Value incorrectly flagged as being possibly None after or predicate ensures it isn't None

[rowillia]

Invoked with: python3.6 -m mypy --warn-no-return --strict-optional test_or_shortcut.py

from typing import Optional, Any


def test_or_shortcut(value: Optional[Any]) -> None:
    if not value:
        pass
    if not value or value.get('foo') == 'hello':
        pass

Fails with:

test_or_shortcut.py:7: error: Item "None" of "Optional[Any]" has no attribute "get"

Oddly if I remove the first branch things typecheck fine:

from typing import Optional, Any


def test_or_shortcut(value: Optional[Any]) -> None:
    if not value or value.get('foo') == 'hello':
        pass

[gvanrossum]

Hm, that looks like a binder bug. But I can't repro it. Does it still fail this way in master?

[rowillia]

@gvanrossum Repro'd on HEAD:

rwilliams at rwilliams-mbp114-2 in ~/src/oss/mypy (master●) (venv)
$ python3.6 -m mypy -V
mypy 0.520-dev-72168faf682722ff8ffa6876d3c2fb94a30d5012

[gvanrossum]

Ah, I did'nt copy the flags you are passing. Sorry. Labeled as bug affecting strict-optional.

[ddfisher]

This looks familiar to me -- I think this might be to do with Anys behavior in the subtyping check (or maybe with is_subtype being used instead of is_proper_subtype).

[ddfisher]

I was wrong -- it's actually related to truthiness tracking. For some reason, after the if not value: pass, the Any part of the union has can_be_true set to False.

[gvanrossum]

What if there is something other than pass in the first if block?

[ddfisher]

I was wondering that too. I tried replacing the pass with an unrelated assignment statement -- the problem persisted.

[ddfisher]

Okay, I think I've tracked this down to the root cause of the problem: true_or_false (in types.py) doesn't behave correctly on Unions. In particular, it sets can_be_true and can_be_false to True on the Union type, but not on any of its items!

[lk-geimfari]

I have the same problem, but with enums

Here is my enum object:

class AllowRandom(EnumMeta):

    def get_random_item(self) -> enum.Enum: # but i'm not sure
        return choice(list(self))

class PrefixSign(enum.Enum, metaclass=AllowRandom):
    POSITIVE = 'positive'
    NEGATIVE = 'negative'

and when i tried this:

def prefix(sign: Optional[PrefixSign] = None) -> str:
    prefixes = {
        'negative': ['...'],
        'positive': ['...'],
    }

    if sign is None:
        sign = PrefixSign.get_random_item()

    if sign in PrefixSign:
        return self.random.choice(prefixes[sign.value])
    else:
        raise NonEnumerableError(PrefixSign)

I got this:

project/test.py:24: error: Item "None" of "Optional[PrefixSign]" has no attribute "value"
project/units.py:25: error: Item "None" of "Optional[PrefixSign]" has no attribute "value"

How to deal with this?

[ilevkivskyi]

@lk-geimfari I think your particular case may be related to the fact that the method get_random_item() is not annotated. If I am right, then this should be fixed by the last commit. Could you please try your code with the current mypy master?

[lk-geimfari]

@ilevkivskyi Unfortunately, it's did not help. I have updated the code which i write, please look above. The problem that i don't know which subclass of Enum will be returned and when i try annotate get_random_item, here even more errors:

project/units.py:24: error: Incompatible types in assignment (expression has type "Enum", variable has type "Optional[PrefixSign]")
project/units.py:25: error: Item "None" of "Optional[PrefixSign]" has no attribute "value"

[ilevkivskyi]

@lk-geimfari You can try self-types instead of just returning Enum.