Wrong type in list after isinstance check

[AlexElvers]

The following code appends a str to a list of ints.

from typing import Iterable, List

def modify(l: Iterable[object]) -> None:
    if isinstance(l, List[object]):
        l.append("abc")

l = List[int]()
modify(l)
print(l)  # ['abc']
1 + l[0]  # TypeError: unsupported operand type(s) for +: 'int' and 'str'

(without mypy errors/warnings)

[rockneurotiko]

And are you sure that this is an error?

Let's see your code:

You have a variable "l" declared as List[int], and you call modify with that list.

"modify" is a function that takes an Iterable with any object inside, that's almost like declare it as Any, so, when you are appending an str, you can, because the mypy "thinks": 'Hey, I want to append a str to a list that contains objects... Can I? Yeah! Of course! Because str is an object.'

So, that function can do it, and outside the function mypy thinks that is still a List[int], and when you do 1 + l[0] thinks that you are taking an int.

What kind of warning error was you waiting?

P.S: That's what happens when have mutability in variables :)

[JukkaL]

This is really working as intended. In order to fix this, we could have something like the wildcard types of Java, and infer a wildcard type for l after the isinstance check, so that calling append would be disallowed. It's probably not worth implementing right now, however, as this does not seem like a very common problem. There are many other ways of running into runtime type errors even when the type checker does not complain (setattr, Any types, cast, etc.).

I flagged this as an enhancement proposal because this is a hole in the type system that we could fix.

[JukkaL]

Closing this as this doesn't seem like a high-priority issue.