custom Python module importer

app.py

from hook import register_import_hook


print('calling to register')
register_import_hook()
print('registered')


def later():
    import test_foo


    print('module', test_foo)
    print('name', test_foo.TEST_NAME)
    print('added thing', test_foo.added_thing)

if __name__ == '__main__':
    later()

hook.py

import imp
import sys
from importlib import import_module


class CustomImporter:

    PATH_TRIGGER = "test_"

    def __init__(self, path_entry):
        print("~~ INIT", path_entry)
        return

    def find_module(self, fullname, path=None):
        print("~~ FIND", type(fullname), path)
        fh, filename, details = imp.find_module(fullname)
        module = imp.load_module(fullname, fh, filename, details)
        module.added_thing = custom_method
        sys.modules[fullname] = module
        print('inside hook')
        module.added_thing()
        return None


def register_import_hook():
    sys.path_hooks.append(CustomImporter)
    sys.path.insert(0, CustomImporter.PATH_TRIGGER)


def custom_method():
    print('i am custom_method')

test_bar.py

TEST_NAME = "BAR"
print("TEST", TEST_NAME)

test_foo.py

TEST_NAME = "FOO"
print("TEST", TEST_NAME)