Fix Type Narrowing for Asymmetric Attributes After Assignment

[mfish33]

This PR fixes the issue where an asymmetric attribute can not be narrowed after it has been assigned in a particular scope.

Take the following for example:

from typing import assert_type

class MyAttr:
   def __set__(self, instance: object, value: float) -> None:
      ...

   def __get__(self, instance: object, owner: type) -> int | None:
      ...

class MyClass:
   attr: MyAttr

MyClass.attr = 1.5

# Should be able to narrow types after initial assignment
assert MyClass.attr is not None
assert_type(MyClass.attr, int)

The current version of Pyright fails since it refuses to narrow MyClass.attr. The solution to this is to save the original type of MyClass.attr when an asymmetric assignment is found. This type can then be retrieved later during flow analysis.

[erictraut]

Pyright is working as intended here. This PR modifies its behavior in a way that is incorrect. For an explanation of the current behavior, refer to this documentation.

[mfish33]

This PR does not implement narrowing based on assignment. The current implementation prevents narrowing the type after assignment. This is a bug.

The following currently works fine with Pyright:

from typing import assert_type

class MyAttr:
   def __set__(self, instance: object, value: float) -> None:
      ...

   def __get__(self, instance: object, owner: type) -> int | None:
      ...

class MyClass:
   attr: MyAttr

# When the assignment is commented out the following type narrowing works as expected
# MyClass.attr = 1.5

# Should be able to narrow types after initial assignment
assert MyClass.attr is not None
assert_type(MyClass.attr, int)

[erictraut]

I still don't consider the current behavior a bug, but I think there is an argument to be made for supporting type narrowing based on type guards for asymmetric attributes.

I'd appreciate it you'd file a feature request so we can discuss it there before posting a PR.

I've reopened the PR for now.

[mfish33]

Hi Eric, happy to open a bug. However, I am unsure if it should be considered a feature request. As I showed above, type guard narrowing is currently supported by Pyright except for when you try to do it after an assignment.