PEP 333 – Python Web Server Gateway Interface v1.0
- Phillip J. Eby <pje at telecommunity.com>
- Web-SIG list
- 07-Dec-2003, 08-Aug-2004, 20-Aug-2004, 27-Aug-2004, 27-Sep-2010
Table of Contents
- Rationale and Goals
- Specification Overview
- Specification Details
- Implementation/Application Notes
- Questions and Answers
- Proposed/Under Discussion
Note: For an updated version of this spec that supports Python 3.x and includes community errata, addenda, and clarifications, please see PEP 3333 instead.
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.
Rationale and Goals
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 . 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.
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
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
The Application/Framework Side
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
__call__ 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:
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"
The Server/Gateway Side
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
sys.stderr and logged by the web
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, exc_info, exc_info 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()
Middleware: Components that Play Both Sides
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:
- Routing a request to different application objects based on the
target URL, after rewriting the
- Allowing multiple applications or frameworks to run side by side in the same process
- Load balancing and remote processing, by forwarding requests and responses over a network
- Perform content postprocessing, such as applying XSL stylesheets
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
text/plain responses to pig Latin, using Joe Strout’s
piglatin.py. (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.)
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))
The application object must accept two positional arguments. For
the sake of illustration, we have named them
start_response, 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
result = application(environ, start_response) as shown above.)
environ parameter is a dictionary object, containing CGI-style
environment variables. This object must be a builtin Python
dictionary (not a subclass,
UserDict 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.
start_response parameter is a callable accepting two
required positional arguments, and one optional argument. For the sake
of illustration, we have named these arguments
exc_info, but they are not required to
have these names, and the application must invoke the
start_response callable using positional arguments (e.g.
status parameter is a status string of the form
"999 Message here", and
response_headers is a list of
(header_name, header_value) tuples describing the HTTP response
header. The optional
exc_info parameter is described below in the
sections on The start_response() Callable and Error Handling.
It is used only when the application has trapped an error and is
attempting to display an error message to the browser.
start_response callable must return a
callable that takes one positional parameter: a string to be written
as part of the HTTP response body. (Note: the
write() 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 Buffering and Streaming section for more
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 Buffering and Streaming 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 Other HTTP Features, below, for more details.)
If a call to
len(iterable) 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
method, it must return an accurate result. (See
the Handling the Content-Length Header section for information
on how this would normally be used.)
If the iterable returned by the application has a
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 PEP 325’s
generator support, and other common iterables with
(Note: the application must invoke the
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
start_response() 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
wsgi.file_wrapper (see Optional
Platform-Specific File Handling). In the general case, only
attributes specified here, or accessed via e.g. the PEP 234 iteration
APIs are acceptable.
environ dictionary is required to contain these CGI
environment variables, as defined by the Common Gateway Interface
specification . 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.
- The HTTP request method, such as
"POST". This cannot ever be an empty string, and so is always required.
- 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.
- 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.
- The portion of the request URL that follows the
"?", if any. May be empty or absent.
- The contents of any
Content-Typefields in the HTTP request. May be empty or absent.
- The contents of any
Content-Lengthfields in the HTTP request. May be empty or absent.
- When combined with
PATH_INFO, these variables can be used to complete the URL. Note, however, that
HTTP_HOST, if present, should be used in preference to
SERVER_NAMEfor reconstructing the request URL. See the URL Reconstruction section below for more detail.
SERVER_PORTcan never be empty strings, and so are always required.
- The version of the protocol the client used to send the request.
Typically this will be something like
"HTTP/1.1"and may be used by the application to determine how to treat any HTTP request headers. (This variable should probably be called
REQUEST_PROTOCOL, 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.)
- Variables corresponding to the client-supplied HTTP request headers
(i.e., variables whose names begin with
"HTTP_"). The presence or absence of these variables should correspond with the presence or absence of the appropriate HTTP header in the request.
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  as are applicable, such as
SSL_PROTOCOL. 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
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
REMOTE_USER when no
authentication has occurred) should be left out of the
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
In addition to the CGI-defined variables, the
may also contain arbitrary operating-system “environment variables”,
and must contain the following WSGI-defined variables:
||A string representing the “scheme” portion of
the URL at which the application is being
invoked. Normally, this will have the value
||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.)|
||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
For many servers,
||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.|
||This value should evaluate true if an equivalent application object may be simultaneously invoked by another process, and should evaluate false otherwise.|
||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).|
environ 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
mod_python might define variables with names like
Input and Error Streams
The input and error streams provided by the server must support the following methods:
The semantics of each method are as documented in the Python Library Reference, except for these notes as listed in the table above:
- The server is not required to read past the client’s specified
Content-Length, 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
- The optional “size” argument to
readline()is not supported, as it may be complex for server authors to implement, and is not often used in practice.
