Standard Handlers

Publisher Handler

The publisher handler is a good way to avoid writing your own handlers and focus on rapid application development. It was inspired by Zope ZPublisher.


To use the handler, you need the following lines in your configuration::

<Directory /some/path>
  SetHandler mod_python
  PythonHandler mod_python.publisher

This handler allows access to functions and variables within a module via URL’s. For example, if you have the following module, called

""" Publisher example """

def say(req, what="NOTHING"):
   return "I am saying %s" % what

A URL would return 'I am saying NOTHING. A URL would return 'I am saying hello.

The Publishing Algorithm

The Publisher handler maps a URI directly to a Python variable or callable object, then, respectively, returns it’s string representation or calls it returning the string representation of the return value.


The Publisher handler locates and imports the module specified in the URI. The module location is determined from the request.filename attribute. Before importing, the file extension, if any, is discarded.

If request.filename is empty, the module name defaults to 'index'.

Once module is imported, the remaining part of the URI up to the beginning of any query data (a.k.a. PATH_INFO) is used to find an object within the module. The Publisher handler traverses the path, one element at a time from left to right, mapping the elements to Python object within the module.

If no path_info was given in the URL, the Publisher handler will use the default value of 'index'. If the last element is an object inside a module, and the one immediately preceding it is a directory (i.e. no module name is given), then the module name will also default to 'index'.

The traversal will stop and HTTP_NOT_FOUND will be returned to the client if:

  • Any of the traversed object’s names begin with an underscore ('_'). Use underscores to protect objects that should not be accessible from the web.

  • A module is encountered. Published objects cannot be modules for security reasons.

If an object in the path could not be found, HTTP_NOT_FOUND is returned to the client.

For example, given the following configuration::

DocumentRoot /some/dir

<Directory /some/dir>
  SetHandler mod_python
  PythonHandler mod_python.publisher

And the following /some/dir/ file::

def index(req):
   return "We are in index()"

def hello(req):
   return "We are in hello()"


Argument Matching and Invocation

Once the destination object is found, if it is callable and not a class, the Publisher handler will get a list of arguments that the object expects. This list is compared with names of fields from HTML form data submitted by the client via POST or GET. Values of fields whose names match the names of callable object arguments will be passed as strings. Any fields whose names do not match the names of callable argument objects will be silently dropped, unless the destination callable object has a **kwargs style argument, in which case fields with unmatched names will be passed in the **kwargs argument.

If the destination is not callable or is a class, then its string representation is returned to the client.


The publisher handler provides simple ways to control access to modules and functions.

At every traversal step, the Publisher handler checks for presence of __auth__ and __access__ attributes (in this order), as well as __auth_realm__ attribute.

If __auth__ is found and it is callable, it will be called with three arguments: the request object, a string containing the user name and a string containing the password. If the return value of __auth__ is false, then HTTP_UNAUTHORIZED is returned to the client (which will usually cause a password dialog box to appear).

If __auth__() is a dictionary, then the user name will be matched against the key and the password against the value associated with this key. If the key and password do not match, HTTP_UNAUTHORIZED is returned. Note that this requires storing passwords as clear text in source code, which is not very secure.

__auth__ can also be a constant. In this case, if it is false (i.e. None, 0, "", etc.), then HTTP_UNAUTHORIZED is returned.

If there exists an __auth_realm__ string, it will be sent to the client as Authorization Realm (this is the text that usually appears at the top of the password dialog box).

If __access__ is found and it is callable, it will be called with two arguments: the request object and a string containing the user name. If the return value of __access__ is false, then HTTP_FORBIDDEN is returned to the client.

If __access__ is a list, then the user name will be matched against the list elements. If the user name is not in the list, HTTP_FORBIDDEN is returned.

Similarly to __auth__, __access__ can be a constant.

In the example below, only user 'eggs' with password 'spam' can access the hello function::

__auth_realm__ = "Members only"

def __auth__(req, user, passwd):

   if user == "eggs" and passwd == "spam" or \
      user == "joe" and passwd == "eoj":
      return 1
      return 0

def __access__(req, user):
   if user == "eggs":
      return 1
       return 0

def hello(req):
   return "hello"

Here is the same functionality, but using an alternative technique::

__auth_realm__ = "Members only"
__auth__ = {"eggs":"spam", "joe":"eoj"}
__access__ = ["eggs"]

def hello(req):
   return "hello"

Since functions cannot be assigned attributes, to protect a function, an __auth__ or __access__ function can be defined within the function, e.g.::

def sensitive(req):

   def __auth__(req, user, password):
      if user == 'spam' and password == 'eggs':
         # let them in
         return 1
         # no access
         return 0

   # something involving sensitive information
   return 'sensitive information`

Note that this technique will also work if __auth__ or __access__ is a constant, but will not work is they are a dictionary or a list.

The __auth__ and __access__ mechanisms exist independently of the standard PythonAuthenHandler. It is possible to use, for example, the handler to authenticate, then the __access__ list to verify that the authenticated user is allowed to a particular function.


In order for mod_python to access __auth__, the module containing it must first be imported. Therefore, any module-level code will get executed during the import even if __auth__ is false. To truly protect a module from being accessed, use other authentication mechanisms, e.g. the Apache mod_auth or with a mod_python PythonAuthenHandler.

Form Data

In the process of matching arguments, the Publisher handler creates an instance of FieldStorage class. A reference to this instance is stored in an attribute member{form} of the request object.

Since a FieldStorage can only be instantiated once per request, one must not attempt to instantiate FieldStorage when using the Publisher handler and should use request.form instead.

WSGI Handler

WSGI handler can run WSGI applications as described in PEP 333.

