Btukfyl

I'm creating a Python wrapper for the Detours library. One piece of the tool is a dispatcher to send all of the hooked API calls to various handlers.

Right now my code looks like this:

if event == 'CreateWindowExW':

    # do something

elif event == 'CreateProcessW':

    # do something

elif ...

This feels ugly. Is there a pattern to create an event dispatcher without my having to create an elif branch for each Windows API function?

One nice way to do this is to define a class which has methods equating to the relevant API function names, plus a dispatch method which dispatches to the correct method. For example:

class ApiDispatcher(object):



    def handle_CreateWindowExW(self):

        # do whatever



    def handle_CreateProcessW(self):

        # do this one



    def dispatch(self, event):

        method = getattr(self, 'handle_%s' % event)

        method()

Those if's will eventually have to go somewhere. Why not do it like this:

handler = get_handler(event)

handler.process()

and in the get_handler you'd have your ifs, each returning an object which does its work in the process method.

An alternative would be a map to callables, like this:

def react_to_create_window_exw():

   # do something with event here

   pass



handlers = {

  "CreateWindowExW" : react_to_create_window_exw

}

and you would use it like this:

handler = handlers[event]

handler()

This way you would not use any if/else conditions.

You can use the dispatch dict method.

def handle_CreateWindowExW():

    print "CreateWindowExW"     

    #do something



events = {

    "CreateWindowExW": handle_CreateWindowExW

}



events[event]()

This way, you can just add events without having to add different if statements.

Usually in such cases when you have a predefined list of actions to take, use a map e.g.

def CreateWindowExW():

    print 'CreateWindowExW'



def CreateProcessW():

    print 'CreateProcessW'



action_map = {

    'CreateWindowExW': CreateWindowExW,

    'CreateProcessW': CreateProcessW

}



for action in ['CreateWindowExW', 'UnkownAction']:

    try:

        action_map[action]()

    except KeyError:

        print action, "Not Found"

Output:

CreateWindowExW

UnkownAction Not Found

so using a map you can create a very powerful dispatcher

I didn't find anything that was as graceful as it could be in this area, so I wrote something that let's you do:

from switcheroo import Switch, default



switch = Switch({

    'foo': lambda x: x+1,

    default: lambda x: x-1,

})



>>> switch['foo'](1)

2

>>> switch['bar'](1)

0

There are some other flavours; docs are here, code is on github, package is on pypi.