- Note that the
readlines()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.
- Since the
errorsstream may not be rewound, servers and gateways are free to forward write operations immediately, without buffering. In this case, the
flush()method may be a no-op. Portable applications, however, cannot assume that output is unbuffered or that
flush()is a no-op. They must call
flush()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.)
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
errors objects. In particular, applications
must not attempt to close these streams, even if they possess
The second parameter passed to the application object is a callable
of the form
start_response(status, response_headers, exc_info=None).
(As with all WSGI callables, the arguments must be supplied
positionally, not by keyword.) The
start_response callable is
used to begin the HTTP response, and it must return a
write(body_data) callable (see the Buffering and Streaming
status argument is an HTTP “status” string like
"404 Not Found". 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 RFC 2616, 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.
response_headers argument is a list of
header_value) tuples. It must be a Python list; i.e.
type(response_headers) is ListType, and the server may change
its contents in any way it desires. Each
header_name must be a
valid HTTP header field-name (as defined by RFC 2616, Section 4.2),
without a trailing colon or other punctuation.
header_value 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
Server: 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
start_response(). (For more specifics on “hop-by-hop” features and
headers, please see the Other HTTP Features section below.)
start_response 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
write() 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
Content-Length 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.
exc_info argument, if supplied, must be a Python
sys.exc_info() tuple. This argument should be supplied by the
application only if
start_response is being called by an error
exc_info is supplied, and no HTTP headers have been
start_response 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
exc_info is provided, and the HTTP headers have already
start_response must raise an error, and should
exc_info tuple. That is:
raise exc_info, exc_info, exc_info
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
start_response, if it called
exc_info. Instead, it should allow
such exceptions to propagate back to the server or gateway. See
Error Handling below, for more details.
The application may call
start_response more than once, if and
only if the
exc_info argument is provided. More precisely, it is
a fatal error to call
start_response without the
start_response 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
should ensure that no reference is held to the
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:
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.
The example CGI gateway provides another illustration of this technique.
If the application does not supply a
Content-Length 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
Content-Length header, or at least
avoid the need to close the client connection. If the application
does not call the
write() callable, and returns an iterable
len() is 1, then the server can automatically determine
Content-Length by taking the length of the first string yielded
by the iterable.
And, if the server and client both support HTTP/1.1
then the server may use chunked encoding to send
a chunk for each
write() call or string yielded by the iterable,
thus generating a
Content-Length 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 RFC 2616
when doing this, or else fall back to one of the other strategies for
dealing with the absence of
(Note: applications and middleware must not apply any kind of
Transfer-Encoding 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 Other HTTP Features below, for
Buffering and Streaming
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:
- Send the entire block to the operating system (and request that any O/S buffers be flushed) before returning control to the application, OR
- Use a different thread to ensure that the block continues to be transmitted while the application produces the next block.
- (Middleware only) send the entire block to its parent gateway/server
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.
Middleware Handling of Block Boundaries
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
write() callable to transmit data that is yielded by an
underlying application. Middleware may only use their parent
write() callable to transmit data that the
underlying application sent using a middleware-provided
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
returned by the
New WSGI applications and frameworks should not use the
write() callable if it is possible to avoid doing so. The
write() 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.
write() callable is returned by the
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
write() 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
An application must return an iterable object, even if it
write() 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
write() from within their return iterable, and therefore any
strings yielded by the iterable are transmitted after all strings
write() have been sent to the client.
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
start_response() as a status or
as response headers must follow RFC 2616 with respect to encoding.
That is, they must either be ISO-8859-1 characters, or use RFC 2047
On Python platforms where the
StringType 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 (
\u00FF, 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
StringType, and must not be of type
UnicodeType. And, even if a given platform allows
for more than 8 bits per character in
only the lower 8 bits may be used, for any value referred to in
this specification as a “string”.
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:
exc_info argument to
start_response. Here is an example
of its use:
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"]
If no output has been written when an exception occurs, the call to
start_response 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,
start_response 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
text/* 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
start_response, 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
write()), allowing the middleware
to capture and modify the error output. These techniques will work as
long as application authors:
- Always provide
exc_infowhen beginning an error response
- Never trap errors raised by
exc_infois being provided
HTTP 1.1 Expect/Continue
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:
- Respond to requests containing an
Expect: 100-continuerequest with an immediate “100 Continue” response, and proceed normally.
- Proceed with the request normally, but provide the application
wsgi.inputstream 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.
- 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.)
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 RFC 2616, sections 8.2.3 and 10.1.1.
Other HTTP Features
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 RFC 2616, section 1.3, for the definition of these terms.)
