Cannot unpack Union of Tuples of different lengths

[nielsbuwen]

• Are you reporting a bug, or opening a feature request?

I am reporting a bug.

• Please insert below the code you are checking with mypy,

def unpack(two_or_three: Union[Tuple[int, int], Tuple[int, int, int]]) -> None:
    try:
        one, two = two_or_three
        three = None
    except ValueError:
        one, two, three = two_or_three

• What is the actual behavior/output?

line:3: error: Too many values to unpack (2 expected, 3 provided)
line:6: error: Need more than 2 values to unpack (3 expected)

• What is the behavior/output you expect?

The type checker should report no error.

• What are the versions of mypy and Python you are using?

mypy==0.750+dev.8ada8e2cdb5432d41f188d2d0610ce21a2c62c12

Python 3.7.3

• What are the mypy flags you are using? (For example --strict-optional)

mypy --strict

• More Information

The call site looks good

unpack((1, 2, 3))  # no error
unpack((1, 2))  # no error
unpack((1, ))  # error as expected 

I also tried to replace the try-except with an if len(...) == 2 but that didn't change the error at all.

Currently, i use this workaround

def unpack(two_or_three: Union[Tuple[int, int], Tuple[int, int, int]]) -> None:
    one, two, *three = two_or_three
    ok = three[0] if three else None

But this is not ideal, because, the first version will actually throw a ValueError during runtime iif more than 4 elements are in the tuple.

[JukkaL]

Unfortunately, the first example is too difficult for mypy to figure out. For example, mypy would have to understand that a mismatch in tuple lengths results in ValueError, but mypy doesn't have any knowledge of which exceptions may be raised by various operations.

However, a if len(...) == x check to narrow narrow down the union would be feasible (tracked in #1178).