the <code class="docutils literal notranslate">environ</code> dictionary may also contain server-defined variables. These variables should be named using only lower-case letters, numbers, dots, and underscores, and should be prefixed with a name that is unique to the defining server or gateway. For example, <code class="docutils literal notranslate">mod_python</code> might define variables with names like <code class="docutils literal notranslate">mod_python.some_variable</code>. <section id="input-and-error-streams"> <h4><a class="toc-backref" href="#input-and-error-streams" role="doc-backlink">Input and Error Streams</a></h4> The input and error streams provided by the server must support the following methods: <table class="docutils align-default"> <thead> <tr class="row-odd"><th class="head">Method</th> <th class="head">Stream</th> <th class="head">Notes</th> </tr> </thead> <tbody> <tr class="row-even"><td><code class="docutils literal notranslate">read(size)</code></td> <td><code class="docutils literal notranslate">input</code></td> <td>1</td> </tr> <tr class="row-odd"><td><code class="docutils literal notranslate">readline()</code></td> <td><code class="docutils literal notranslate">input</code></td> <td>1, 2</td> </tr> <tr class="row-even"><td><code class="docutils literal notranslate">readlines(hint)</code></td> <td><code class="docutils literal notranslate">input</code></td> <td>1, 3</td> </tr> <tr class="row-odd"><td><code class="docutils literal notranslate">__iter__()</code></td> <td><code class="docutils literal notranslate">input</code></td> <td></td> </tr> <tr class="row-even"><td><code class="docutils literal notranslate">flush()</code></td> <td><code class="docutils literal notranslate">errors</code></td> <td>4</td> </tr> <tr class="row-odd"><td><code class="docutils literal notranslate">write(str)</code></td> <td><code class="docutils literal notranslate">errors</code></td> <td></td> </tr> <tr class="row-even"><td><code class="docutils literal notranslate">writelines(seq)</code></td> <td><code class="docutils literal notranslate">errors</code></td> <td></td> </tr> </tbody> </table> The semantics of each method are as documented in the Python Library Reference, except for these notes as listed in the table above: <ol class="arabic simple"> <li>The server is not required to read past the client’s specified <code class="docutils literal notranslate">Content-Length</code>, and is allowed to simulate an end-of-file condition if the application attempts to read past that point. The application should not attempt to read more data than is specified by the <code class="docutils literal notranslate">CONTENT_LENGTH</code> variable.</li> <li>The optional “size” argument to <code class="docutils literal notranslate">readline()</code> is not supported, as it may be complex for server authors to implement, and is not often used in practice.</li> <li>Note that the <code class="docutils literal notranslate">hint</code> argument to <code class="docutils literal notranslate">readlines()</code> is optional for both caller and implementer. The application is free not to supply it, and the server or gateway is free to ignore it.</li> <li>Since the <code class="docutils literal notranslate">errors</code> stream may not be rewound, servers and gateways are free to forward write operations immediately, without buffering. In this case, the <code class="docutils literal notranslate">flush()</code> method may be a no-op. Portable applications, however, cannot assume that output is unbuffered or that <code class="docutils literal notranslate">flush()</code> is a no-op. They must call <code class="docutils literal notranslate">flush()</code> if they need to ensure that output has in fact been written. (For example, to minimize intermingling of data from multiple processes writing to the same error log.)</li> </ol> The methods listed in the table above must be supported by all servers conforming to this specification. Applications conforming to this specification must not use any other methods or attributes of the <code class="docutils literal notranslate">input</code> or <code class="docutils literal notranslate">errors</code> objects. In particular, applications must not attempt to close these streams, even if they possess <code class="docutils literal notranslate">close()</code> methods. </section> </section> <section id="the-start-response-callable"> <h3><a class="toc-backref" href="#the-start-response-callable" role="doc-backlink">The <code class="docutils literal notranslate">start_response()</code> Callable</a></h3> The second parameter passed to the application object is a callable of the form <code class="docutils literal notranslate">start_response(status, response_headers, exc_info=None)</code>. (As with all WSGI callables, the arguments must be supplied positionally, not by keyword.) The <code class="docutils literal notranslate">start_response</code> callable is used to begin the HTTP response, and it must return a <code class="docutils literal notranslate">write(body_data)</code> callable (see the <a class="reference internal" href="#buffering-and-streaming">Buffering and Streaming</a> section, below). The <code class="docutils literal notranslate">status</code> argument is an HTTP “status” string like <code class="docutils literal notranslate">"200 OK"</code> or <code class="docutils literal notranslate">"404 Not Found"</code>. That is, it is a string consisting of a Status-Code and a Reason-Phrase, in that order and separated by a single space, with no surrounding whitespace or other characters. (See <a class="rfc reference external" href="https://datatracker.ietf.org/doc/html/rfc2616.html">RFC 2616</a>, Section 6.1.1 for more information.) The string must not contain control characters, and must not be terminated with a carriage return, linefeed, or combination thereof. The <code class="docutils literal notranslate">response_headers</code> argument is a list of <code class="docutils literal notranslate">(header_name, header_value)</code> tuples. It must be a Python list; i.e. <code class="docutils literal notranslate">type(response_headers) is ListType</code>, and the server may change its contents in any way it desires. Each <code class="docutils literal notranslate">header_name</code> must be a valid HTTP header field-name (as defined by <a class="rfc reference external" href="https://datatracker.ietf.org/doc/html/rfc2616.html">RFC 2616</a>, Section 4.2), without a trailing colon or other punctuation. Each <code class="docutils literal notranslate">header_value</code> must not include any control characters, including carriage returns or linefeeds, either embedded or at the end. (These requirements are to minimize the complexity of any parsing that must be performed by servers, gateways, and intermediate response processors that need to inspect or modify response headers.) In general, the server or gateway is responsible for ensuring that correct headers are sent to the client: if the application omits a header required by HTTP (or other relevant specifications that are in effect), the server or gateway must add it. For example, the HTTP <code class="docutils literal notranslate">Date:</code> and <code class="docutils literal notranslate">Server:</code> headers would normally be supplied by the server or gateway. (A reminder for server/gateway authors: HTTP header names are case-insensitive, so be sure to take that into consideration when examining application-supplied headers!) Applications and middleware are forbidden from using HTTP/1.1 “hop-by-hop” features or headers, any equivalent features in HTTP/1.0, or any headers that would affect the persistence of the client’s connection to the web server. These features are the exclusive province of the actual web server, and a server or gateway should consider it a fatal error for an application to attempt sending them, and raise an error if they are supplied to <code class="docutils literal notranslate">start_response()</code>. (For more specifics on “hop-by-hop” features and headers, please see the <a class="reference internal" href="#other-http-features">Other HTTP Features</a> section below.) The <code class="docutils literal notranslate">start_response</code> callable must not actually transmit the response headers. Instead, it must store them for the server or gateway to transmit only after the first iteration of the application return value that yields a non-empty string, or upon the application’s first invocation of the <code class="docutils literal notranslate">write()</code> callable. In other words, response headers must not be sent until there is actual body data available, or until the application’s returned iterable is exhausted. (The only possible exception to this rule is if the response headers explicitly include a <code class="docutils literal notranslate">Content-Length</code> of zero.) This delaying of response header transmission is to ensure that buffered and asynchronous applications can replace their originally intended output with error output, up until the last possible moment. For example, the application may need to change the response status from “200 OK” to “500 Internal Error”, if an error occurs while the body is being generated within an application buffer. The <code class="docutils literal notranslate">exc_info</code> argument, if supplied, must be a Python <code class="docutils literal notranslate">sys.exc_info()</code> tuple. This argument should be supplied by the application only if <code class="docutils literal notranslate">start_response</code> is being called by an error handler. If <code class="docutils literal notranslate">exc_info</code> is supplied, and no HTTP headers have been output yet, <code class="docutils literal notranslate">start_response</code> should replace the currently-stored HTTP response headers with the newly-supplied ones, thus allowing the application to “change its mind” about the output when an error has occurred. However, if <code class="docutils literal notranslate">exc_info</code> is provided, and the HTTP headers have already been sent, <code class="docutils literal notranslate">start_response</code> must raise an error, and should raise the <code class="docutils literal notranslate">exc_info</code> tuple. That is: <div class="highlight-default notranslate"><div class="highlight"><pre>raise exc_info[0], exc_info[1], exc_info[2] </pre></div> </div> This will re-raise the exception trapped by the application, and in principle should abort the application. (It is not safe for the application to attempt error output to the browser once the HTTP headers have already been sent.) The application must not trap any exceptions raised by <code class="docutils literal notranslate">start_response</code>, if it called <code class="docutils literal notranslate">start_response</code> with <code class="docutils literal notranslate">exc_info</code>. Instead, it should allow such exceptions to propagate back to the server or gateway. See <a class="reference internal" href="#error-handling">Error Handling</a> below, for more details. The application may call <code class="docutils literal notranslate">start_response</code> more than once, if and only if the <code class="docutils literal notranslate">exc_info</code> argument is provided. More precisely, it is a fatal error to call <code class="docutils literal notranslate">start_response</code> without the <code class="docutils literal notranslate">exc_info</code> argument if <code class="docutils literal notranslate">start_response</code> has already been called within the current invocation of the application. (See the example CGI gateway above for an illustration of the correct logic.) Note: servers, gateways, or middleware implementing <code class="docutils literal notranslate">start_response</code> should ensure that no reference is held to the <code class="docutils literal notranslate">exc_info</code> parameter beyond the duration of the function’s execution, to avoid creating a circular reference through the traceback and frames involved. The simplest way to do this is something like: <div class="highlight-default notranslate"><div class="highlight"><pre>def start_response(status, response_headers, exc_info=None): if exc_info: try: # do stuff w/exc_info here finally: exc_info = None # Avoid circular ref. </pre></div> </div> The example CGI gateway provides another illustration of this technique. <section id="handling-the-content-length-header"> <h4><a class="toc-backref" href="#handling-the-content-length-header" role="doc-backlink">Handling the <code class="docutils literal notranslate">Content-Length</code> Header</a></h4> If the application does not supply a <code class="docutils literal notranslate">Content-Length</code> header, a server or gateway may choose one of several approaches to handling it. The simplest of these is to close the client connection when the response is completed. Under some circumstances, however, the server or gateway may be able to either generate a <code class="docutils literal notranslate">Content-Length</code> header, or at least avoid the need to close the client connection. If the application does not call the <code class="docutils literal notranslate">write()</code> callable, and returns an iterable whose <code class="docutils literal notranslate">len()</code> is 1, then the server can automatically determine <code class="docutils literal notranslate">Content-Length</code> by taking the length of the first string yielded by the iterable. And, if the server and client both support HTTP/1.1 <a class="rfc reference external" href="https://datatracker.ietf.org/doc/html/rfc2616.html#section-3.6.1">“chunked encoding”</a>, then the server may use chunked encoding to send a chunk for each <code class="docutils literal notranslate">write()</code> call or string yielded by the iterable, thus generating a <code class="docutils literal notranslate">Content-Length</code> header for each chunk. This allows the server to keep the client connection alive, if it wishes to do so. Note that the server must comply fully with <a class="rfc reference external" href="https://datatracker.ietf.org/doc/html/rfc2616.html">RFC 2616</a> when doing this, or else fall back to one of the other strategies for dealing with the absence of <code class="docutils literal notranslate">Content-Length</code>. (Note: applications and middleware must not apply any kind of <code class="docutils literal notranslate">Transfer-Encoding</code> to their output, such as chunking or gzipping; as “hop-by-hop” operations, these encodings are the province of the actual web server/gateway. See <a class="reference internal" href="#other-http-features">Other HTTP Features</a> below, for more details.) </section> </section> <section id="buffering-and-streaming"> <h3><a class="toc-backref" href="#buffering-and-streaming" role="doc-backlink">Buffering and Streaming</a></h3> Generally speaking, applications will achieve the best throughput by buffering their (modestly-sized) output and sending it all at once. This is a common approach in existing frameworks such as Zope: the output is buffered in a StringIO or similar object, then transmitted all at once, along with the response headers. The corresponding approach in WSGI is for the application to simply return a single-element iterable (such as a list) containing the response body as a single string. This is the recommended approach for the vast majority of application functions, that render HTML pages whose text easily fits in memory. For large files, however, or for specialized uses of HTTP streaming (such as multipart “server push”), an application may need to provide output in smaller blocks (e.g. to avoid loading a large file into memory). It’s also sometimes the case that part of a response may be time-consuming to produce, but it would be useful to send ahead the portion of the response that precedes it. In these cases, applications will usually return an iterator (often a generator-iterator) that produces the output in a block-by-block fashion. These blocks may be broken to coincide with multipart boundaries (for “server push”), or just before time-consuming tasks (such as reading another block of an on-disk file). WSGI servers, gateways, and middleware must not delay the transmission of any block; they must either fully transmit the block to the client, or guarantee that they will continue transmission even while the application is producing its next block. A server/gateway or middleware may provide this guarantee in one of three ways: <ol class="arabic simple"> <li>Send the entire block to the operating system (and request that any O/S buffers be flushed) before returning control to the application, OR</li> <li>Use a different thread to ensure that the block continues to be transmitted while the application produces the next block.