Python Patch/Mock class method but still call original method

Question

I want to use patch to record all function calls made to a function in a class for a unittest, but need the original function to still run as expected. I created a dummy code example below:

from mock import patch

class A(object):
    def __init__(self):
        self._a = 1

class B(A):
    def __init__(self):
        super(B, self).__init__() # TypeError: super() argument 1 must be type, not MagicMock
        self._b = 11

    def bar(self, b):
        self._b = self._b + 1 + b

    def foo(self, b):
        self.bar(b)

class MockB(B):
    def foo(self, b):
        super(MockB, self).foo(self, b)

@patch('main.B')
def main(b_class):
    b_class.side_effect = MockB

    b = B()

    print b._b # 11
    b.foo(0)
    print b._b # 12

main()

In my case, the instance of the class b = B() is not actually in the main function but in another module, so I can't Mock the instance. I need it to generically be a decorator for all instances of B.

Summary: I am not sure how to individually mock the class method on it's own, but still call the original method. After, I want to use something like call_args_list where I can see all calls made to foo().

What are you trying to achieve here, what's actually under test? — jonrsharpe, Apr 29 '19 at 07:36
@jonrsharpe Thanks it wasn't relevant. I have removed the tag. — mojo1mojo2, Apr 29 '19 at 07:39
What's not relevant? If you're not testing, why are you mocking anything? — jonrsharpe, Apr 29 '19 at 07:59
The relevant part is that I need to mock a class method and keep the original method working. I want to mock because it comes with call_args_list and other useful functionality. — mojo1mojo2, Apr 29 '19 at 23:43

progmatico · Accepted Answer · 2021-06-12T19:00:32.453

25

I think you are looking for the wraps Mock parameter. Search the official documentation for wraps. Accessing attributes returns a mock object. Calling methods gives the real method result instead, if a return value is not configured for the mock.

edited Jun 12 '21 at 19:00

answered May 02 '19 at 22:03

progmatico

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10

A code snippet with an example wouldn't hurt. https://stackoverflow.com/a/36724642/8843585 – Ramon Dias Aug 17 '22 at 23:14
2

This works exactly how I wanted it to, e.g., `mock.patch('os.remove', wraps=os.remove)`. This way, I have mock with which I can call `assert_called_once_with()`, etc., and `os.remove()` still gets called as is. – Chris Wong Oct 29 '22 at 02:57

score 3 · Answer 2 · answered May 23 '20 at 10:37

More details can be found here: https://stackoverflow.com/a/61963740/1731460

import mock
from contextlib import contextmanager


@contextmanager
def mock_patch_method_original(mock_path, original_method, results):
    results = iter(results)

    def side_effect(self, *args, **kwargs):
        value = next(results)
        if value == '__original__':
            side_effect.self = self
            return original_method(self, *args, **kwargs)
        else:
            return value

    patcher = mock.patch(mock_path, autospec=True, side_effect=side_effect)
    yield patcher.start()
    patcher.stop()

score 2 · Answer 3 · edited Jul 23 '21 at 02:06

Just keep a reference to your original, non-mocked/patched instance around.

I had an issue that I was patching ctypes.windll.winmm.mciSendStringW, but I still wanted to be able to access that original function. Initially, I had tried:

from ctypes import windll
originalWindll = windll

# Later on trying to call the original mciSendStringW when it's patched...
originalWindll.winmm.mciSendStringW(...)  # Didn't work! Called the patched version of the function!

The correct thing to do was actually this... since it was the function being mocked/patched, it was the function that I needed to keep a reference to.

from ctypes import windll
originalMCISendStringW = windll.winmm.mciSendStringW

# Later on trying to call the original mciSendStringW when it's patched...
originalMCISendStringW(...)  # Works! Calls the original, non-patched version of this function!

Python Patch/Mock class method but still call original method

3 Answers3