[Python] How can I apply side effects to a function of an instance that is created in a...

I have a custom class in the file MyClass.py

class MyClass:
def__init__(self):
self.fetched_data = False

def get_data(self):
self.fetched_data = True
return [1, 2, 3]


I import and use that class in the file function_to_test.py

from MyClass import MyClass

def do_data_calculations():
my_class = MyClass()
data = my_class.get_data()
print(data)
print(f"Data fetched: {my_class.fetched_data}")
return True


This is the code I got and I cannot change it. I am writing unit tests for it using the unittest library, and since I cannot execute the get_data functions properly I have to mock them. To do this I have this code

from function_to_test import do_data_calculations
import unittest
from unittest.mock import patch

def get_data_side_effect(self):
self.fetched_data = True
return [10, 9, 8]

@patch('function_to_test.MyClass.get_data')
class TestMyClass(unittest.TestCase):
def test_greet(self, mock_get_data):
mock_get_data.side_effect = get_data_side_effect
do_data_calculations()
self.assertTrue(mock_get_data.called)

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


running the unittest I get the error

Traceback (most recent call last):
File "C:\Users\user4219\AppData\Local\miniconda3\envs\azure\lib\unittest\mock.py", line 1379, in patched
return func(*newargs, **newkeywargs)
File "c:\Users\user4219\source\projects\mockingtest\test_function.py", line 14, in test_greet
do_data_calculations()
File "c:\Users\user4219\source\projects\mockingtest\function_to_test.py", line 5, in do_data_calculations
data = my_class.get_data()
File "C:\Users\user4219\AppData\Local\miniconda3\envs\azure\lib\unittest\mock.py", line 1114, in call
return self._mock_call(*args, **kwargs)
File "C:\Users\user4219\AppData\Local\miniconda3\envs\azure\lib\unittest\mock.py", line 1118, in _mock_call
return self._execute_mock_call(*args, **kwargs)
File "C:\Users\user4219\AppData\Local\miniconda3\envs\azure\lib\unittest\mock.py", line 1179, in _execute_mock_call
result = effect(*args, **kwargs)
TypeError: get_data_side_effect() missing 1 required positional argument: 'self'


When I read the documentation it seems to be the case that the side effect gets the parameters with which the function is called. But when I try it it does not seem to be the case. If I ommit the 'self' parameter (and the self.fetched_data = True line), It works fine and it prints the [10, 9, 8] data. When I use the @patch line at the function level, using the new=callable parameter, I get it to work, but since I have to write loads of tests patching this same function, I want to keep it at the class level (and I cannot use the new=callable line since each function has to call it with different parameters).

How can I change this to be able to apply the side effects to the class instance?

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