reset_mock resets MagicMock's magic methods in an unexpected way

[mentalAdventurer]

Bug report

Bug description:

The reset_mock(return_value=True) method behaves in a wrong/inconsistent way.
When used with MagicMock, the method reset_mock(return_value=True) does not reset the return values of the magic methods. Only if you call for example __str__ and then call the reset_mock function, the return value will be reset, but not to the default value.

from unittest import mock

mm = mock.MagicMock()
print(type(mm.__str__()))
mm.reset_mock(return_value=True)
print(type(mm.__str__()))
print(type(mm.__hash__()))
mm.reset_mock(return_value=True)
print(type(mm.__hash__()))

Output

<class 'str'>
<class 'unittest.mock.MagicMock'>
<class 'int'>
<class 'unittest.mock.MagicMock'>

Since Python 3.9 PR reset_mock now also resets child mocks. This explains the behaviour. Calling the __str__ method creates a child MagicMock with a set return value. Since this child mock now exists, its return value is reset when reset_mock(return_value=True) is called.
Although this can be logically explained, it's counter-intuitive and annoying as I'm never sure which values are being reset.

I would expect the same behaviour as Mock. The return value of __str__ and other magic methods should not be effected.

from unittest import mock

m = mock.Mock()
print(type(m.__str__()))
m.reset_mock(return_value=True)
print(type(m.__str__()))
print(type(m.__hash__()))
m.reset_mock(return_value=True)
print(type(m.__hash__()))

Output

<class 'str'>
<class 'str'>
<class 'int'>
<class 'int'>

CPython versions tested on:

3.10

Operating systems tested on:

Linux

Linked PRs

• gh-123934: Fix MagicMock not to reset magic method return values #124038
• [3.13] gh-123934: Fix MagicMock not to reset magic method return values (GH-124038) #124231
• [3.12] gh-123934: Fix MagicMock not to reset magic method return values (GH-124038) #124232

[sobolevn]

Thank you for the report. I will take a look soon :)

[sobolevn]

Since this child mock now exists, its return value is reset when reset_mock(return_value=True) is called.

Yes, this is correct.

I would expect the same behaviour as Mock. The return value of str and other magic methods should not be effected.

Mock does not mock magic methods at all, so they are unaffected at any case. Compare:

print(type(mock.Mock().__str__))
# <class 'method-wrapper'>

print(type(mock.MagicMock().__str__))
# <class 'unittest.mock.MagicMock'>

But, I agree that retuning <class 'unittest.mock.MagicMock'> from __str__ is not correct.
So, my proposed solution is to ignore cases when:

1. child is a MagicMock instance
2. key is a magic method
3. return_value is True

[hroncok]

I see that this test in mu-editor fails with infinite RecursionError:

https://github.com/mu-editor/mu/blob/7641d7ada71a986f895ebd8a26d2ee2f52c3f1fe/tests/modes/test_python3.py#L486-L490

    with mock.patch("builtins.super") as mock_super:
        pm = PythonMode(editor, view)
        pm.set_buttons = mock.MagicMock()
        mock_super.reset_mock()

I believe this is because it is calling reset_mock on a mock of super and after this PR reset_mock calls super.

I think that mocking super is probably dangerous (and always has been), but I thought I should mention this here.

[sobolevn]

Hm, I think that we can not use super() for this feature technically. But, I think that mocking super() is dangerous indeed.

What do others think? Should we fix this?

[hroncok]

FWIW this particular use case was simple enough to fix: mu-editor/mu#2528

[hugovk]

Triage: PR merged and backported.

The output on 3.12 and 3.14 is now as expected:

<class 'str'>
<class 'str'>
<class 'int'>
<class 'int'>

Please re-open if there's more to do.