Narrowing techniques perform very well when the condition is calculated inside the if condition. When these are made outside, I can understand that mypy cannot guarantee that the narrow will be ok as the boolean value of the condition might be changed before reaching the if statement. However, when the value of this condition is given outside of the if statement but defined as a Final type, the narrow should be working

Minimal code to repdroduce it:

from typing import Optional, Final

def example_func(value: Optional[int] = None):

    condition: Final = value is not None

    if condition: 
        reveal_type(value)  # Revealed type is "Union[builtins.int, None]"
        value = value + 1   # MYPY ERROR

    if value is not None:
        reveal_type(value)  # Revealed type is "builtins.int"
        value = value + 1   # MYPY OK
    return 0

Expected Behavior
The type of value for the first if statement should be builtins.int, as the condition variable should remain with the same value that was previously assigned to it.

Actual Behavior
Narrowed not performed, and thus the type of value is still inferred as Optional[int]

Your Environment

• Mypy version used: 0.931
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: Python 3.8.10
• Operating system and version: Ubuntu 20