</li> <li>(Middleware only) send the entire block to its parent gateway/server</li> </ol> By providing this guarantee, WSGI allows applications to ensure that transmission will not become stalled at an arbitrary point in their output data. This is critical for proper functioning of e.g. multipart “server push” streaming, where data between multipart boundaries should be transmitted in full to the client. <section id="middleware-handling-of-block-boundaries"> <h4><a class="toc-backref" href="#middleware-handling-of-block-boundaries" role="doc-backlink">Middleware Handling of Block Boundaries</a></h4> In order to better support asynchronous applications and servers, middleware components must not block iteration waiting for multiple values from an application iterable. If the middleware needs to accumulate more data from the application before it can produce any output, it must yield an empty string. To put this requirement another way, a middleware component must yield at least one value each time its underlying application yields a value. If the middleware cannot yield any other value, it must yield an empty string. This requirement ensures that asynchronous applications and servers can conspire to reduce the number of threads that are required to run a given number of application instances simultaneously. Note also that this requirement means that middleware must return an iterable as soon as its underlying application returns an iterable. It is also forbidden for middleware to use the <code class="docutils literal notranslate">write()</code> callable to transmit data that is yielded by an underlying application. Middleware may only use their parent server’s <code class="docutils literal notranslate">write()</code> callable to transmit data that the underlying application sent using a middleware-provided <code class="docutils literal notranslate">write()</code> callable. </section> <section id="the-write-callable"> <h4><a class="toc-backref" href="#the-write-callable" role="doc-backlink">The <code class="docutils literal notranslate">write()</code> Callable</a></h4> Some existing application framework APIs support unbuffered output in a different manner than WSGI. Specifically, they provide a “write” function or method of some kind to write an unbuffered block of data, or else they provide a buffered “write” function and a “flush” mechanism to flush the buffer. Unfortunately, such APIs cannot be implemented in terms of WSGI’s “iterable” application return value, unless threads or other special mechanisms are used. Therefore, to allow these frameworks to continue using an imperative API, WSGI includes a special <code class="docutils literal notranslate">write()</code> callable, returned by the <code class="docutils literal notranslate">start_response</code> callable. New WSGI applications and frameworks should not use the <code class="docutils literal notranslate">write()</code> callable if it is possible to avoid doing so. The <code class="docutils literal notranslate">write()</code> callable is strictly a hack to support imperative streaming APIs. In general, applications should produce their output via their returned iterable, as this makes it possible for web servers to interleave other tasks in the same Python thread, potentially providing better throughput for the server as a whole. The <code class="docutils literal notranslate">write()</code> callable is returned by the <code class="docutils literal notranslate">start_response()</code> callable, and it accepts a single parameter: a string to be written as part of the HTTP response body, that is treated exactly as though it had been yielded by the output iterable. In other words, before <code class="docutils literal notranslate">write()</code> returns, it must guarantee that the passed-in string was either completely sent to the client, or that it is buffered for transmission while the application proceeds onward. An application must return an iterable object, even if it uses <code class="docutils literal notranslate">write()</code> to produce all or part of its response body. The returned iterable may be empty (i.e. yield no non-empty strings), but if it does yield non-empty strings, that output must be treated normally by the server or gateway (i.e., it must be sent or queued immediately). Applications must not invoke <code class="docutils literal notranslate">write()</code> from within their return iterable, and therefore any strings yielded by the iterable are transmitted after all strings passed to <code class="docutils literal notranslate">write()</code> have been sent to the client. </section> </section> <section id="unicode-issues"> <h3><a class="toc-backref" href="#unicode-issues" role="doc-backlink">Unicode Issues</a></h3> HTTP does not directly support Unicode, and neither does this interface. All encoding/decoding must be handled by the application; all strings passed to or from the server must be standard Python byte strings, not Unicode objects. The result of using a Unicode object where a string object is required, is undefined. Note also that strings passed to <code class="docutils literal notranslate">start_response()</code> as a status or as response headers must follow <a class="rfc reference external" href="https://datatracker.ietf.org/doc/html/rfc2616.html">RFC 2616</a> with respect to encoding. That is, they must either be ISO-8859-1 characters, or use <a class="rfc reference external" href="https://datatracker.ietf.org/doc/html/rfc2047.html">RFC 2047</a> MIME encoding. On Python platforms where the <code class="docutils literal notranslate">str</code> or <code class="docutils literal notranslate">StringType</code> type is in fact Unicode-based (e.g. Jython, IronPython, Python 3000, etc.), all “strings” referred to in this specification must contain only code points representable in ISO-8859-1 encoding (<code class="docutils literal notranslate">\u0000</code> through <code class="docutils literal notranslate">\u00FF</code>, inclusive). It is a fatal error for an application to supply strings containing any other Unicode character or code point. Similarly, servers and gateways must not supply strings to an application containing any other Unicode characters. Again, all strings referred to in this specification must be of type <code class="docutils literal notranslate">str</code> or <code class="docutils literal notranslate">StringType</code>, and must not be of type <code class="docutils literal notranslate">unicode</code> or <code class="docutils literal notranslate">UnicodeType</code>. And, even if a given platform allows for more than 8 bits per character in <code class="docutils literal notranslate">str</code>/<code class="docutils literal notranslate">StringType</code> objects, only the lower 8 bits may be used, for any value referred to in this specification as a “string”. </section> <section id="error-handling"> <h3><a