While loop prevents triggering "no return statement"

[sebsheep]

Bug Report

A while loop in a function prevents mypy to detect a missing return statement.

To Reproduce https://mypy-play.net/?mypy=latest&python=3.12&flags=strict&gist=363627f969a9a127616dd5ee7ea2a235

def f() -> int:
    b = None
    while not b:
        b = 1

Expected Behavior

In strict mode, mypy should raise an error because f doesn't return Actual Behavior

Mypy successes!

Your Environment

• Mypy version used: 1.8.0
• Mypy command-line flags: --stric
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.12

[KotlinIsland]

More generally, the type of the while subject does not get updated from the body:

b = None
reveal_type(b)  # None from ? | None
while not b:
    reveal_type(b)  # None from ? | None
    b = 1
    reveal_type(b)  # int from int | None
reveal_type(b)  # unreachable

https://mypy-play.net/?mypy=basedmypy-latest&python=3.12&flags=strict&gist=363627f969a9a127616dd5ee7ea2a235

• I think this is the same issues as #16909

[sebsheep]

even if the while subject we had a true infinite loop, this function never returns... Is this the expected behavior?

def f() -> int:
    while True:
        pass

type checks!

[KotlinIsland]

Yes, that is intended, because it is technically correct, consider:

def f() -> Never:
    while True: ...

This doesn't produce any error because the function will Never return.

But perhaps it should warn here when the return type of the function is not Never, something like:

def f() -> int:
   while True:  # this loop will never return
       ...

[davidhalter]

@sebsheep I agree that this code should raise an error. Note however, that you have not enabled --warn-unreachable. I think it's a mistake that that's not enabled in --strict.

In your case that won't help. However it will help at least in

def f() -> int:
    while True:
        pass
    return 1