zip vararg

[dfroger]

mypy 0.950 (compiled: yes) and Python 3.10.4:

z1 = list(zip((1, 10), (2, 20), (3, 30)))  # correct: list[Tuple[int, int, int]]
z2 = list(zip(*[(1, 10), (2, 20), (3, 30)]))  # list[tuple[Any, ...]] but I expect list[tuple[int, int, int]]

Any chance to get mypy infer the type of zip(*l)?

Thanks for reading!

For the story it comes from here https://github.com/joelgrus/data-science-from-scratch/blob/d5d0f117f41b3ccab3b07f1ee1fa21cfcf69afa1/first-edition/code-python3/machine_learning.py#L18 !

[pranavrajpal]

I think the problem with inferring the number of elements in each tuple is that mypy is that we don't track the length of lists in general.

In your simplified example we could determine the length of the list since it's a literal, but a fairly straightforward workaround is using a tuple instead:

z2 = list(zip(*[(1, 10), (2, 20), (3, 30)]))
reveal_type(z2) # N: Revealed type is "builtins.list[builtins.tuple[Any, ...]]"

z3 = list(zip(*((1, 10), (2, 20), (3, 30))))
reveal_type(z3) # N: Revealed type is "builtins.list[Tuple[builtins.int, builtins.int, builtins.int]]"

Supporting the general case well enough to support the code you linked would probably involve some form of https://peps.python.org/pep-0646/#shape-arithmetic to track the length of the input list across calls.

The revealed type including Any seems like a bug though. The inferred type should be list[tuple[int, ...]].

[hauntsaninja]

Pranav is correct. The remaining inference issue he mentions can be improved in typeshed, transferring the issue there.