Python: find all classes which inherit from this one?

Is there any way in python to query a namespace for classes which inherit from a particular class? Given a class widget I'd like to be able to call something like inheritors(widget) to get a list of all my different kinds of widget.

You want to use Widget.__subclasses__() to get a list of all the subclasses. It only looks for direct subclasses though so if you want all of them you'll have to do a bit more work:

def inheritors(klass):
    subclasses = set()
    work = [klass]
    while work:
        parent = work.pop()
        for child in parent.__subclasses__():
            if child not in subclasses:
                subclasses.add(child)
                work.append(child)
    return subclasses

N.B. If you are using Python 2.x this only works for new-style classes.

You can track inheritance with your own metaclass

import collections

class A(object):
    class __metaclass__(type):
        __inheritors__ = defaultdict(list)

        def __new__(meta, name, bases, dct):
            klass = type.__new__(meta, name, bases, dct)
            for base in klass.mro()[1:-1]:
                meta.__inheritors__[base].append(klass)
            return klass

class B(A):
    pass

class C(B):
    pass

>>> A.__inheritors__
defaultdict(<type 'list'>, {<class '__main__.A'>: [<class '__main__.B'>, <class '__main__.C'>], <class '__main__.B'>: [<class '__main__.C'>]})

Anything that is inherited from A or it's derived classes will be tracked. You will get full inheritance map when all the modules in your application are loaded.

You have to walk through all objects in the global namespace (globals()) and check if the related object/class is a subclass of the some other class (check the Python docs for issubclass()).

# return list of tuples (objclass, name) containing all subclasses in callers' module
def FindAllSubclasses(classType):
    import sys, inspect
    subclasses = []
    callers_module = sys._getframe(1).f_globals['__name__']
    classes = inspect.getmembers(sys.modules[callers_module], inspect.isclass)
    for name, obj in classes:
        if (obj is not classType) and (classType in inspect.getmro(obj)):
            subclasses.append((obj, name))
    return subclasses