Type declarations of base class attributes not carried over to subclasses if None is present

[Vgr255]

Simplest repro:

class BaseClass:
    my_attr = () # type: tuple

class Subclass(BaseClass):
    my_attr = None

Output:

testing.py: note: In class "Subclass":
testing.py:5: error: Need type annotation for variable

However, if None isn't the sole value but part of something else, this is likely not the desired behaviour. Consider this:

from typing import Tuple, Any

class BaseClass:
    my_attr = ( ("", None), ) # type: Tuple[Tuple[str, Any]]

class Subclass(BaseClass):
    my_attr = (
        ("one",  set ),
        ("two",  str ),
        ("three",None),
    )

Output:

testing.py: note: In class "Subclass":
testing.py:8: error: Need type annotation for variable

Changing None for anything else, adding a # type: ignore comment on the ("one", set), line (but not on any other line of the assignment) silences the error. This is somewhat related to #1338 (and, to a lesser extent, #1032).

[gvanrossum]

Yeah, definitely a bug. I think it's related to #1338, the type of class attributes is just not checked against the base classes at all (unlike methods).

[gvanrossum]

Looking into this again (now that #2510 has been merged) I think there's still something fishy here: I think the first example (in the absence of --strict-optional) should be silent, since it's essentially equivalent to

x = None  # type: int
x = None

which is silent.

(However the second example contains an error, since the subclass uses attr instead of my_attr. Fixing that, the example correctly complains that the top-level Tuple type in the subclass (of three items) is incompatible with the type from the base class (of 1 item).)

[Vgr255]

Whoops, I probably messed up when I copied the repro over. Edited the comment to correct this. I sadly no longer have a use case for this, as the affected code is gone. I still think it's worth fixing, though, and I hope you figure it out. Cheers!

[AlexWaygood]

The example code no longer causes a mypy error if you use the --no-strict-optional option (which was the default at the time of the bug being filed). If you don't use the --no-strict-optional option, you just need to change the annotations to this:

class BaseClass:
    my_attr = () # type: tuple | None

class Subclass(BaseClass):
    my_attr = None

And mypy is fine with it.