Use function for mocked class' method return value

I am new to mock and and trying to work with side_effects.

I am trying to set the return value of a method of a mocked class based on the argument said method was called with. In the below code, I am trying to set the return value of some_function when having mocked MyClass.

# application.py
from my_module.my_submodule import MyClass

def my_function(var1):
    instance = MyClass()

    instance.some_function(var1)

and my testing file

# test_application.py
import mock
import application

def test_my_function():
    with mock.patch('application.MyClass') as MockClass:
        MockClass.return_value.my_function.return_value = some_return

        application.my_function(var1)

This works such that some_function now returns some_return, but I would like to have a function in place of some_return which takes the argument var1 that the function is called with.

The problem is that I don't know how to define the mock to anticipate the calling argument of some_function.

I have experimented with what is discussed in this post changing the side effect of a mock object's method created with patch, but I can't for the life of me figure out how to format it.

I have tried something like this

# test_application.py
import mock
import application

def test_my_function():
    with mock.patch('application.MyClass') as MockClass:
        MockClass.return_value.my_function.return_value = some_return  

        # Breaking very long line, in my code it's actually one line.
        MockDataPrep.return_value.extract_preprocessed_citizen_data.\
        side_effect =\
        mock.MagicMock(side_effect=my_side_effect)

        application.my_function(var1)

where the function my_side_effect looks like this:

def my_side_effect(var1):
    return_val = some_manipulation_of_var1(var1)

    if something:
        return `abc`
    else:
        raise LookupError

but it doesn't seem that the my_side_effect is ever entered (tried with print statements inside of it). How would I format this?