Mocking 3rd party function in another module with python, pytest, mock

Question

I have a function func1() that is in production and cannot be modified. It calls a function ,function_to_be_mocked(), in another module. This takes input parameters.

I have another function func2() which calls func1().

I am writing unit tests to test func2(), and trying to mock function_to_be_mocked (as it depends on some keys I don't have (and should't have) on my local system). The only thing I can modify is test_func2().

I have a set up like the following (minimum example):

from othermodule import function_to_be_mocked
import pytest
import mock

def func1():
    function_to_be_mocked(None)

def func2():
    ret = func1()
    print (ret)

@mock.patch('othermodule.function_to_be_mocked', return_value = 3)
def test_func2(mocker):
    func2()

And othermodule.py is:

def function_to_be_mocked(arg1):
    if not arg1 == 'foo':
        raise ValueError

My output:

Calling func2 directly:

Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/Users/blah/temp.py", line 9, in func2
    ret = func1()
  File "/Users/blah/temp.py", line 6, in func1
    function_to_be_mocked(None)
  File "/Users/blah/othermodule.py", line 3, in function_to_be_mocked
    raise ValueError
ValueError

Calling test_func2() which I would expect to be mocked:

Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/Users/blah/venv/lib/python2.7/site-packages/mock/mock.py", line 1305, in patched
    return func(*args, **keywargs)
  File "/Users/blah/temp.py", line 14, in test_func2
    func2()
  File "/Users/blah/temp.py", line 9, in func2
    ret = func1()
  File "/Users/blah/temp.py", line 6, in func1
    function_to_be_mocked(None)
  File "/Users/blah/othermodule.py", line 3, in function_to_be_mocked
    raise ValueError
ValueError

So the mock doesn't seem to be working. Does anyone have any thoughts how to achieve this?

============ Edited below this line ===========

It doesn't sound like I can do what I thought I could (as I cannot modify anything related to function 1, or 2 in fact. All I have control over is the test.

Let me pose the following problem then as perhaps more experienced eyes than mine can see a way forward.

I have a function:

def function_to_be_tested(args):
    # Some processing steps

    # Function call that works locally
    function_that_returns_something_1()

    # Some logic

    # Function call that works locally
    function_that_returns_something_2()

    # Function that raises an exception when running locally,
    # and since I need to test the logic after this function 
    # (and cannot edit this code here to bypass it) I would 
    # (naively) like to mock it.
    function_I_would_like_to_mock()

    # Much more logic that follows this function. 
    # And this logic needs to be unit tested.
    return some_value_based_on_the_logic

Tests:

def test_function_to_be_tested():
    assert function_to_be_tested(args) == some_expected_value

I can easily unit test anything before function_I_would_like_to_mock().

But since this function crashes locally (and I cannot edit the code to stop it crashing locally), I feel like the correct approach would be to mock it and force a sensible return value. So that I can unit tests the code paths beyond this.

What would you suggest as a good approach?

Please note, the only thing I can modify is the test function. I can't add even decorators to the main functions.

Pablo Henkowski · Accepted Answer · 2018-07-13 05:40:55Z

4

Option A)

The function you are willing to mock is loaded into func1. Therefore you have to apply the @patch decorator to func1

import pytest
from unittest import mock

@mock.patch('othermodule.function_to_be_mocked', return_value = 3)
def func1(mocker):
    from othermodule import function_to_be_mocked
    function_to_be_mocked(None)

def func2():
    ret = func1()
    print (ret)

def test_func2():
    func2()

test_func2()

=========Edit===========

Option B)

import pytest
from unittest import mock


def func1():
    from othermodule import function_to_be_mocked
    function_to_be_mocked(None)

def func2():
    ret = func1()
    print (ret)

def test_func2():
    with mock.patch('othermodule.function_to_be_mocked', return_value = 3) as irrelevant:
        func2()

test_func2()

edited Jul 13, 2018 at 5:40

answered Jul 12, 2018 at 18:34

Pablo Henkowski

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5 Comments

Joey O Over a year ago

Thanks for this Pablo, but I cannot make any amendments to anything other than the test function. I have added a wider description of what I'm trying to achieve, maybe it's not possible.

Pablo Henkowski Over a year ago

Hi, I fail to understand while you can’t change func1. That would make sense if you actually import it from a module in your test script, do you?

Joey O Over a year ago

Yes it's in another module. I just brought everything into a very short example for the purposes of this post,. Thanks a lot for your help.

Joey O Over a year ago

One final question. If I move the import statement "from othermodule import function_to_be_mocked' to the top of the file instead of inside func2(), this doesn't work any longer. Why is this? Is there a way around this, as in my source code all the imports are done at the top of the file? Thanks

Joey O Over a year ago

Never mind, I figured it out. I needed to modify the path to the function to be mocked as it's already imported into the module, and that's the one I need to patch. Thanks :)

István Siroki · Accepted Answer · 2020-11-02 14:20:19Z

The Where to patch section of the official "unittest.mock — mock object library" documentation explains this quite clearly:

a.py
    -> Defines SomeClass

b.py
    -> from a import SomeClass
    -> some_function instantiates SomeClass

Now we want to test some_function but we want to mock out SomeClass using patch(). The problem is that when we import module b, which we will have to do then it imports SomeClass from module a. If we use patch() to mock out a.SomeClass then it will have no effect on our test; module b already has a reference to the real SomeClass and it looks like our patching had no effect.

The key is to patch out SomeClass where it is used (or where it is looked up ). In this case some_function will actually look up SomeClass in module b, where we have imported it. The patching should look like:

@patch('b.SomeClass')

So I think in your case the patching should look like:

@patch("module_of_func2.function_to_be_mocked_as_it_is_imported_there", return_value=3)
def test_func2():
    ...

Vasanth L · Accepted Answer · 2022-02-26 03:51:27Z

1

If suppose you want to mock a function from module inside function in another module in python you can try this.

 # application2.py
 def app2_func(a):
           print(a)

 # application1.py
 import application2
def app1_func(a):
    application2.app2_func(a) # func to be mocked

the test file to test the function

   # application_test.py
    import application1
    def test_app1_func(mocker):
       app2_mocker = mocker.patch('application1.application2.app2_func')
       application1.app1_func('mock call')
       app2_mocker.assert_called_once() # to check if mocked function is called once

edited Feb 26, 2022 at 3:51

answered Feb 26, 2022 at 3:46

Vasanth L

112 bronze badges

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