Python 3 unittest cannot mock method used inside functions

Question

I am trying unit test in Python.

test.py

from unittest import TestCase,main
from unittest.mock import patch

import file

def mock_return(*args):
    return -1

class Tests(TestCase):
    @patch("file.Foo.a", side_effect=mock_return)
    def test_1(self, mock_fun):
        self.assertEqual(file.Foo().a(), -1)

    @patch("os.urandom", side_effect=mock_return)
    def test_2(self, mock_fun):
        self.assertEqual(file.Foo().b(), -1)

if __name__ == "__main__":
    main()

file.py

from os import urandom

class Foo:
    def a(self):
        return 1

    def b(self):
        return urandom(1)

Why test_1 passed but test_2 failed? Is there a way to mock a method used by other class?

Terry Jan Reedy · Accepted Answer · 2018-01-04 02:15:15Z

1

You must patch the binding used by the function being tested.

from os import urandom  # in file.py

binds name urandom to the function os.urandom in the file module. Foo.b accesses the function via the file.urandom binding. So the test for Foo.b must patch file.urandom, not os.urandom.

answered Jan 4, 2018 at 2:15

Terry Jan Reedy

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2 Comments

Joshua Over a year ago

This is the answer I need. from os import urandom needs file.urandom and import os need os.urandom

Terry Jan Reedy Over a year ago

I was thinking of changing @patch("os.urandom",... to @patch("file.urandom",..., and both test pass after that. An alternative is to change from os import urandom to import os and change the call in Foo.b to os.urandom. I personally prefer the former as I prefer doing the patching in the tested code, rather than the stdlib. But the important point is that the test, including mocking, must carefully match the tested code.

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