class="toc-backref" href="#error-handling" role="doc-backlink">Error Handling</a></h3> In general, applications should try to trap their own, internal errors, and display a helpful message in the browser. (It is up to the application to decide what “helpful” means in this context.) However, to display such a message, the application must not have actually sent any data to the browser yet, or else it risks corrupting the response. WSGI therefore provides a mechanism to either allow the application to send its error message, or be automatically aborted: the <code class="docutils literal notranslate">exc_info</code> argument to <code class="docutils literal notranslate">start_response</code>. Here is an example of its use: <div class="highlight-default notranslate"><div class="highlight"><pre>try: # regular application code here status = "200 Froody" response_headers = [("content-type", "text/plain")] start_response(status, response_headers) return ["normal body goes here"] except: # XXX should trap runtime issues like MemoryError, KeyboardInterrupt # in a separate handler before this bare 'except:'... status = "500 Oops" response_headers = [("content-type", "text/plain")] start_response(status, response_headers, sys.exc_info()) return ["error body goes here"] </pre></div> </div> If no output has been written when an exception occurs, the call to <code class="docutils literal notranslate">start_response</code> will return normally, and the application will return an error body to be sent to the browser. However, if any output has already been sent to the browser, <code class="docutils literal notranslate">start_response</code> will reraise the provided exception. This exception should not be trapped by the application, and so the application will abort. The server or gateway can then trap this (fatal) exception and abort the response. Servers should trap and log any exception that aborts an application or the iteration of its return value. If a partial response has already been written to the browser when an application error occurs, the server or gateway may attempt to add an error message to the output, if the already-sent headers indicate a <code class="docutils literal notranslate">text/*</code> content type that the server knows how to modify cleanly. Some middleware may wish to provide additional exception handling services, or intercept and replace application error messages. In such cases, middleware may choose to not re-raise the <code class="docutils literal notranslate">exc_info</code> supplied to <code class="docutils literal notranslate">start_response</code>, but instead raise a middleware-specific exception, or simply return without an exception after storing the supplied arguments. This will then cause the application to return its error body iterable (or invoke <code class="docutils literal notranslate">write()</code>), allowing the middleware to capture and modify the error output. These techniques will work as long as application authors: <ol class="arabic simple"> <li>Always provide <code class="docutils literal notranslate">exc_info</code> when beginning an error response</li> <li>Never trap errors raised by <code class="docutils literal notranslate">start_response</code> when <code class="docutils literal notranslate">exc_info</code> is being provided</li> </ol> </section> <section id="http-1-1-expect-continue"> <h3><a class="toc-backref" href="#http-1-1-expect-continue" role="doc-backlink">HTTP 1.1 Expect/Continue</a></h3> Servers and gateways that implement HTTP 1.1 must provide transparent support for HTTP 1.1’s “expect/continue” mechanism. This may be done in any of several ways: <ol class="arabic simple"> <li>Respond to requests containing an <code class="docutils literal notranslate">Expect: 100-continue</code> request with an immediate “100 Continue” response, and proceed normally.</li> <li>Proceed with the request normally, but provide the application with a <code class="docutils literal notranslate">wsgi.input</code> stream that will send the “100 Continue” response if/when the application first attempts to read from the input stream. The read request must then remain blocked until the client responds.</li> <li>Wait until the client decides that the server does not support expect/continue, and sends the request body on its own. (This is suboptimal, and is not recommended.)</li> </ol> Note that these behavior restrictions do not apply for HTTP 1.0 requests, or for requests that are not directed to an application object. For more information on HTTP 1.1 Expect/Continue, see <a class="rfc reference external" href="https://datatracker.ietf.org/doc/html/rfc2616.html">RFC 2616</a>, sections 8.2.3 and 10.1.1. </section> <section id="other-http-features"> <h3><a class="toc-backref" href="#other-http-features" role="doc-backlink">Other HTTP Features</a></h3> In general, servers and gateways should “play dumb” and allow the application complete control over its output. They should only make changes that do not alter the effective semantics of the application’s response. It is always possible for the application developer to add middleware components to supply additional features, so server/gateway developers should be conservative in their implementation. In a sense, a server should consider itself to be like an HTTP “gateway server”, with the application being an HTTP “origin server”. (See <a class="rfc reference external" href="https://datatracker.ietf.org/doc/html/rfc2616.html">RFC 2616</a>, section 1.3, for the definition of these terms.) However, because WSGI servers and applications do not communicate via HTTP, what <a class="rfc reference external" href="https://datatracker.ietf.org/doc/html/rfc2616.html">RFC 2616</a> calls “hop-by-hop” headers do not apply to WSGI internal communications. WSGI applications must not generate any <a class="rfc reference external" href="https://datatracker.ietf.org/doc/html/rfc2616.html#section-13.5.1">“hop-by-hop” headers</a>, attempt to use HTTP features that would require them to generate such headers, or rely on the content of any incoming “hop-by-hop” headers in the <code class="docutils literal notranslate">environ</code> dictionary. WSGI servers must handle any supported inbound “hop-by-hop” headers on their own, such as by decoding any inbound <code class="docutils literal notranslate">Transfer-Encoding</code>, including chunked encoding if applicable. Applying these principles to a variety of HTTP features, it should be clear that a server may handle cache validation via the <code class="docutils literal notranslate">If-None-Match</code> and <code class="docutils literal notranslate">If-Modified-Since</code> request headers and the <code class="docutils literal notranslate">Last-Modified</code> and <code class="docutils literal notranslate">ETag</code> response headers. However, it is not required to do this, and the application should perform its own cache validation if it wants to support that feature, since the server/gateway is not required to do such validation. Similarly, a server may re-encode or transport-encode an application’s response, but the application should use a suitable content encoding on its own, and must not apply a transport encoding. A server may transmit byte ranges of the application’s response if requested by the client, and the application doesn’t natively support byte ranges. Again, however, the application should perform this function on its own if desired. Note that these restrictions on applications do not necessarily mean that every application must reimplement every HTTP feature; many HTTP features can be partially or fully implemented by middleware components, thus freeing both server and application authors from implementing the same features over and over again. </section> <section id="thread-support"> <h3><a class="toc-backref" href="#thread-support" role="doc-backlink">Thread Support</a></h3> Thread support, or lack thereof, is also