mypy doesn't understand that Optional is no longer Optional in an 'if' clause

[1st1]

mypy doesn't understand that Optional is no longer Optional in an 'if' clause:

test.py:

from typing import *


class Item:
    pass


class Foo:
    def __init__(self, ignore: Optional[AbstractSet[Item]] = None) -> None:
        self.ignore = ignore

    def do(self, commands: Sequence[Item]) -> None:
        if self.ignore:
            commands = list(filter(lambda c: c in self.ignore, commands))

$ mypy --version
mypy 0.740

$ mypy --strict test.py
test.py:14: error: Unsupported right operand type for in ("Optional[AbstractSet[Item]]")
Found 1 error in 1 file (checked 1 source file)

[emmatyping]

Hi Yury! You need to do if self.ignore is None for mypy to pick up that you want self.ignore to be non-None.

There has been previous discussion about allowing if var to narrow var, but I believe we decided to not go down that route as it could get complicated?

[msullivan]

We do support narrowing with `if var`. The issue here is that the use is in a lambda, which could get called after the value is changed.

…

On Thu, Nov 21, 2019, 16:01 Ethan Smith ***@***.***> wrote: Hi Yury! You need to do if self.ignore is None for mypy to pick up that you want self.ignore to be non-None. There has been previous discussion about allowing if var to narrow var, but I believe we decided to not go down that route as it could get complicated? — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#8000?email_source=notifications&email_token=AACTC7NF4ZSVQ3DDW2RJ6WDQU4OOFA5CNFSM4JQJQVSKYY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGOEE4CEVA#issuecomment-557326932>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AACTC7MJRHLA6IMTB5GUEIDQU4OOFANCNFSM4JQJQVSA> .

[1st1]

Hi Yury! You need to do if self.ignore is None

Hi!

We do support narrowing with if var.

Yeah, changing to if is not None doesn't help.

a lambda, which could get called after the value is changed.

Not sure I follow. self.ignore isn't changed anywhere. It's non-None inside the if clause and the filter() function is called right away.

Upd: Ideally mypy should infer that the lambda is used synchronously in this case. Would it be possible to fix this? Or should our code be rewritten to close over a temp variable?

[ilevkivskyi]

Duplicate of #4973 (which is itself a special case of #2608).

[emmatyping]

We do support narrowing with if var

Oh huh, I must have missed that, apologies!

[ilevkivskyi]

Would it be possible to fix this?

Definitelly possible, but it may be non-trivial and I am not sure we will have time for this, but PRs are very welcome.

Or should our code be rewritten to close over a temp variable?

Yep, a temp variable is a quick and simple workaround.

[1st1]

Thank you guys for a quick turnaround!

[alan-isaac]

Alternatively, adding an assertion also seems to satisfy mypy. Is that reliable?

I'm finding this problem generally emerges when an instance variable must be initialized to None but will never be gotten until after a value is set. What is the preferred way to communicate this to mypy?

[hauntsaninja]

Yes, an assertion works well.

For the instance variable thing you can lie to mypy and just give it a non-None type and use type ignore. Then it's on you to make sure your init logic is right.

Alternatively, you could use a @Property that contains an assert and returns a non-Optional type