[Request]: Warn for None types then using Union[None, ...]

[gcoimbra]

I think it'll be a good idea to warn the developer to check for None types when he/she indicate that the types of the variable can be None. Like this:

def func(var: Union[None, int]) -> int:
    return (var + 1)
                 ^ - Operation could be done on a None.

Or something like that...

[JelleZijlstra]

That code should already throw an error in mypy. Are you seeing that it doesn't?

[gcoimbra]

That code should already throw an error in mypy. Are you seeing that it doesn't?

Yes. It's true, but when you do type check like this:

from typing import *
def func(var: Union[None, int]) -> int:
    if type(var) is int:
        return (var + 1)
    else:
        return 0

It still prints an error:

m.py:4: error: Unsupported operand types for + ("None" and "int")
m.py:4: note: Left operand is of type "Optional[int]"

Then this is, instead of a feature request, an bugfix?

EDIT:
fix number of lines in code with import

[JukkaL]

Mypy doesn't recognize type(x) is int as a valid type check, since var could be an instance of a subclass of int and it would evaluate to false. If you use isinstance(var, int), everything should work as expected.

[JukkaL]

Created #7260 to discuss what we should do about this (if anything).