How to Find Subclasses of a Python Class Using `__subclasses__`?

Finding Subclasses of a Class Using Python's subclasses Method

Finding all the subclasses of a given class is a common need in Python object-oriented programming. Python provides an elegant solution through the __subclasses__ method.

New-style classes in Python (i.e., those that inherit from the object class) have a __subclasses__ method that returns a list of their subclasses. Consider the following class hierarchy:

class Foo(object):
    pass

class Bar(Foo):
    pass

class Baz(Foo):
    pass

class Bing(Bar):
    pass

To obtain the subclasses of Foo, simply call Foo.__subclasses__():

print(Foo.__subclasses__())
# Output: [<class '__main__.Bar'>, <class '__main__.Baz'>]

Recursive Subclass Retrieval

If you want to find all subclasses, including subsubclasses, recursion is necessary:

def all_subclasses(cls):
    return set(cls.__subclasses__()).union(
        [s for c in cls.__subclasses__() for s in all_subclasses(c)])

print(all_subclasses(Foo))
# Output: {<class '__main__.Bar'>, <class '__main__.Baz'>, <class '__main__.Bing'>}

Dealing with Class Name Strings

You mentioned finding subclasses given the class name as a string. Since Python classes are first-class objects, it's recommended to use the class directly rather than a string representation. However, if you do have a class name string, you can retrieve the class using the globals() dictionary:

name = 'Foo'
cls = globals()[name]
print(cls.__subclasses__())

For classes defined in different modules, you can use importlib to retrieve the class by importing its module and accessing the corresponding attribute:

name = 'pkg.module.Foo'
import importlib
modname, _, clsname = name.rpartition('.')
mod = importlib.import_module(modname)
cls = getattr(mod, clsname)
print(cls.__subclasses__())

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