server-dependent. Servers that can run multiple requests in parallel, should also provide the option of running an application in a single-threaded fashion, so that applications or frameworks that are not thread-safe may still be used with that server. </section> </section> <section id="implementation-application-notes"> <h2><a class="toc-backref" href="#implementation-application-notes" role="doc-backlink">Implementation/Application Notes</a></h2> <section id="server-extension-apis"> <h3><a class="toc-backref" href="#server-extension-apis" role="doc-backlink">Server Extension APIs</a></h3> Some server authors may wish to expose more advanced APIs, that application or framework authors can use for specialized purposes. For example, a gateway based on <code class="docutils literal notranslate">mod_python</code> might wish to expose part of the Apache API as a WSGI extension. In the simplest case, this requires nothing more than defining an <code class="docutils literal notranslate">environ</code> variable, such as <code class="docutils literal notranslate">mod_python.some_api</code>. But, in many cases, the possible presence of middleware can make this difficult. For example, an API that offers access to the same HTTP headers that are found in <code class="docutils literal notranslate">environ</code> variables, might return different data if <code class="docutils literal notranslate">environ</code> has been modified by middleware. In general, any extension API that duplicates, supplants, or bypasses some portion of WSGI functionality runs the risk of being incompatible with middleware components. Server/gateway developers should not assume that nobody will use middleware, because some framework developers specifically intend to organize or reorganize their frameworks to function almost entirely as middleware of various kinds. So, to provide maximum compatibility, servers and gateways that provide extension APIs that replace some WSGI functionality, must design those APIs so that they are invoked using the portion of the API that they replace. For example, an extension API to access HTTP request headers must require the application to pass in its current <code class="docutils literal notranslate">environ</code>, so that the server/gateway may verify that HTTP headers accessible via the API have not been altered by middleware. If the extension API cannot guarantee that it will always agree with <code class="docutils literal notranslate">environ</code> about the contents of HTTP headers, it must refuse service to the application, e.g. by raising an error, returning <code class="docutils literal notranslate">None</code> instead of a header collection, or whatever is appropriate to the API. Similarly, if an extension API provides an alternate means of writing response data or headers, it should require the <code class="docutils literal notranslate">start_response</code> callable to be passed in, before the application can obtain the extended service. If the object passed in is not the same one that the server/gateway originally supplied to the application, it cannot guarantee correct operation and must refuse to provide the extended service to the application. These guidelines also apply to middleware that adds information such as parsed cookies, form variables, sessions, and the like to <code class="docutils literal notranslate">environ</code>. Specifically, such middleware should provide these features as functions which operate on <code class="docutils literal notranslate">environ</code>, rather than simply stuffing values into <code class="docutils literal notranslate">environ</code>. This helps ensure that information is calculated from <code class="docutils literal notranslate">environ</code> after any middleware has done any URL rewrites or other <code class="docutils literal notranslate">environ</code> modifications. It is very important that these “safe extension” rules be followed by both server/gateway and middleware developers, in order to avoid a future in which middleware developers are forced to delete any and all extension APIs from <code class="docutils literal notranslate">environ</code> to ensure that their mediation isn’t being bypassed by applications using those extensions! </section> <section id="application-configuration"> <h3><a class="toc-backref" href="#application-configuration" role="doc-backlink">Application Configuration</a></h3> This specification does not define how a server selects or obtains an application to invoke. These and other configuration options are highly server-specific matters. It is expected that server/gateway authors will document how to configure the server to execute a particular application object, and with what options (such as threading options). Framework authors, on the other hand, should document how to create an application object that wraps their framework’s functionality. The user, who has chosen both the server and the application framework, must connect the two together. However, since both the framework and the server now have a common interface, this should be merely a mechanical matter, rather than a significant engineering effort for each new server/framework pair. Finally, some applications, frameworks, and middleware may wish to use the <code class="docutils literal notranslate">environ</code> dictionary to receive simple string configuration options. Servers and gateways should support this by allowing an application’s deployer to specify name-value pairs to be placed in <code class="docutils literal notranslate">environ</code>. In the simplest case, this support can consist merely of copying all operating system-supplied environment variables from <code class="docutils literal notranslate">os.environ</code> into the <code class="docutils literal notranslate">environ</code> dictionary, since the deployer in principle can configure these externally to the server, or in the CGI case they may be able to be set via the server’s configuration files. Applications should try to keep such required variables to a minimum, since not all servers will support easy configuration of them. Of course, even in the worst case, persons deploying an application can create a script to supply the necessary configuration values: <div class="highlight-default notranslate"><div class="highlight"><pre>from the_app import application def new_app(environ, start_response): environ['the_app.configval1'] = 'something' return application(environ, start_response) </pre></div> </div> But, most existing applications and frameworks will probably only need a single configuration value from <code class="docutils literal notranslate">environ</code>, to indicate the location of their application or framework-specific configuration file(s). (Of course, applications should cache such configuration, to avoid having to re-read it upon each invocation.) </section> <section id="url-reconstruction"> <h3><a class="toc-backref" href="#url-reconstruction" role="doc-backlink">URL Reconstruction</a></h3> If an application wishes to reconstruct a request’s complete URL, it may do so using the following algorithm, contributed by Ian Bicking: <div class="highlight-default notranslate"><div class="highlight"><pre>from urllib import quote url = environ['wsgi.url_scheme']+'://' if environ.get('HTTP_HOST'): url += environ['HTTP_HOST'] else: url += environ['SERVER_NAME'] if environ['wsgi.url_scheme'] == 'https': if environ['SERVER_PORT'] != '443': url += ':' + environ['SERVER_PORT'] else: if environ['SERVER_PORT'] != '80': url += ':' + environ['SERVER_PORT'] url += quote(environ.get('SCRIPT_NAME', '')) url += quote(environ.get('PATH_INFO', '')) if environ.get('QUERY_STRING'): url += '?' + environ['QUERY_STRING'] </pre></div> </div> Note that such a reconstructed URL may not be precisely the same URI as requested by the client. Server rewrite rules, for example, may have modified the client’s originally requested URL to place