Question about usage of union of Tuples

[tacaswell]

I just started trying to apply typing to a code base, so I may be greatly miss-understanding what I am doing, but here we go :)

We have a case where we get a tuple back from a user supplied function which must be (None, None), (some_not_none, None), or (some_not_none, other_not_none), that the second value can only be not None if the first value is also not None. (This is why I did not do Tuple[Optional[int], Optional[int]]).

A minimal example to reproduce the issues is

from typing import Tuple, Union

InputType = Union[Tuple[None, None],
                  Tuple[int, None],
                  Tuple[int, int]]


def f(a: InputType) -> None:
    b, c = a

which gives

(dd36) ✔ ~ 
17:32 $ mypy --version
mypy 0.471
(dd36) ✔ ~ 
17:32 $ mypy /tmp/test.py
/tmp/test.py:9: error: 'Union[Tuple[void, void], Tuple[builtins.int, void], Tuple[builtins.int, builtins.int]]' object is not iterable

[ilevkivskyi]

@tacaswell Oh, this is a well known bug in mypy, see e.g. python/mypy#1855 and python/mypy#1575. There is even a PR for this python/mypy#2219 but I don't know when it will land.

Probably you could use your workaround with less precise type, or maybe use an overload in stub file, something like

@overload
def f(a: Tuple[None, None]) -> None: ...
@overload
def f(a: Tuple[int, None]) -> None: ...
@overload
def f(a: Tuple[int, int]) -> None: ...

might work (overloads will be allowed also outside stubs, i.e. in source code, see python/mypy#2603, but it is also not clear when this lands).

[tacaswell]

Thanks! Sorry for the noise!

[ilevkivskyi]

Sorry for the noise!

No problem at all :-)