freeze_time class decoration causes problems with subclasses and Python 3.11.1

[mathiasertl]

Hi,

I have discovered that the way freezegun.freeze_time decorates unittest.TestCase classes causes Problems in Python 3.11.1 (yes, the micro version) if you create a subclass of a class with the decorator and you use unittest.TestCase.addClassCleanup() in setUpClass() (as Djangos SimpleTestCase classes do, that's how I stumbled on this problem).

The change in Python 3.11.1 is python/cpython#99645. While the problem only surfaces in that special combination, I do believe the right place to fix this is this library, as the freezegun decorator is what triggers this behavior.

To demonstrate the problem, let me first show a contrived example of test case classes:

@freeze_time("2019-04-14 12:26:00")
class FrozenTestCase(unittest.TestCase):
    @classmethod
    def setUpClass(cls):
        print("BaseTest.setUpClass:", cls)
        super().setUpClass()
    
    def test_something(self):
        print(self.id(), datetime.utcnow().strftime("%Y-%m-%d %H:%M:%S"))


class FrozenSubTestCase(FrozenTestCase):
    def test_something_else(self):
        print(self.id(), datetime.utcnow().strftime("%Y-%m-%d %H:%M:%S"))

Now if you run the test suite, you'll see that both test methods will always get the frozen time (as expected), but setUpClass() will get the base class even in the subclass. Here's the output (full class paths removed for brevity):

BaseTest.setUpClass: <class 'FrozenTestCase'>
FrozenSubTestCase.test_something 2019-04-14 12:26:00
FrozenSubTestCase.test_something_else 2019-04-14 12:26:00
BaseTest.setUpClass: <class 'FrozenTestCase'>
FrozenTestCase.test_something 2019-04-14 12:26:00

I have checked that just removing the freeze_time decorator restores the expected behavior (setUpClass receives the subclass when run for the subclass) and that this also happens if the method is defined in yet a base class (e.g. Djangos SimpleTestCase).

While certainly odd, The above class works the same way as it does in Python 3.11.1. The problem starts if you have a call to addClassCleanup() in your base class. python/cpython#99645 means that only those registered for the exact class are called, and setUpClass() for the subclass now receives the wrong class (the parent). Our sub-testcase now registers a second cleanup for the parent instead of its own class.

If we add a cleanup function to the base class:

@freeze_time("2019-04-14 12:26:00")
class FrozenTestCase(unittest.TestCase):
    @classmethod
    def custom_cleanup(cls):
        print("Custom cleanup!", cls)

    @classmethod
    def setUpClass(cls):
        print("BaseTest.setUpClass:", cls)
        super().setUpClass()
        cls.addClassCleanup(cls.custom_cleanup)
...

and run the test suite again, you get the following output (again trimmed the output for brevity):

BaseTest.setUpClass: <class 'FrozenTestCase'>
FrozenSubTestCase.test_something 2019-04-14 12:26:00
FrozenSubTestCase.test_something_else 2019-04-14 12:26:00
BaseTest.setUpClass: <class 'FrozenTestCase'>
FrozenTestCase.test_something 2019-04-14 12:26:00
Custom cleanup! <class 'FrozenTestCase'>
Custom cleanup! <class 'FrozenTestCase'>

Notice that the cleanup function is now called twice for the base class, but not at all for the subclass. Sure enough, if you run this with 3.11.0, you get:

BaseTest.setUpClass: <class 'FrozenTestCase'>
FrozenSubTestCase.test_something 2019-04-14 12:26:00
FrozenSubTestCase.test_something_else 2019-04-14 12:26:00
Custom cleanup! <class 'FrozenTestCase'>
BaseTest.setUpClass: <class 'FrozenTestCase'>
FrozenTestCase.test_something 2019-04-14 12:26:00
Custom cleanup! <class 'FrozenTestCase'>

... sure enough, setUpClass() still gets the wrong class, but it's not a problem yet, as cleanups for all base classes are called.

Unfortunately I'm not quite sure how to correctly solve this issue, as I have not found a way to "correctly" decorate a class and wrap class methods so that they get the correct classes.

[blochsbek]

I think this is also happening in 3.10 as well, getting an import error with Freezetime and test set up....

[radekholy24]

In my case it causes really weird and impossible-to-debug Django transactional issues.