Cannot determine type of attribute due to circular reference:

Is this a bug?

The following code (playground link)...

class X:
    def __init__(self, value: str | None = None, parent: X | None = None) -> None:
        if value is None:
            if parent is not None and parent.value is not None:
                value = "something"
            else:
                value = "something_else"
        reveal_type(value)
        self.value = value

Produces this output...

main.py:4: error: Cannot determine type of "value"
main.py:8: note: Revealed type is "builtins.str"

Here, it's clear that the type of X.value is a string, as indicated by reveal_type, but MyPy is unable to realize this by the time we reach parent.value.

The solution is to forward declare the type of X.value:

class X:
    def __init__(self, value: str | None = None, parent: X | None = None) -> None:
        self.value: str
        if value is None:
            if parent is not None and parent.value is not None:
                value = "something"
            else:
                value = "something_else"
        self.value = value

But that doesn't seem necessary given that resolving the circular reference is not strictly required to figure out what the type of X.value is.

Start with the linked mypy playground and reproduce the example under mypy 0.910, Python 3.10, and strict mode. Trace the inference around parent.value and self.value in the relevant type-checking path. Done means the example no longer reports that value's type cannot be determined while retaining the revealed str type.