Assuming there exists the following minimal WSGI app residing in a file named mysite/ in directory /path/to/mysite (so that the full path to is /path/to/mysite/mysite/

def application(environ, start_response):
   status = '200 OK'
   output = 'Hello World!'

   response_headers = [('Content-type', 'text/plain'),
                       ('Content-Length', str(len(output)))]
   start_response(status, response_headers)

   return [output]

It can be executed using the WSGI handler by adding the following to the Apache configuration:

PythonHandler mod_python.wsgi
PythonOption mod_python.wsgi.application mysite.wsgi
PythonPath "sys.path+['/path/to/mysite']"

The above configuration will import a module named mysite.wsgi and will look for an application callable in the module.

An alternative name for the callable can be specified by appending it to the module name separated by '::', e.g.:

PythonOption mod_python.wsgi.application mysite.wsgi::my_application

If you would like your application to appear under a base URI, it can be specified by wrapping your configuration in a <Location> block. It can also be specified via the mod_python.wsgi.base_uri option, but the <Location> method is recommended, also because it has a side-benefit of informing mod_python to skip the map-to-storage processing phase and thereby improving performance.

For example, if you would like the above application to appear under '/wsgiapps', you could specify:

<Location /wsgiapps>
   PythonHandler mod_python.wsgi
   PythonOption mod_python.wsgi.application mysite.wsgi
   PythonPath "sys.path+['/path/to/mysite']"

With the above configuration, content formerly under becomes available under

If both <Location> and mod_python.wsgi.base_uri exist, then mod_python.wsgi.base_uri takes precedence. mod_python.wsgi.base_uri cannot be '/' or end with a '/'. “Root” (or no base_uri) is a blank string, which is the default. (Note that it is allowed for <Location> path to be "/" or have a trailing slash, it will automatically be removed by mod_python before computing PATH_INFO).


PEP 333 describes SCRIPT_NAME and PATH_INFO environment variables which are core to the specification. Most WSGI-supporting frameworks currently in existence use the value of PATH_INFO as the request URI.

The two variable’s name and function originate in CGI (RFC 3875), which describes an environment wherein a script (or any executable’s) output could be passed on by the web server as content. A typical CGI script resides somewhere on the filesystem to which the request URI maps. As part of serving the request the server traverses the URI mapping each element to an element of the filesystem path to locate the script. Once the script is found, the portion of the URI used thus far is assigned to the SCRIPT_NAME variable, while the remainder of the URI gets assigned to PATH_INFO.

Because the relationship between Python modules and files on disk is largely tangential, it is not very clear what exactly PATH_INFO and SCRIPT_NAME ought to be. Even though Python modules are most often files on disk located somewhere in the Python path, they don’t have to be (they could be code objects constructed on-the-fly), and their location in the filesystem has no relationship to the URL structure at all.

The mismatch between CGI and WSGI results in an ambiguity which requires that the split between the two variables be explicitely specified, which is why mod_python.wsgi.base_uri exists. In essence mod_python.wsgi.base_uri (or the path in surrounding <Location>) is the SCRIPT_NAME portion of the URI and defaults to ''.

An important detail is that SCRIPT_NAME + PATH_INFO should result in the original URI (encoding issues aside). Since SCRIPT_NAME (in its original CGI definition) referrs to an actual file, its name never ends with a slash. The slash, if any, always ends up in PATH_INFO. E.g. /path/to/myscrip/foo/bar splits into /path/to/myscript and /foo/bar. If the whole site is served by an app or a script, then SCRIPT_NAME is a blank string '', not a '/'.

PSP Handler

PSP handler is a handler that processes documents using the PSP class in mod_python.psp module.

To use it, simply add this to your httpd configuration:

AddHandler mod_python .psp
PythonHandler mod_python.psp

For more details on the PSP syntax, see Section psp – Python Server Pager.

If PythonDebug server configuration is On, then by appending an underscore ('_') to the end of the url you can get a nice side-by-side listing of original PSP code and resulting Python code generated by the psp} module. This is very useful for debugging. You’ll need to adjust your httpd configuration::

AddHandler mod_python .psp .psp_
PythonHandler mod_python.psp
PythonDebug On


Leaving debug on in a production environment will allow remote users to display source code of your PSP pages!

CGI Handler

CGI handler is a handler that emulates the CGI environment under mod_python.

Note that this is not a 'true' CGI environment in that it is emulated at the Python level. stdin and stdout are provided by substituting sys.stdin and sys.stdout, and the environment is replaced by a dictionary. The implication is that any outside programs called from within this environment via os.system, etc. will not see the environment available to the Python program, nor will they be able to read/write from standard input/output with the results expected in a 'true' CGI environment.

The handler is provided as a stepping stone for the migration of legacy code away from CGI. It is not recommended that you settle on using this handler as the preferred way to use mod_python for the long term. This is because the CGI environment was not intended for execution within threads (e.g. requires changing of current directory with is inherently not thread-safe, so to overcome this cgihandler maintains a thread lock which forces it to process one request at a time in a multi-threaded server) and therefore can only be implemented in a way that defeats many of the advantages of using mod_python in the first place.

To use it, simply add this to your .htaccess file::

SetHandler mod_python
PythonHandler mod_python.cgihandler

As of version 2.7, the cgihandler will properly reload even indirectly imported module. This is done by saving a list of loaded modules (sys.modules) prior to executing a CGI script, and then comparing it with a list of imported modules after the CGI script is done. Modules (except for whose whose __file__ attribute points to the standard Python library location) will be deleted from sys.modules thereby forcing Python to load them again next time the CGI script imports them.

If you do not want the above behavior, edit the file and comment out the code delimited by ###.

Tests show the cgihandler leaking some memory when processing a lot of file uploads. It is still not clear what causes this. The way to work around this is to set the Apache MaxRequestsPerChild to a non-zero value.