generator in dict: No overloads for "__init__" ...

[ewerybody]

Hello! With Pylance I get a typing error highlighted for the following code:

line = 'NAME|a=2345|b=SDFdss sdfsdf|ignored|c=another'
parts = line.strip().split('|')
attrs = dict(p.split('=', 1) for p in parts if '=' in p) # < HERE

No overloads for "__init__" match the provided arguments
  Argument types: (Generator[list[str], None, ...

I got the code working like that for aaaages!! But I see: a list can have different lengths and the ways I am making sure
it is like len == 2 is not obvious to the type checker, right?

I figured adding this into the builtins.pyi under class dict... makes the error go away:

    @overload
    def __init__(self, iterable: Iterable[Iterable[str]]) -> None: ...

But is there another way to make this more type safe?
Cheers:
ëRiC

[Akuli]

There is no way to say "a list with exactly 2 elements", but we could add another __init__ overload for lists in general. It would mean that mypy won't complain if you do dict([[1, 2], [3, 4, 5]]), but that's fine. We generally prefer false negatives (type checker doesn't complain about wrong code) over false positives (type checker complains about correct code).

[srittau]

See also python/typing#592 and python/typing#786 for suggestions to add fixed-length sequences. I agree that we should probably add another overload, although I would suggest to allow sequences in general, not only lists.

[Akuli]

Sequences in general would mean that arbitrary-length tuples are allowed too.