Shared via mypy Playground

main.py

from typing import Optional

# executed with --warn-unreachable

class A:
    x: Optional[int]

    def execute(self) -> None:
        self.x = None
        for i in range(3):
            if self.x is None:
                self.x = 10
            else:
                self.x = 20  # error: Statement is unreachable  [unreachable]
            print(self.x)


a = A()
a.execute()