Patch bounded method of pre-existing instance

patch_bounded_method.py

from unittest.mock import patch
from contextlib import contextmanager


class MyClass:
    def print_args(self, *args):
        print(f'original: {self=}, {args=}')


@contextmanager
def patch_bounded_method(obj, method_name, new_method):
    def wrapper(*args, **kwargs):
        return new_method(obj, *args, **kwargs)
    with patch.object(obj, method_name, wrapper) as p:
        yield p


def main(obj):
    obj.print_args(1)

    def mock_method(self, *args):
        print(f'mock: {self=}, {args=}')

    with patch_bounded_method(obj, 'print_args', mock_method):
        obj.print_args(1)


if __name__ == '__main__':
    pre_existing_instance = MyClass()
    main(pre_existing_instance)
    
"""
original: self=<__main__.MyClass object at 0x00000243F389D630>, args=(1,)
mock: self=<__main__.MyClass object at 0x00000243F389D630>, args=(1,)
"""