it in a canonical form. </section> <section id="supporting-older-2-2-versions-of-python"> <h3><a class="toc-backref" href="#supporting-older-2-2-versions-of-python" role="doc-backlink">Supporting Older (<2.2) Versions of Python</a></h3> Some servers, gateways, or applications may wish to support older (<2.2) versions of Python. This is especially important if Jython is a target platform, since as of this writing a production-ready version of Jython 2.2 is not yet available. For servers and gateways, this is relatively straightforward: servers and gateways targeting pre-2.2 versions of Python must simply restrict themselves to using only a standard “for” loop to iterate over any iterable returned by an application. This is the only way to ensure source-level compatibility with both the pre-2.2 iterator protocol (discussed further below) and “today’s” iterator protocol (see <a class="pep reference internal" href="../pep-0234/" title="PEP 234 – Iterators">PEP 234</a>). (Note that this technique necessarily applies only to servers, gateways, or middleware that are written in Python. Discussion of how to use iterator protocol(s) correctly from other languages is outside the scope of this PEP.) For applications, supporting pre-2.2 versions of Python is slightly more complex: <ul class="simple"> <li>You may not return a file object and expect it to work as an iterable, since before Python 2.2, files were not iterable. (In general, you shouldn’t do this anyway, because it will perform quite poorly most of the time!) Use <code class="docutils literal notranslate">wsgi.file_wrapper</code> or an application-specific file wrapper class. (See <a class="reference internal" href="#optional-platform-specific-file-handling">Optional Platform-Specific File Handling</a> for more on <code class="docutils literal notranslate">wsgi.file_wrapper</code>, and an example class you can use to wrap a file as an iterable.)</li> <li>If you return a custom iterable, it must implement the pre-2.2 iterator protocol. That is, provide a <code class="docutils literal notranslate">__getitem__</code> method that accepts an integer key, and raises <code class="docutils literal notranslate">IndexError</code> when exhausted. (Note that built-in sequence types are also acceptable, since they also implement this protocol.)</li> </ul> Finally, middleware that wishes to support pre-2.2 versions of Python, and iterates over application return values or itself returns an iterable (or both), must follow the appropriate recommendations above. (Note: It should go without saying that to support pre-2.2 versions of Python, any server, gateway, application, or middleware must also use only language features available in the target version, use 1 and 0 instead of <code class="docutils literal notranslate">True</code> and <code class="docutils literal notranslate">False</code>, etc.) </section> <section id="optional-platform-specific-file-handling"> <h3><a class="toc-backref" href="#optional-platform-specific-file-handling" role="doc-backlink">Optional Platform-Specific File Handling</a></h3> Some operating environments provide special high-performance file-transmission facilities, such as the Unix <code class="docutils literal notranslate">sendfile()</code> call. Servers and gateways may expose this functionality via an optional <code class="docutils literal notranslate">wsgi.file_wrapper</code> key in the <code class="docutils literal notranslate">environ</code>. An application may use this “file wrapper” to convert a file or file-like object into an iterable that it then returns, e.g.: <div class="highlight-default notranslate"><div class="highlight"><pre>if 'wsgi.file_wrapper' in environ: return environ['wsgi.file_wrapper'](filelike, block_size) else: return iter(lambda: filelike.read(block_size), '') </pre></div> </div> If the server or gateway supplies <code class="docutils literal notranslate">wsgi.file_wrapper</code>, it must be a callable that accepts one required positional parameter, and one optional positional parameter. The first parameter is the file-like object to be sent, and the second parameter is an optional block size “suggestion” (which the server/gateway need not use). The callable must return an iterable object, and must not perform any data transmission until and unless the server/gateway actually receives the iterable as a return value from the application. (To do otherwise would prevent middleware from being able to interpret or override the response data.) To be considered “file-like”, the object supplied by the application must have a <code class="docutils literal notranslate">read()</code> method that takes an optional size argument. It may have a <code class="docutils literal notranslate">close()</code> method, and if so, the iterable returned by <code class="docutils literal notranslate">wsgi.file_wrapper</code> must have a <code class="docutils literal notranslate">close()</code> method that invokes the original file-like object’s <code class="docutils literal notranslate">close()</code> method. If the “file-like” object has any other methods or attributes with names matching those of Python built-in file objects (e.g. <code class="docutils literal notranslate">fileno()</code>), the <code class="docutils literal notranslate">wsgi.file_wrapper</code> may assume that these methods or attributes have the same semantics as those of a built-in file object. The actual implementation of any platform-specific file handling must occur after the application returns, and the server or gateway checks to see if a wrapper object was returned. (Again, because of the presence of middleware, error handlers, and the like, it is not guaranteed that any wrapper created will actually be used.) Apart from the handling of <code class="docutils literal notranslate">close()</code>, the semantics of returning a file wrapper from the application should be the same as if the application had returned <code class="docutils literal notranslate">iter(filelike.read, '')</code>. In other words, transmission should begin at the current position within the “file” at the time that transmission begins, and continue until the end is reached. Of course, platform-specific file transmission APIs don’t usually accept arbitrary “file-like” objects. Therefore, a <code class="docutils literal notranslate">wsgi.file_wrapper</code> has to introspect the supplied object for things such as a <code class="docutils literal notranslate">fileno()</code> (Unix-like OSes) or a <code class="docutils literal notranslate">java.nio.FileChannel</code> (under Jython) in order to determine if the file-like object is suitable for use with the platform-specific API it supports. Note that even if the object is not suitable for the platform API, the <code class="docutils literal notranslate">wsgi.file_wrapper</code> must still return an iterable that wraps <code class="docutils literal notranslate">read()</code> and <code class="docutils literal notranslate">close()</code>, so that applications using file wrappers are portable across platforms. Here’s a simple platform-agnostic file wrapper class, suitable for old (pre 2.2) and new Pythons alike: <div class="highlight-default notranslate"><div class="highlight"><pre>class FileWrapper: def __init__(self, filelike, blksize=8192): self.filelike = filelike self.blksize = blksize if hasattr(filelike, 'close'): self.close = filelike.close def __getitem__(self, key): data = self.filelike.read(self.blksize) if data: return data raise IndexError </pre></div> </div> and here is a snippet from a server/gateway that uses it to provide access to a platform-specific API: <div class="highlight-default notranslate"><div class="highlight"><pre>environ['wsgi.file_wrapper'] = FileWrapper result = application(environ, start_response) try: if isinstance(result, FileWrapper): # check if result.filelike is usable w/platform-specific # API, and if so, use that API to transmit the