NWSGI 2.0: Advanced Dispatching

Function Callables

Let’s say we’re dealing with a much larger Python application, coolapp, that is installed at C:\coolapp-1.1\. Unfortunately, coolapp doesn’t provide a ready-made .wsgi file for us, but it does provide a function(coolapp.dispatchers.wsgi_application) that we can use by writing a little wrapper that looks something like:

import sys
sys.path.append("C:\coolapp-1.1")
import coolapp.dispatchers.wsgi_application
application = coolapp.dispatchers.wsgi_application

We could save this as coolapp.wsgi and follow either of two methods from last time, but since we’re not doing anything fancy in the wrapper, NWSGI provides a shortcut:

<wsgi>
    <pythonPaths>
        <path path="C:\coolapp-1.1" />
    </pythonPaths>
    <scriptMappings>
        <scriptMapping scriptName="coolapp.wsgi" callable="coolapp.dispatchers.wsgi_application" />
    </scriptMappings>
</wsgi>

First, we add C:\coolapp-1.1 to Python’s path so that we can import it. Next, we tell it that any requests for coolapp.wsgi should be dispatched to the function coolapp.dispatchers.wsgi_application. This is completely equivalent to the wrapper file, but saves us from creating it. Visiting http://example.com/coolapp.wsgi/ will run coolapp.

Class Callables

Functions aren’t the only things in Python that are callable; classes are also callable (it creates an instance of the class). Let’s say we upgrade coolapp to version 2.0 (installed in C:\coolapp-2.0). The team has made some changes, and their WSGI application is now a class, WsgiApplication. This class has a __call__ method (making its instances callable as well!), so our WSGI application is an instance of the WsgiApplication class. We could use a coolapp.wsgi wrapper again:

import sys
sys.path.append("C:\coolapp-2.1")
import coolapp.dispatchers.WsgiApplication
application = coolapp.dispatchers.WsgiApplication()

Again, our little wrapper doesn’t do much, so NWSGI provides a shortcut:

<wsgi>
    <pythonPaths>
        <path path="C:\coolapp-2.0" />
    </pythonPaths>
    <scriptMappings>
        <scriptMapping scriptName="coolapp.wsgi" callable="coolapp.dispatchers.WsgiApplication()" />
    </scriptMappings>
</wsgi>

This time, NWSGI will create an instance of WsgiApplication, and then call the instance and return the result to IIS. This shortcut will only work if the class doesn’t require any arguments; if it does, it needs to be wrapped in a .wsgi file (when in doubt, you can always use a wrapper file). Again, visit http://example.com/coolapp.wsgi/ to run the application.

There’s actually one more case: Python classes that implement the iterator protocol, but I don’t think two many applications are implemented that, so I’ll skip it. PEP 333 gives an example if you’re really interested.