error: 'Union[Any, Tuple[Any, Any]]' object is not iterable

[gvanrossum]

Code:

from typing import *
Pair = Tuple[Any, Any]
def f(a: Union[Any, Pair]):
    x, y = a  # E: 'Union[Any, Tuple[Any, Any]]' object is not iterable

If the type of argument is either just Any or just Pair the code passes. But with the union it gives the error.

UPDATE: I don't think the unreduced union is at fault -- an example with two different Tuple types of length 2 also fails. It's more that the unpack operation isn't tried for each variant of the union.

[gnprice]

We've seen this a couple of times internally.

[elazarg]

Could be related to #2128 ?

[gvanrossum]

Quite possibly the same root cause, which I suspect has to do with unions.

[elazarg]

Turns out it's not directly the same cause: the union case was not handled and not tested.

I believe it's the same root cause but in a deeper sense: the matching of multi-assignment and function calls define is typechecking and should be handled through the subtype module. Many subtype-related decisions are scattered through the checker (such as itemwise matching, and the iteration over union elements that I just added).

But even if it's not moved to subtype.py, I think it'll help to have a single matcher, common to function calls, multi assignment, multi-iteration (what's the terminology for that?) etc. If it was the case, my fix would have probably fixed #2128 too (I did not study it yet, though; It's just a guess).

[nickpresta]

EDIT: I'm running mypy 0.511 -- I'm trying with the latest master now.

We're seeing this problem manifest itself with our code like this (no OSS code to link):

class User(NamedTuple):
    id: UUID
    # more attributes
    

class UserService:
    def get_or_create(self, user_id: UUID, user_data: Optional[dict] = None) -> Tuple[User, bool]:
        return User(id=uuid4()), False


class Authentication:
    def authenticate(self, request: Request) -> Tuple[User, None]:
        user, _ = UserService().get_or_create(user_id=uuid4())  # ERROR HERE
        return user, None

The error I get is:

error: 'Union[Any, Tuple[Tuple[uuid.UUID, datetime.datetime, datetime.datetime, datetime.datetime, builtins.bool, builtins.str, builtins.str, builtins.str, fallback=users.domain.User], builtins.bool]]' object is not iterable

The Tuple[uuid.UUID, datetime.datetime, ...] bit is our User namedtuple.

As soon as I do something like:

class Authentication:
    def authenticate(self, request: Request) -> Tuple[User, None]:
        some_id = uuid4()
        user = UserService().get_or_create(user_id=some_id)[0]  # explicitly unpack first element
        return user, None

The error then goes away.

Right now we're working around it with the explicit index but any other insight would be much appreciated.

Thanks