result. # If not, fall through to normal iterable handling # loop below. for data in result: # etc. finally: if hasattr(result, 'close'): result.close() </pre></div> </div> </section> </section> <section id="questions-and-answers"> <h2><a class="toc-backref" href="#questions-and-answers" role="doc-backlink">Questions and Answers</a></h2> <ol class="arabic"> <li>Why must <code class="docutils literal notranslate">environ</code> be a dictionary? What’s wrong with using a subclass?The rationale for requiring a dictionary is to maximize portability between servers. The alternative would be to define some subset of a dictionary’s methods as being the standard and portable interface. In practice, however, most servers will probably find a dictionary adequate to their needs, and thus framework authors will come to expect the full set of dictionary features to be available, since they will be there more often than not. But, if some server chooses not to use a dictionary, then there will be interoperability problems despite that server’s “conformance” to spec. Therefore, making a dictionary mandatory simplifies the specification and guarantees interoperability. Note that this does not prevent server or framework developers from offering specialized services as custom variables inside the <code class="docutils literal notranslate">environ</code> dictionary. This is the recommended approach for offering any such value-added services. </li> <li>Why can you call <code class="docutils literal notranslate">write()</code> and yield strings/return an iterable? Shouldn’t we pick just one way?If we supported only the iteration approach, then current frameworks that assume the availability of “push” suffer. But, if we only support pushing via <code class="docutils literal notranslate">write()</code>, then server performance suffers for transmission of e.g. large files (if a worker thread can’t begin work on a new request until all of the output has been sent). Thus, this compromise allows an application framework to support both approaches, as appropriate, but with only a little more burden to the server implementor than a push-only approach would require. </li> <li>What’s the <code class="docutils literal notranslate">close()</code> for?When writes are done during the execution of an application object, the application can ensure that resources are released using a try/finally block. But, if the application returns an iterable, any resources used will not be released until the iterable is garbage collected. The <code class="docutils literal notranslate">close()</code> idiom allows an application to release critical resources at the end of a request, and it’s forward-compatible with the support for try/finally in generators that’s proposed by <a class="pep reference internal" href="../pep-0325/" title="PEP 325 – Resource-Release Support for Generators">PEP 325</a>. </li> <li>Why is this interface so low-level? I want feature X! (e.g. cookies, sessions, persistence, …)This isn’t Yet Another Python Web Framework. It’s just a way for frameworks to talk to web servers, and vice versa. If you want these features, you need to pick a web framework that provides the features you want. And if that framework lets you create a WSGI application, you should be able to run it in most WSGI-supporting servers. Also, some WSGI servers may offer additional services via objects provided in their <code class="docutils literal notranslate">environ</code> dictionary; see the applicable server documentation for details. (Of course, applications that use such extensions will not be portable to other WSGI-based servers.) </li> <li>Why use CGI variables instead of good old HTTP headers? And why mix them in with WSGI-defined variables?Many existing web frameworks are built heavily upon the CGI spec, and existing web servers know how to generate CGI variables. In contrast, alternative ways of representing inbound HTTP information are fragmented and lack market share. Thus, using the CGI “standard” seems like a good way to leverage existing implementations. As for mixing them with WSGI variables, separating them would just require two dictionary arguments to be passed around, while providing no real benefits. </li> <li>What about the status string? Can’t we just use the number, passing in <code class="docutils literal notranslate">200</code> instead of <code class="docutils literal notranslate">"200 OK"</code>?Doing this would complicate the server or gateway, by requiring them to have a table of numeric statuses and corresponding messages. By contrast, it is easy for an application or framework author to type the extra text to go with the specific response code they are using, and existing frameworks often already have a table containing the needed messages. So, on balance it seems better to make the application/framework responsible, rather than the server or gateway. </li> <li>Why is <code class="docutils literal notranslate">wsgi.run_once</code> not guaranteed to run the app only once?Because it’s merely a suggestion to the application that it should “rig for infrequent running”. This is intended for application frameworks that have multiple modes of operation for caching, sessions, and so forth. In a “multiple run” mode, such frameworks may preload caches, and may not write e.g. logs or session data to disk after each request. In “single run” mode, such frameworks avoid preloading and flush all necessary writes after each request. However, in order to test an application or framework to verify correct operation in the latter mode, it may be necessary (or at least expedient) to invoke it more than once. Therefore, an application should not assume that it will definitely not be run again, just because it is called with <code class="docutils literal notranslate">wsgi.run_once</code> set to <code class="docutils literal notranslate">True</code>. </li> <li>Feature X (dictionaries, callables, etc.) are ugly for use in application code; why don’t we use objects instead?All of these implementation choices of WSGI are specifically intended to decouple features from one another; recombining these features into encapsulated objects makes it somewhat harder to write servers or gateways, and an order of magnitude harder to write middleware that replaces or modifies only small portions of the overall functionality. In essence, middleware wants to have a “Chain of Responsibility” pattern, whereby it can act as a “handler” for some functions, while allowing others to remain unchanged. This is difficult to do with ordinary Python objects, if the interface is to remain extensible. For example, one must use <code class="docutils literal notranslate">__getattr__</code> or <code class="docutils literal notranslate">__getattribute__</code> overrides, to ensure that extensions (such as attributes defined by future WSGI versions) are passed through. This type of code is notoriously difficult to get 100% correct, and few people will want to write it themselves. They will therefore copy other people’s implementations, but fail to update them when the person they copied from corrects yet another corner case. Further, this necessary boilerplate would be pure excise, a developer tax paid by middleware developers to support a slightly prettier API for application framework developers. But, application framework developers will typically only be updating one framework to support WSGI, and in a very limited part of their framework as a whole. It will likely be their first (and maybe their only) WSGI implementation, and thus they will likely implement with this specification ready to hand. Thus, the effort of making the API “prettier” with object attributes and suchlike would likely be wasted for this audience. We encourage those who want a prettier (or otherwise improved) WSGI interface for use in direct web application programming (as opposed to web framework development) to develop APIs or frameworks that wrap WSGI for convenient use by application developers. In this way, WSGI can remain conveniently low-level for server and middleware authors, while not being “ugly” for application developers. </li> </ol> </section> <section id="proposed-under-discussion"> <h2><a class="toc-backref" href="#proposed-under-discussion" role="doc-backlink">Proposed/Under Discussion</a></h2> These items are currently being discussed on the Web-SIG and elsewhere, or are on the PEP author’s “to-do” list: <ul class="simple"> <li>Should <code class="docutils literal notranslate">wsgi.input</code> be an iterator instead of a file? This would help for asynchronous applications and chunked-encoding input streams.