Common checks and union-attr error

Bug Report / Discussion

I want to write the code shown below where the types of the attributes are checked in a centralised consistent way to avoid duplication. However, this triggers the union-attr error because the type inference within the checking method doesn't propagate to the outer method.

I'm pretty sure that mypy can't (or won't) do anything about this, but I'm not clear about what the proper resolution for this is that avoids (too much) code duplication or lots of extra code (see below for potential workarounds). Maybe something needs to be documented (e.g. via this issue) and possibly added to the common issues.

To Reproduce

class X:
    def __init__(self, x: str | None = None) -> None:
        self.x = x

    def ready(self) -> bool:
        return isinstance(self.x, str) and bool(self.x)

    def show(self) -> None:
        if not self.ready():
            raise RuntimeError("Object is not configured")

        print(repr(self.x.split()))  # <-- union-attr error

This is a simple example where the duplication would be limited, but it's fairly easy to get into situations where the ready method is far more complicated, e.g. might need to check several fields or combinations of fields, and where it's reused many times. Note that the ready method is useful in its own right.

Expected Behavior

Should be ok, in an ideal world.

Actual Behavior

x.py:12: error: Item "None" of "str | None" has no attribute "split"  [union-attr]
Found 1 error in 1 file (checked 1 source file)

Potential Workarounds

Workarounds are to write one of the following:

    def show(self) -> None:
        if not self.ready():
            raise RuntimeError("Object is not configured")

        print(repr(self.x.split())  # type: ignore[union-attr]  # might ignore other potential errors

or

    def show(self) -> None:
        if self.x is None or not self.ready():  # Duplicates logic in ready method
            raise RuntimeError("Object is not configured")

        print(repr(self.x.split()))

or (as suggested in (https://github.com/python/mypy/issues/4805#issuecomment-1018892112))

    def show(self) -> None:
        if not self.ready():
            raise RuntimeError("Object is not configured")
        assert isinstance(self.x, str)  # Duplicates logic in ready method
        # although it's nice that this assert is skipped when using `python -O`

        print(repr(self.x.split())

or (as I think is suggested in https://github.com/python/mypy/issues/15207#issuecomment-1537519614)

    @property
    def x_checked(self) -> str:
        # Do I have to write a property for every attribute?
        if isinstance(self.x, str) and bool(self.x):
            return self.x
        else:
            raise RuntimeError("Object is not configured")

    def ready(self) -> bool:
        try:  # Clumsy
            self.x_checked
        except Exception:
            return False
        else:
            return True

    def show(self) -> None:
        print(repr(self.x_checked.split()))

Note that the following is not a workaround, since it's an implicit Optional.

    def __init__(self, x: str = None) -> None:
        self.x = x

Your Environment

• Mypy version used: 1.5.0 (compiled: yes)
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.10.12

Inizia con l’esempio minimo di X e confronta gli approcci proposti nelle discussioni mypy collegate #4805 e #15207. Una risoluzione utile stabilirebbe se il narrowing tra metodi è supportato; altrimenti, documenta il workaround consigliato oppure aggiungi il comportamento necessario.