Type constraints on self are not correctly checked

[SquidDev]

Version Info

• Python 3.10.12
• pytype 2024.04.11

Details

Consider the following code:

from typing import Generic, TypeVar

T = TypeVar("T", int, str)

class Example(Generic[T]):
    value: T

    def as_str(self: "Example[str]") -> str:
        return self.value # <- errors

The type annotation on self should constrain the type variable T to be a str. However, this constraint is not applied, and instead T is assumed to be int | str:

File "example.py", line 9, in as_str: bad option 'int' in return type [bad-return-type]
           Expected: str
  Actually returned: Union[int, str]

This only happens when T has constraints defined. If T is left unconstrained (T = TypeVar("T")), then this code checks as expected.

[SquidDev]

A somewhat similar bug can be seen when using Any¹ as one of the constraints to TypeVar. Happy to report this as a separate issue, but it's possible the root cause is the same so wanted to check first:

---

from typing import Generic, TypeVar, Any

T = TypeVar("T", str, Any)

class Example(Generic[T]):
    value: T

def get_value(x: Example[Any]) -> int:
    return x.value # <- errors

The above code fails to type check with the following error:

File "example.py", line 9, in get_value: bad option 'str' in return type [bad-return-type]
           Expected: int
  Actually returned: Any

If we replace x.value with an actual Any value, then this code will type check.

Footnotes

1. To clarify, we're not actually using Any here (that would be a bit silly!), but a type in an external (untyped) Python module. From my understanding, this then gets replaced with Any. ↩