Random failure of returning the desired value for a patch.object about pytest-mock HOT 3 CLOSED

Hi @baudren!

I can reproduce the problem, thanks for the detailed explanation.

I checked that the mocker fixture is calling mock.patch.object at the appropriate places, so that seems fine.

I dug down into the source code for unittest.mock and noticed that the line that actually installs the mock into the target object sometimes fail:

# contents of mock.py:1239
setattr(self.target, self.attribute, new_attr)

In the first mocker.patch.object call from your example, this line is executed and correctly replaces QMessage.question staticmethod by a new MagicMock instance. The second time this code is executed it sometimes fails to replace the existing MagicMock instance by the new one, which can be verified by adding print(getattr(self.target, self.attribute)) right after it.

Changing your code slightly to add another level of indirection, like this:

class Simple(QFrame):

    def query(self, notebook):
        reply = self._question(notebook, self)
        ...

    @staticmethod
    def _question(notebook, parent):
        return QMessageBox.question(
            parent, 'Message', 'Deleting %s, are you sure?' % notebook,
            QMessageBox.Yes | QMessageBox.No, QMessageBox.No)

def test_Qt(qtbot, mocker):
   ...
   mocker.patch.object(simple, '_question',
                        return_value=QMessageBox.No))

Seems to fix the problem. My bet is that PySide's bindings might be at fault here when one tries to change an staticmethod attribute of a class.

Having said all that, an easy workaround is for you to reuse the mocked object:

def test_Qt(qtbot, mocker):
    simple = Simple()
    qtbot.addWidget(simple)

    question = mocker.patch.object(QMessageBox, 'question',
                        return_value=QMessageBox.No)
    simple.query('toto')
    assert simple.answer == 'no'

    question.return_value = QMessageBox.Yes
    simple.query('toto')
    assert simple.answer == 'yes'

Answering your questions:

Is it somehow forbidden to use two mock.patch.object statements in a row, without doing some sort of cleaning?

Shouldn't be necessary, but if you must you can use mocker.stopall().

And that it is pure luck that this works under 2.7?

No, it seems a consistent failure.

It seemed to me that the original mock module uses a Python
with ...: syntax, probably for house cleaning. There is no such thing in this
module, right?

No, there isn't... if you want to use a with statement, you can use mock directly. This was the motivation for changing the fixture name to mocker, so you can use both in your tests if needed.

Or is it something due to the fact that now mock is integrated in py.test, since Python 3.3?

Don't think so, I believe it is the same source code. (I believe you meant Python above, of course 😁)

Thanks for the detailed explanation and bug report!

Cheers, 🍻

from pytest-mock.

Thanks, one more time, for the detailed explanation. I will go along the route of reusing the object - I should have thought about that myself. So you would say it is rather a problem directly with PySide, not with mock?

Best,

from pytest-mock.

Seems like it, this test fails for me in Python 3, but works in Python 2:

def test_foo():
    for i in range(10):
        QtGui.QMessageBox.question = i
        assert QtGui.QMessageBox.question == i

Cheers!

from pytest-mock.