Introspection for class discovery in Python

introspect.py

'''
Usage (from one directory up):
    $ python -m introspection.introspect

'''
# The local package's name.
import introspection

import re
import os
import sys

# Taken shamelessly from unittest/loader.py.
VALID_MODULE_NAME = re.compile(r'[_a-z]\w*\.py$', re.IGNORECASE)

class Fooer(object):
    def foo(self):
        raise NotImplementedError('I don\'t know how to foo!')

class Fooer42(Fooer):
    def foo(self):
        return 'The answer is 42'

class FortyTwoFooer(Fooer):
    def foo(self):
        return 'Forty-two is the answer'

def get_name_from_path(path, top_level_dir):
    path = os.path.splitext(os.path.normpath(path))[0]
    _relpath = os.path.relpath(path, top_level_dir)
    return _relpath.replace(os.path.sep, '.')

def get_named_module(name):
    __import__(name)
    return sys.modules[name]

def dispatch():
    files = [ 
        f for sublist in 
        [os.listdir(p) for p in introspection.__path__]
        for f in sublist
    ]
    module_names = [
        get_name_from_path(os.path.abspath(m), os.path.abspath(os.path.curdir)) 
        for m in files if VALID_MODULE_NAME.match(m)
    ]
    modules = [
        get_named_module('introspection.'+m) for m in module_names
    ]
    for module in modules:
        for name in dir(module):
            obj = getattr(module, name)
            if isinstance(obj, type) and \
                    issubclass(obj, introspection.introspect.Fooer) and \
                    obj is not introspection.introspect.Fooer:
                instance = Fooer.__new__(obj)
                print instance.foo()

if __name__ == "__main__":
    dispatch()