Python mock patch a function called by another function

Question

def f1():
    return 10, True

def f2():
    num, stat = f1()
    return 2*num, stat

How do I use python's mock library to patch f1() and return a custom result so I could test f2()?

Edited: Is there something wrong with my test? This doesn't seem to be working, all the tests failed with AssertionError

from foo.bar import f2
from mock import patch

class MyTest(TestCase):

    def test_f2_1(self):
        with patch('project.module.f1') as some_func:
            some_func.return_value = (20, False)
            num, stat = f2()
            self.assertEqual((num, stat), (40, False))

   @patch('project.module.f1')
   def test_f2_2(self, some_func):
       some_func.return_value = (20, False)
       num, stat = f2()
       self.assertEqual((num, stat), (40, False))

blong · Accepted Answer · 2023-05-09 14:58:16Z

64

First example suggests that f1() and f2() defined in the same module. Hence the following should work:

from foo.bar import f2
from unittest.mock import patch

class MyTest(TestCase):

    @patch('foo.bar.f1')
    def test_f2_2(self, some_func):
        some_func.return_value = (20, False)
        num, stat = f2()
        self.assertEqual((num, stat), (40, False))

Patch is on the same as import: @patch('foo.bar.f1')

Here is a good answer on the issue:

http://bhfsteve.blogspot.nl/2012/06/patching-tip-using-mocks-in-python-unit.html

edited May 9, 2023 at 14:58

blong

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answered Mar 15, 2015 at 9:43

Vasiliy

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3 Comments

HeyWatchThis Over a year ago

Thanks for recommending the bhfsteve.blogspot.nl link about patching and import styles. That solved one of my problems.

Henry Over a year ago

How would this be different if f1 and f2 were in different modules?

kta Over a year ago

@HenryWoody: I believe no difference because it won't change the line @patch('foo.bar.f1') even if f1 is defined from another module because foo.bar has to imported f1 to use and we patch the imported one, not where it was defined.

Mariusz Jamro · Accepted Answer · 2012-04-23 09:10:03Z

32

Assuming that you're using this mock libary:

def f1():
    return 10, True

def f2():
    num, stat = f1()
    return 2*num, stat

import mock

print f2()   # Unchanged f1 -> prints (20, True)

with mock.patch('__main__.f1') as MockClass:       # replace f1 with MockClass 
    MockClass.return_value = (30, True)     # Change the return value

    print f2()     # f2 with changed f1 -> prints (60, True)

If your code is divided into modules you would probably need to replace __main__.f1 with the path to your module/function.

answered Apr 23, 2012 at 9:10

Mariusz Jamro

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4 Comments

Nam Ngo Over a year ago

Would it make any difference if f1() accepts args? E.g: f1(arg)

Mariusz Jamro Over a year ago

The mocking function can be called with any number of arguments, it'll always return the return_value.

Nam Ngo Over a year ago

Thanks a lot @Secator. I have updated my question with a test case and it doesn't seem to be passing. What am I doing wrong?

Nam Ngo Over a year ago

The test failed with AssertionError

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