However, because WSGI servers and applications do not communicate via
HTTP, what RFC 2616 calls “hop-by-hop” headers do not apply to WSGI
internal communications. WSGI applications must not generate any
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
WSGI servers must handle any supported inbound “hop-by-hop” headers
on their own, such as by decoding any inbound
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
If-Modified-Since request headers and the
ETag 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.
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.
Server Extension APIs
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
mod_python might wish to expose
part of the Apache API as a WSGI extension.
In the simplest case, this requires nothing more than defining an
environ variable, such as
mod_python.some_api. 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
environ variables, might return different data if
environ 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
environ, 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
environ about the contents of HTTP headers, it must refuse service
to the application, e.g. by raising an error, returning
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
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
environ. Specifically, such middleware should provide these
features as functions which operate on
environ, rather than simply
stuffing values into
environ. This helps ensure that information
is calculated from
environ after any middleware has done any URL
rewrites or other
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
environ to ensure that their mediation isn’t
being bypassed by applications using those extensions!
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
environ 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
environ. In the simplest case, this support can consist merely of
copying all operating system-supplied environment variables from
os.environ into the
environ 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
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:
from the_app import application def new_app(environ, start_response): environ['the_app.configval1'] = 'something' return application(environ, start_response)
But, most existing applications and frameworks will probably only need
a single configuration value from
environ, 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.)
If an application wishes to reconstruct a request’s complete URL, it may do so using the following algorithm, contributed by Ian Bicking:
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']
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.
Supporting Older (<2.2) Versions of Python
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 PEP 234).
(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:
- 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
wsgi.file_wrapperor an application-specific file wrapper class. (See Optional Platform-Specific File Handling for more on
wsgi.file_wrapper, and an example class you can use to wrap a file as an iterable.)
- If you return a custom iterable, it must implement the pre-2.2
iterator protocol. That is, provide a
__getitem__method that accepts an integer key, and raises
IndexErrorwhen exhausted. (Note that built-in sequence types are also acceptable, since they also implement this protocol.)
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
Optional Platform-Specific File Handling
Some operating environments provide special high-performance
file-transmission facilities, such as the Unix
Servers and gateways may expose this functionality via an optional
wsgi.file_wrapper key in the
environ. An application
may use this “file wrapper” to convert a file or file-like object
into an iterable that it then returns, e.g.:
if 'wsgi.file_wrapper' in environ: return environ['wsgi.file_wrapper'](filelike, block_size) else: return iter(lambda: filelike.read(block_size), '')
If the server or gateway supplies
wsgi.file_wrapper, 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
read() method that takes an optional size argument.
It may have a
close() method, and if so, the iterable returned
wsgi.file_wrapper must have a
close() method that
invokes the original file-like object’s
close() method. If the
“file-like” object has any other methods or attributes with names
matching those of Python built-in file objects (e.g.
wsgi.file_wrapper 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
close(), the semantics of returning a
file wrapper from the application should be the same as if the
application had returned
iter(filelike.read, ''). 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
Of course, platform-specific file transmission APIs don’t usually
accept arbitrary “file-like” objects. Therefore, a
wsgi.file_wrapper has to introspect the supplied object for
things such as a
fileno() (Unix-like OSes) or a
java.nio.FileChannel (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,
wsgi.file_wrapper must still return an iterable that wraps
close(), 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:
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
and here is a snippet from a server/gateway that uses it to provide access to a platform-specific API:
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()
Questions and Answers
- Why must
environbe 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
environdictionary. This is the recommended approach for offering any such value-added services.
- Why can you call
write()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
write(), 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.
- What’s the
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
close()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 PEP 325.
- 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
environdictionary; see the applicable server documentation for details. (Of course, applications that use such extensions will not be portable to other WSGI-based servers.)
- 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.
- What about the status string? Can’t we just use the number,
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.
- Why is
wsgi.run_oncenot 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
- 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
__getattribute__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.
These items are currently being discussed on the Web-SIG and elsewhere, or are on the PEP author’s “to-do” list:
wsgi.inputbe an iterator instead of a file? This would help for asynchronous applications and chunked-encoding input streams.
- Optional extensions are being discussed for pausing iteration of an application’s output until input is available or until a callback occurs.
- Add a section about synchronous vs. asynchronous apps and servers, the relevant threading models, and issues/design goals in these areas.
Thanks go to the many folks on the Web-SIG mailing list whose thoughtful feedback made this revised draft possible. Especially:
- Gregory “Grisha” Trubetskoy, author of
mod_python, 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.
- 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.
- Tony Lownds, who came up with the concept of a
start_responsefunction that took the status and headers, returning a
writefunction. His input also guided the design of the exception handling facilities, especially in the area of allowing for middleware that overrides application error messages.
- 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
- 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.
This document has been placed in the public domain.
Last modified: 2023-09-09 17:39:29 GMT