Code snippet on how to implement appropriate method mocking of inaccessible instance.

method_mocking_example.py

import unittest.mock

class Callee:
    def __init__(self, member):
        self.member = member

    def do_something(self, method_argument):
        pass

def do_call():
    Callee("Bar").do_something("Foo")

mocked_do_something_method = unittest.mock.Mock(wraps=Callee.do_something)

with unittest.mock.patch.object(Callee, 'do_something', new=lambda *args, **kwargs: mocked_do_something_method(*args, **kwargs)):
    do_call()

assert mocked_do_something_method.call_count == 1
method_args, method_kwargs = mocked_do_something_method.call_args
callee_self, callee_method_argument = method_args
assert callee_self.member == "Bar"
assert callee_method_argument == "Foo"