Unpacking tuples of variable length

[mihneagiurgea]

Given the following code:

from typing import Any, Dict, Tuple, Union

VarTuple = Union[Tuple[int, int], Tuple[int, int, int]]

def make_tuple():
    # type: () -> VarTuple
    return None

x = make_tuple()
a = b = c = 0
if len(x) == 3:
    a, b, c = x
else:
    a, b = x
    c = 0

mypy complains about the following error:

sandbox.py:12: error: 'Union[Tuple[builtins.int, builtins.int], Tuple[builtins.int, builtins.int, builtins.int]]' object is not iterable
sandbox.py:14: error: 'Union[Tuple[builtins.int, builtins.int], Tuple[builtins.int, builtins.int, builtins.int]]' object is not iterable

[JukkaL]

The error message is incorrect. However, mypy may not be able to type check this properly since it doesn't recognize the len(x) == 3 check. So there seem to be two related issues:

• Union types aren't recognized as iterables when type checking assignment statements.
• len(x) == n checks can't be used to narrow down tuple types or unions. These could work similarly to isinstance checks.

The recommended way to work around the issue is to use casts:

if len(x) == 3:
    a, b, c = cast(Tuple[int, int, int], x)
else:
    a, b = cast(Tuple[int, int], x)
    c = 0

[JukkaL]

Also, mypy could recognize len(x) when x is a variable-length tuple. Example:

def f(x: Tuple[int, ...]) -> None:
    assert len(x) == 3 
    # Type of x should be Tuple[int, int, int] now
    ...

[neiljp]

I just ran into this issue and was going to post the following test case, but didn't realise that len(x) wasn't yet a way to distinguish among unions:

def f_4(a: Tuple[int, int, int, int]) -> None:
    pass

def f_2_4(a: Union[Tuple[int, int], Tuple[int, int, int, int]]) -> None:
    if len(a) == 2:
        aa = a + (0,0)
    else:
        aa = a
    f_4(aa)