What is the reason that mock.patch ignores a fully imported function?

Question

Today I realized that it matters for unittest.mock.patch how I import a function. Depending on the used way a mock.patch call worked or was ignored. In Python we typically import a function with:

an import statement like import os or
a from ... import ... statement like from os import system

A mock.patch works like a charm if I use import os, but it was ignored if I patch a from os import system.

Example 1: Using import

import os
from unittest import mock


def echo():
    os.system('echo "Hello"')


with mock.patch('os.system') as mocked:
    print(mocked)
    mocked.side_effect = Exception('Patch works!')
    echo()

Output of example 1

<MagicMock name='system' id='140037358656760'>

Traceback (most recent call last):
  File "/.../config/scratches/scratch_7.py", line 12, in <module>
    echo()
  File "/.../config/scratches/scratch_7.py", line 6, in echo
    os.system('echo "Hello"')
  File "/.../python3.5/unittest/mock.py", line 917, in __call__
    return _mock_self._mock_call(*args, **kwargs)
  File "/.../python3.5/unittest/mock.py", line 973, in _mock_call
    raise effect
Exception: Patch works!

Example 2: Using a full function import and from-import

When I fully import os.system the mock.patch ignores the mocked.side_effect.

from os import system
from unittest import mock


def echo():
    system('echo "Hello"')


with mock.patch('os.system') as mocked:
    print(mocked)
    mocked.side_effect = Exception('Patching does not work!')
    echo()

    print('Patch was ignored!')

Output of example 2

<MagicMock name='system' id='139851175427376'>
Hello
Patch was ignored!

In both cases I don't receive an error and mock could find os.system as a valid path. However, in the second case the function is not properly patched.

Why mock.patch does not patch the function in the second example?
Are there any implementation specific reasons why the second patch did not work?

Generally you would use [`patch.object`](https://docs.python.org/3/library/unittest.mock.html#unittest.mock.patch.object) when the object is already imported in the current namespace. It likely won't work on `system` though, since that will be built-in/read only. — wim, Jul 31 '18 at 14:25

score 7 · Accepted Answer · answered Jul 31 '18 at 14:18

When you do from os import system, you get a variable named system pointing to os.system function. Later, you assign, via patching, a different function to os.system, but system keeps to point to the old function. This is the same reason why the following works:

tmp = a
a = b
b = tmp

It doesn't happen in the first example, because you reference os.system before it is mocked. To fix your second example, I'd go with the following:

from os import system
from unittest import mock

def echo():
    system('echo "Hello"')

with mock.patch('__main__.system') as mocked:
    print(mocked)
    mocked.side_effect = Exception('Patching does not work!')
    echo()

    print('Patch was ignored!')

This way you make sure you patch the right reference. This is a rather common pattern. If echo function were in a file named echo.py, the patch call would look like with mock.patch('echo.system').

What is the reason that mock.patch ignores a fully imported function?

1 Answers1