Documentation for wsgi_intercept

Installs a WSGI application in place of a real host for testing.


Testing a WSGI application sometimes involves starting a server at a local host and port, then pointing your test code to that address. Instead, this library lets you intercept calls to any specific host/port combination and redirect them into a WSGI application importable by your test program. Thus, you can avoid spawning multiple processes or threads to test your Web app.

Supported Libaries

wsgi_intercept works with a variety of HTTP clients in Python 2.7, 3.4 and beyond, and in pypy.

  • urllib2
  • urllib.request
  • httplib
  • http.client
  • httplib2
  • requests
  • urllib3

How Does It Work?

wsgi_intercept works by replacing httplib.HTTPConnection with a subclass, wsgi_intercept.WSGI_HTTPConnection. This class then redirects specific server/port combinations into a WSGI application by emulating a socket. If no intercept is registered for the host and port requested, those requests are passed on to the standard handler.

The easiest way to use an intercept is to import an appropriate subclass of ~wsgi_intercept.interceptor.Interceptor and use that as a context manager over web requests that use the library associated with the subclass. For example:

import httplib2
from wsgi_intercept.interceptor import Httplib2Interceptor
from mywsgiapp import app

def load_app():
    return app

http = httplib2.Http()
with Httplib2Interceptor(load_app, host='', port=80) as url:
    response, content = http.request('%s%s' % (url, '/path'))
    assert response.status == 200

The interceptor class may aslo be used directly to install intercepts. See the module documentation for more information.

Older versions required that the functions add_wsgi_intercept(host, port, app_create_fn, script_name='') and remove_wsgi_intercept(host,port) be used to specify which URLs should be redirected into what applications. These methods are still available, but the Interceptor classes are likely easier to use for most use cases.


app_create_fn is a function object returning a WSGI application; script_name becomes SCRIPT_NAME in the WSGI app’s environment, if set.


If http_proxy or https_proxy is set in the environment this can cause difficulties with some of the intercepted libraries. If requests or urllib is being used, these will raise an exception if one of those variables is set.


If wsgi_intercept.STRICT_RESPONSE_HEADERS is set to True then response headers sent by an application will be checked to make sure they are of the type str native to the version of Python, as required by pep 3333. The default is False (to preserve backwards compatibility)


pip install -U wsgi_intercept

Packages Intercepted

Unfortunately each of the HTTP client libraries use their own specific mechanism for making HTTP call-outs, so individual implementations are needed. At this time there are implementations for httplib2, urllib3 and requests in both Python 2 and 3, urllib2 and httplib in Python 2 and urllib.request and http.client in Python 3.

If you are using Python 2 and need support for a different HTTP client, require a version of wsgi_intercept<0.6. Earlier versions include support for webtest, webunit and zope.testbrowser.

The best way to figure out how to use interception is to inspect the tests. More comprehensive documentation available upon request.


Pursuant to Ian Bicking’s “best Web testing framework” post, Titus Brown put together an in-process HTTP-to-WSGI interception mechanism for his own Web testing system, twill. Because the mechanism is pretty generic – it works at the httplib level – Titus decided to try adding it into all of the other Python Web testing frameworks.

The Python 2 version of wsgi-intercept was the result. Kumar McMillan later took over maintenance.

The current version is tested with Python 2.7, 3.4, 3.5, 3.6, and pypy and was assembled by Chris Dent. Testing and documentation improvements from Sasha Hart.

Project Home

This project lives on GitHub. Please submit all bugs, patches, failing tests, et cetera using the Issue Tracker.

Additional documentation is available on Read The Docs.

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