</li> <li>Optional extensions are being discussed for pausing iteration of an application’s output until input is available or until a callback occurs.</li> <li>Add a section about synchronous vs. asynchronous apps and servers, the relevant threading models, and issues/design goals in these areas.</li> </ul> </section> <section id="acknowledgements"> <h2><a class="toc-backref" href="#acknowledgements" role="doc-backlink">Acknowledgements</a></h2> Thanks go to the many folks on the Web-SIG mailing list whose thoughtful feedback made this revised draft possible. Especially: <ul class="simple"> <li>Gregory “Grisha” Trubetskoy, author of <code class="docutils literal notranslate">mod_python</code>, who beat up on the first draft as not offering any advantages over “plain old CGI”, thus encouraging me to look for a better approach.</li> <li>Ian Bicking, who helped nag me into properly specifying the multithreading and multiprocess options, as well as badgering me to provide a mechanism for servers to supply custom extension data to an application.</li> <li>Tony Lownds, who came up with the concept of a <code class="docutils literal notranslate">start_response</code> function that took the status and headers, returning a <code class="docutils literal notranslate">write</code> function. His input also guided the design of the exception handling facilities, especially in the area of allowing for middleware that overrides application error messages.</li> <li>Alan Kennedy, whose courageous attempts to implement WSGI-on-Jython (well before the spec was finalized) helped to shape the “supporting older versions of Python” section, as well as the optional <code class="docutils literal notranslate">wsgi.file_wrapper</code> facility.</li> <li>Mark Nottingham, who reviewed the spec extensively for issues with HTTP RFC compliance, especially with regard to HTTP/1.1 features that I didn’t even know existed until he pointed them out.</li> </ul> </section> <section id="references"> <h2><a class="toc-backref" href="#references" role="doc-backlink">References</a></h2> <aside class="footnote-list brackets"> <aside class="footnote brackets" id="id4" role="doc-footnote"> <dt class="label" id="id4">[<a href="#id1">1</a>]</dt> <dd>The Python Wiki “Web Programming” topic (<a class="reference external" href="http://www.python.org/cgi-bin/moinmoin/WebProgramming">http://www.python.org/cgi-bin/moinmoin/WebProgramming</a>)</aside> <aside class="footnote brackets" id="id5" role="doc-footnote"> <dt class="label" id="id5">[<a href="#id2">2</a>]</dt> <dd>The Common Gateway Interface Specification, v 1.1, 3rd Draft (<a class="reference external" href="https://datatracker.ietf.org/doc/html/draft-coar-cgi-v11-03">https://datatracker.ietf.org/doc/html/draft-coar-cgi-v11-03</a>)</aside> <aside class="footnote brackets" id="id6" role="doc-footnote"> <dt class="label" id="id6">[<a href="#id3">3</a>]</dt> <dd>mod_ssl Reference, “Environment Variables” (<a class="reference external" href="http://www.modssl.org/docs/2.8/ssl_reference.html#ToC25">http://www.modssl.org/docs/2.8/ssl_reference.html#ToC25</a>)</aside> </aside> </section> <section id="copyright"> <h2><a class="toc-backref" href="#copyright" role="doc-backlink">Copyright</a></h2> This document has been placed in the public domain. </section> </section> <hr class="docutils" /> Source: <a class="reference external" href="https://github.com/python/peps/blob/main/peps/pep-0333.rst">https://github.com/python/peps/blob/main/peps/pep-0333.rst</a> Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0333.rst">2025-02-01 08:59:27 GMT</a> </article> <nav id="pep-sidebar"> <h2>Contents</h2> <ul> <li><a class="reference internal" href="#preface">Preface</a></li> <li><a class="reference internal" href="#abstract">Abstract</a></li> <li><a class="reference internal" href="#rationale-and-goals">Rationale and Goals</a></li> <li><a class="reference internal" href="#specification-overview">Specification Overview</a><ul> <li><a class="reference internal" href="#the-application-framework-side">The Application/Framework Side</a></li> <li><a class="reference internal" href="#the-server-gateway-side">The Server/Gateway Side</a></li> <li><a class="reference internal" href="#middleware-components-that-play-both-sides">Middleware: Components that Play Both Sides</a></li> </ul> </li> <li><a class="reference internal" href="#specification-details">Specification Details</a><ul> <li><a class="reference internal" href="#environ-variables"><code class="docutils literal notranslate">environ</code> Variables</a><ul> <li><a class="reference internal" href="#input-and-error-streams">Input and Error Streams</a></li> </ul> </li> <li><a class="reference internal" href="#the-start-response-callable">The <code class="docutils literal notranslate">start_response()</code> Callable</a><ul> <li><a class="reference internal" href="#handling-the-content-length-header">Handling the <code class="docutils literal notranslate">Content-Length</code> Header</a></li> </ul> </li> <li><a class="reference internal" href="#buffering-and-streaming">Buffering and Streaming</a><ul> <li><a class="reference internal" href="#middleware-handling-of-block-boundaries">Middleware Handling of Block Boundaries</a></li> <li><a class="reference internal" href="#the-write-callable">The <code class="docutils literal notranslate">write()</code> Callable</a></li> </ul> </li> <li><a class="reference internal" href="#unicode-issues">Unicode Issues</a></li> <li><a class="reference internal" href="#error-handling">Error Handling</a></li> <li><a class="reference internal" href="#http-1-1-expect-continue">HTTP 1.1 Expect/Continue</a></li> <li><a class="reference internal" href="#other-http-features">Other HTTP Features</a></li> <li><a class="reference internal" href="#thread-support">Thread Support</a></li> </ul> </li> <li><a class="reference internal" href="#implementation-application-notes">Implementation/Application Notes</a><ul> <li><a class="reference internal" href="#server-extension-apis">Server Extension APIs</a></li> <li><a class="reference internal" href="#application-configuration">Application Configuration</a></li> <li><a class="reference internal" href="#url-reconstruction">URL Reconstruction</a></li> <li><a class="reference internal" href="#supporting-older-2-2-versions-of-python">Supporting Older (<2.2) Versions of Python</a></li> <li><a class="reference internal" href="#optional-platform-specific-file-handling">Optional Platform-Specific File Handling</a></li> </ul> </li> <li><a class="reference internal" href="#questions-and-answers">Questions and Answers</a></li> <li><a class="reference internal" href="#proposed-under-discussion">Proposed/Under Discussion</a></li> <li><a class="reference internal" href="#acknowledgements">Acknowledgements</a></li> <li><a class="reference internal" href="#references">References</a></li> <li><a class="reference internal" href="#copyright">Copyright</a></li> </ul> <a id="source" href="https://github.com/python/peps/blob/main/peps/pep-0333.rst">Page Source (GitHub)</a> </nav> </section> <script src="../_static/colour_scheme.js"></script> <script src="../_static/wrap_tables.js"></script> <script 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