Mocking a method inside class in python test

Question

I am new to Python and writing a unit test for the already existing code.

I have the following Python code structure:

root_project:
-- src 
 -- signUp 
 -- __init__.py 
 -- abc.py 
 -- xyz.py

abc.py contains the following code:

from . import xyz
class Abc: 
 def __init__(self):
 pass 
 
 def start_process(self):
 xyz.start_timer()

I want to mock xyz.start_timer() inside method start_process of class Abc so that start_timer() mock version can be used.

How can I achieve this?

• jonrsharpe: This would be much easier if you inverted the dependency - if xyz was a parameter to the __init__ of Abc rather than an import, you could trivially inject a test double when instantiating Abc in the tests.

Answer 1

You can simply patch the start_timer function using mock.patch().

abc.py

import xyz
class Abc: 
 def __init__(self):
 pass 
 
 def start_process(self):
 xyz.start_timer()

test_abc.py

from unittest import mock, TestCase
from abc import Abc
class Test_Abc(TestCase):
 @mock.patch("abc.start_timer")
 def test_start_process(self, mock_start_timer):
 # your test code

Notice that I have patched abc.start_timer and not xyz.start_timer.

Answer 2

You just need to patchxyz.start_timer(), I just added return to start_process to show that the patching really does what it should:

import xyz
import unittest
from unittest import mock
class Abc:
 def __init__(self):
 pass
 def start_process(self):
 return xyz.start_timer()
class Test_Abc(unittest.TestCase):
 @mock.patch('xyz.start_timer', return_value=True)
 def test_start_process(self, start_process):
 a = Abc()
 self.assertTrue(a.start_process())
if __name__ == '__main__':
 unittest.main()

Out:

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