[attribute-error] when usage is protected by an if statement

[cjw296]

I have this class method:

    @classmethod
    def _adjust_instance_using_tzinfo(cls, instance: datetime) -> datetime:
        if cls._mock_tzinfo:
            offset = cls._mock_tzinfo.utcoffset(instance)
            if offset is None:
                raise TypeError('tzinfo with .utcoffset() returning None is not supported')
            instance = instance - offset
        return instance

...where pytype complains but mypy does not, and it feels like the pytype complaint is erroneous:

[10/35] check testfixtures.datetime
FAILED: /Users/chris/vcs/git/testfixtures/.pytype/pyi/testfixtures/datetime.pyi 
/Users/chris/virtualenvs/testfixtures_312/bin/python -m pytype.main --disable import-error --imports_info /Users/chris/vcs/git/testfixtures/.pytype/imports/testfixtures.datetime.imports --module-name testfixtures.datetime --platform darwin -V 3.12 -o /Users/chris/vcs/git/testfixtures/.pytype/pyi/testfixtures/datetime.pyi --analyze-annotated --nofail --quick /Users/chris/vcs/git/testfixtures/testfixtures/datetime.py
/Users/chris/vcs/git/testfixtures/testfixtures/datetime.py:264:22: error: in _adjust_instance_using_tzinfo: No attribute 'utcoffset' on None [attribute-error]
  In Optional[datetime.tzinfo]

            offset = cls._mock_tzinfo.utcoffset(instance)
                     ~~~~~~~~~~~~~~~~~~~~~~~~~~

Even if I change the guarding if block to be if cls._mock_tzinfo is not None, the complaint remains.

[martindemello]

pytype doesn't assume an attribute access returns the same object when called twice in succession. the workaround is to assign it to a local variable first, i.e. x = cls.mock_tzinfo; if x: ...

[cjw296]

@martindemello - hmm, that seems quite an unfortunate choice, do you know if there are plans to change that?
(the fact you refer to it as a workaround gives me a little hope...)