How to patch global class in python correctly

Question

My project structure:

├── core   
|    └── service
|         └── file.py
└── tests 
     └── test_file.py

I have a function fun that uses a class method.

file.py

from another_module import myClass
cls_instance=myClass()

def fun(): 
    return cls_instance.request()

And I want to test the function fun by patch as showed below:

test_file.py

from core.service.file import fun
class TestCase(unittest.TestCase): 
    @patch("core.service.file.cls_instance")
    def setUp(self, mock): 
        self.mock = mock.return_value

    def test_fun(self):
        self.mock.request.return_value = 1
        resp=fun()
        self.assertEqual(resp, 1)

Test Failed with the message:

Expected : None
Actual   : 1

What am I doing wrong here?

User051209 · Accepted Answer · 2025-03-27 15:58:03Z

0

You have to use the @patch( ) in the test_fun() method and not the setUp() method. So your test file becomes:

import unittest
from unittest.mock import patch
from core.service.file import fun

class MyTestCase(unittest.TestCase):
    # ---REMOVE THE setUp() METHOD ---
    #@patch("core.service.file.cls_instance")
    #def setUp(self, mock): 
    #    self.mock = mock.return_value

    @patch("core.service.file.cls_instance")
    def test_fun(self, mock):
        mock.request.return_value = 1
        resp = fun()
        self.assertEqual(resp, 1)

if __name__ == '__main__':
    unittest.main()

As you can see in my test code I have removed the method setUp().

In this way your test passes:

.
----------------------------------------------------------------------
Ran 1 test in 0.000s

OK

edited Mar 27 at 15:58

answered Mar 26 at 17:25

User051209

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5 Comments

user2357112 Mar 27 at 14:58

Why do you say that the first argument to assertEqual is the expected value? They're documented as just first and second.

User051209 Mar 27 at 15:46

When I use PyCharm Community to execute a test and it fails the error message calls Expected the first argument and Actual the second argument. It is the same with JUnit (Java Framework); I didn't read this part the documentation; Thank you I'm going to read it now.

User051209 Mar 27 at 15:57

@user2357112 You are right in the documentation are indicated as first and second arguments I'm going to edit my answer and remove the paragraph.

rhar Apr 7 at 14:22

@User051209 Can you please tell why patching in setup won't work? Isn't setUp run before each test?

User051209 Apr 7 at 14:48

In general the patch decorator is used for the test method and not for the setUp method. Now I can't retest your code. Is important for you put patch before setUp()? I have never done this on my test! If my code works could be an acceptable answer for you.

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