cls.py

class A:
    def foo(self):
        print("foo")

ext.py

from cls import A
from trait import extend


@extend(A)
class B:
    def bar(self):
        print("extension method bar")


def bar(b):
    b.bar()  # ok

main.py

from cls import A
from ext import bar

a = A()
bar(a)  # ok
a.bar()  # error

trait.py

import sys
from inspect import stack


def extend(cls1):
    if not hasattr(cls1, "__extensions__"):
        cls1.__extensions__ = {}
        orig_getattribute = cls1.__getattribute__

        def ext_getattr(self, name):
            exts = type(self).__extensions__
            module = stack(0)[1].filename
            ext = exts.get(module)
            if ext:
                try:
                    return getattr(ext, name)
                except AttributeError:
                    pass
            return orig_getattribute(self, name)

        cls1.__getattribute__ = ext_getattr

    def patch(cls2):
        module = sys.modules[cls2.__module__].__file__
        cls1.__extensions__[module] = cls2()

    return patch