Expected meaning of <Alias>: typing.TypeAlias = typing.ClassVar[<type>], with inheritance?

[bzoracler]

For the following code snippet,

from typing import TypeAlias, ClassVar, get_type_hints

ClassAlias: TypeAlias = ClassVar[str]

class A:
    i: ClassAlias = "g"     # No errors

class B:
    i: ClassVar[str] = "g"  # No errors

class A2(A):
    i: ClassVar[str] = "g"  # <<<<<<< pylance and mypy both flag this as an error

class B2(B):
    i: ClassVar[str] = "g"  # No errors

mypy and pylance are both flagging A2.i as a class variable overriding a parent class's instance variable (A.i):

• Class variable "i" overrides instance variable of same name in class "A" Pylance[reportIncompatibleVariableOverride]
• Mypy: Cannot override instance variable (previously declared on base class "A") with class variable

However, typing.get_type_hints show that the type hints on A and B are identical:

>>> print("type hints of A:", get_type_hints(A))
type hints of A: {'i': typing.ClassVar[str]}
>>> print("type hints of B:", get_type_hints(B))
type hints of B: {'i': typing.ClassVar[str]}

I didn't pick up anything about a restricted scope of typing.ClassVar in PEP 526 nor anything about the interaction of typing.TypeAlias with typing.ClassVar in PEP 613, so assumed that we can type alias a ClassVar[...] at the module level and use it as a type annotation in the class. What's going on here?

[erictraut]

ClassVar should not be used outside of a variable declaration within a class. ClassVar is not an annotation that describes the type of a variable, but it describes a (non-type) attribute of a variable. Most other languages offer keywords for such attributes, but Python overloads the type annotation mechanism to handle these cases. Other examples include Final, InitVar, Required, and NotRequired. These special forms should not be used within type aliases because they are not type annotations.

The use of ClassVar should be flagged as an error when used in a type alias like this. When I implemented support for ClassVar in pyright, it never occurred to me that someone would attempt to use it in a type alias, so there is no check for this error condition. But you should assume that it will not work when used in this manner.

[erictraut]

I've filed an enhancement request in pyright to properly flag this usage as an error.

[bzoracler]

Thanks for the explanation about special forms - it means that this is the same reason why mypy and pylance are giving the same override errors for <...>: Alias = ClassVar[<...>] within a class as well.

from typing import TypeAlias, ClassVar

class A:
    ClassAlias: TypeAlias = ClassVar[int]  # No flagged errors
    i: ClassAlias = 2                      # No flagged errors

class B(A):
    i: ClassVar[int] = 3  # Class variable "i" overrides instance variable of same name in class "A"

[erictraut]

Yes, the next release of pyright and pylance will properly flag the use of ClassVar in a TypeAlias declaration as an error. You might want to file a similar enhancement request for mypy so it detects and reports this error condition.