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
– jonrsharpe, Commented Jul 5, 2016 at 8:52
mock_a.method_a.return_value = 'Mocked A' => mock_a().method_a.return_value = 'Mocked A' should be better :) — Ali SAID OMAR
– Ali SAID OMAR, Commented Jul 5, 2016 at 8:53
@AliSAIDOMAR is precisely correct, it's the return value from calling mock_a that should have the method, not mock_a itself. — jonrsharpe
– jonrsharpe, Commented Jul 5, 2016 at 8:54
@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
– Mehdi Jafarnia Jahromi, Commented Jul 5, 2016 at 8:59

jonrsharpe · Accepted Answer · 2024-11-13 15:27:37Z

222

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 (and, as mentioned in the comments, avoiding adding an entry to mock_a's call list):

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.

edited Nov 13, 2024 at 15:27

answered Jul 5, 2016 at 9:05

jonrsharpe

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11 Comments

jonrsharpe Over a year ago

@imsrgadich do you need a mock for that? Just build the appropriate dataframe, treat it as a test value.

Sharanya Over a year ago

This explanation is way better than any python documentation. Thank you!

Sławomir Lenart Over a year ago

and if you're asking the reason why it works ... that mock_a() return a singleton

WestCoastProjects Over a year ago

I've set the return_value and the mock is returned anyways. These mocks are not easy to set up properly

Rainbolt Over a year ago

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.

|

ndrini · Accepted Answer · 2022-06-01 16:33:44Z

4

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 1, 2022 at 16:33

ndrini

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Comments

natielle · Accepted Answer · 2023-03-31 16:34:37Z

0

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)

answered Mar 31, 2023 at 16:34

natielle

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