mypy should understand that setters can accept wider types than their getter returns

[warsaw]

Bug Report

Apologies in advance if this has already been reported. I tried to search the tracker but my search-fu must not be good enough.

mypy should allow property setters to accept compatible but wider types than their property getter returns. mypy 1.50.0 returns this error:

lock.py:11: error: Incompatible types in assignment (expression has type "timedelta | int", variable has type "timedelta")  [assignment]
Found 1 error in 1 file (checked 1 source file)

but it should be allowed, since self._lifetime is always guaranteed to be a timedelta and therefore the getter's return type should always be correct.

To Reproduce

from datetime import timedelta

type Interval = timedelta | int

DEFAULT_LIFETIME = timedelta(seconds=3)


class Lock:
    def __init__(self, lifetime: Interval | None = None):
        self._lifetime: timedelta
        self.lifetime = DEFAULT_LIFETIME if lifetime is None else lifetime

    @property
    def lifetime(self) -> timedelta:
        return self._lifetime

    @lifetime.setter
    def lifetime(self, lifetime: Interval) -> None:
        if isinstance(lifetime, timedelta):
            self._lifetime = lifetime
        else:
            self._lifetime = timedelta(seconds=lifetime)

Expected Behavior

I wouldn't expect mypy to complain about line 11.

Actual Behavior

See above for the error. It's easy enough to add a typing: ignore[assignment] to line 11, but that doesn't seem like it should be necessary.

Your Environment

• Mypy version used: 1.15.0 (compiled: yes)
• Mypy command-line flags: mypy lock.py
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: Python 3.13.3

[jorenham]

I believe this has been fixed in #18510

[warsaw]

Thanks! Looks like that will be part of 1.16 so I will try it again when that is released.

[A5rocks]

Since the reproducer works in mypy playground w/ the master branch version, I'm going to close this. (I assume that's due to the PR mentioned above)

If things are still broken after the next mypy release, it should be some other interaction.