Shared via mypy Playground

main.py

x: int | None = 1

# Either of these two type checks produces identical results:
if isinstance(x, int):
# if x is not None:

    # Revealed type is "builtins.int"
    reveal_type(x)
    
    def f() -> int: 
         # Revealed type is "Union[builtins.int, None]"
        reveal_type(x)
        
        # error: Unsupported operand types for + ("None" and "int")  [operator]
        # note: Left operand is of type "int | None"
        # return x + 2 
        
        # No errors here:
        return 1 + 2
    
    # error: Unsupported operand types for + ("None" and "int")  [operator]
    # note: Left operand is of type "int | None"
    # (lambda: x + 2)()
    
    # No errors here:
    (lambda: 1 + 2)()
    
    # However, defining a lambda without calling it does not cause errors:
    l = lambda: x + 2