Qfyilyi

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?

You are mocking wrong method. my_function is a function of application module, not a method of MyClass and hence what you want to mock is some_function instead.

import mock

import application





def my_side_effect(*args, **kwargs):

    print("my_side_effect called")

    print(args, kwargs)





def test_my_function():

    with mock.patch("application.MyClass") as MockClass:

        MockClass.return_value.some_function.side_effect = my_side_effect



        application.my_function(arg1)

This way arg1 will be passed to my_side_effect within *args.

Also, you may want to mock only a specific method, not the entire class.

def test_my_function():

    with mock.patch("application.MyClass.some_function") as mock_some_function:

        mock_some_function.side_effect = my_side_effect



        application.my_function(arg1)

It looks you turned to side_effects because you couldn't do it with a return_value.

But you can use the wraps mock parameter. It wraps an object with a mock. You can only use existing methods in the object (although you can add more) and the returned result is the result of the execution of the real method. return_value is ignored:

from unittest.mock import Mock



class Mirror:

    def func(self,x):

        return x



mirror = Mirror()

m = Mock(wraps=mirror)

print(m.func(20))

If instead you want to test how a mock was called, with what args, how many times and so on, there are inspection methods in the mock objects too.