Python returns MagicMock object instead of return_value

Question

I have a python file a.py which contains two classes A and B.

class A(object):
    def method_a(self):
        return "Class A method a"

class B(object):
    def method_b(self):
        a = A()
        print a.method_a()

I would like to unittest method_b in class B by mocking A. Here is the content of the file testa.py for this purpose:

import unittest
import mock
import a


class TestB(unittest.TestCase):

    @mock.patch('a.A')
    def test_method_b(self, mock_a):
        mock_a.method_a.return_value = 'Mocked A'
        b = a.B()
        b.method_b()


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

I expect to get Mocked A in the output. But what I get is:

<MagicMock name='A().method_a()' id='4326621392'>

Where am I doing wrong?

When testing, `A()` returns the `return_value` from `mock_A` (a regular `MagicMock`, as you haven't specified anything else), which is not an instance of the class `A`. You need to set that `return_value` to be something that has a defined `method_a`. — jonrsharpe, Jul 05 '16 at 08:52
mock_a.method_a.return_value = 'Mocked A' => mock_a().method_a.return_value = 'Mocked A' should be better :) — Ali SAID OMAR, Jul 05 '16 at 08:53
@AliSAIDOMAR is precisely correct, it's the return value from calling `mock_a` that should have the method, not `mock_a` itself. — jonrsharpe, Jul 05 '16 at 08:54
@jonrsharpe . Thanks for your explanation. I just tried. Both `mock_a().method_a.return_value = 'Mocked A'` and `mock_a.return_value.method_a.return_value = 'Mocked A'` worked. Thanks a lot for your comments. Would you please go ahead and put it as an answer? — Mehdi Jafarnia Jahromi, Jul 05 '16 at 08:59
could something like this also work ? `mocker.patch('a.A.some_method_a', RETURN_VALUE="foo")` that's what im stuck on at the moment. seems that it still swaps magic mock for class A — Sonic Soul, Feb 17 '20 at 18:09

score 198 · Accepted Answer · answered Jul 05 '16 at 09:05

198

When you @mock.patch('a.A'), you are replacing the class A in the code under test with mock_a.

In B.method_b you then set a = A(), which is now a = mock_a() - i.e. a is the return_value of mock_a. As you haven't specified this value, it's a regular MagicMock; this isn't configured either, so you get the default response (yet another MagicMock) when calling methods on it.

Instead, you want to configure the return_value of mock_a to have the appropriate method, which you can do as either:

mock_a().method_a.return_value = 'Mocked A' 
    # ^ note parentheses

or, perhaps more explicitly:

mock_a.return_value.method_a.return_value = 'Mocked A'

Your code would have worked in the case a = A (assigning the class, not creating an instance), as then a.method_a() would have triggered your mock method.

answered Jul 05 '16 at 09:05

jonrsharpe

115,751
26
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@imsrgadich do you need a *mock* for that? Just build the appropriate dataframe, treat it as a test value. – jonrsharpe Sep 08 '18 at 09:54
@jonrsharpe yeah, I could do that but I'm also performing df.drop in my called method which I need to assert and over that I'm not returning the dataframe back from called method. That creates a problem. I found a way using `mock_data.configure_mock(columns='my_column')` which solves it. Thanks for the reply though. (ref: https://bradmontgomery.net/blog/how-world-do-you-mock-name-attribute/) – Mr. Unnormalized Posterior Sep 08 '18 at 10:36
This doesn't seem to work when you mock two SQLAlchemy models in the same test. The first one works OK, but the second one will return a MagicMock regardless of what you define. – Juha Untinen Oct 23 '18 at 21:34
2

This explanation is way better than any python documentation. Thank you! – Sharanya Aug 09 '19 at 22:27
I am confused what does the parentheses do? @jonrsharpe – user13758558 Jun 07 '21 at 20:06
@user13758558 I don't know which parentheses you mean, but In Python they're either grouping (which doesn't appear in this post) or _calling_. – jonrsharpe Jun 07 '21 at 20:07
@jonrsharpe could you please help me with this? https://stackoverflow.com/questions/67878856/how-to-mock-function-calls-of-s3-boto3-python – user13758558 Jun 07 '21 at 21:31
2

and if you're asking the reason why it works ... that `mock_a()` return a singleton – Sławomir Lenart Sep 02 '21 at 15:09
You sir, are a gentleman and a scholar. THANK YOU! – Astockwell Dec 03 '21 at 20:04
1

I've set the `return_value` and the `mock` is returned anyways. These mocks are not easy to set up properly – WestCoastProjects Jan 10 '23 at 14:08
1

The first solution, `mock_a().method_a.return_value`, invokes `mock_a` in such a way that `mock_a.assert_called()` becomes true even if `mock_a` is never invoked by the code being tested. The second solution, `mock_a.return_value.method_a.return_value`, does not invoke `mock_a` in that way (`mock_a.assert_called()` remains false). For this reason, I prefer the second solution. – Rainbolt Mar 23 '23 at 16:26

score 2 · Answer 2 · answered Jun 01 '22 at 16:33

2

In case of mocking an object I use this syntax:

@mock.patch.object(
    a.A,
    'method_a',
    lambda a: "Mocked A")
def test_method_b(self):
    b = a.B()
    b.method_b()

In this case the method_a is mocked by the lambda function.

answered Jun 01 '22 at 16:33

ndrini

81
8

score 0 · Answer 3 · answered Mar 31 '23 at 16:34

In my case, my return_value was this {"key1": "value1"}. So I needed to cast this with dict() before applying the patch.

Like this:

@fixture
def mock_something(mocker: MockFixture, request: SubRequest) -> MagicMock:
    method_to_mock = f"your_module.YourClass.some_method"
    return_value = dict(request.param)
    if isinstance(return_value, Exception):
        return mocker.patch(method_to_mock, side_effect=return_value)
    return mocker.patch(method_to_mock, return_value=return_value)

Python returns MagicMock object instead of return_value

3 Answers3

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