itertools.groupby should be typesafe

[winstxnhdw]

Currently, Pyright is unable to infer the type of a element from a groupby. Thinking about it, this is probably impossible to implement unless the Pyright team decides to do a custom check for this. However, since this is a built-in module, I think it is still something we should consider.

from itertools import groupby

unformatted_number_list = [[16, 72], 69, 76, 76, 79, [16, 49]]
list_of_number_lists: list[list[int]] = []

for key, group in groupby(unformatted_number_list, key=lambda x: isinstance(x, list)):
    group_list = list(group)

    if key:  # Ideally, should act as a TypeGuard
       # Argument of type "list[list[int] | int]" cannot be assigned to parameter "__iterable"
        list_of_number_lists.extend(group_list)  # should resolve to list[list[int]]

    else:
        # Argument of type "list[list[int] | int]" cannot be assigned to parameter "__object"
        list_of_number_lists.append(group_list)  # should resolve to list[int]

print(list_of_number_lists) # [[16, 72], [69, 76, 76, 79], [16, 49]]

[erictraut]

I don't see any practical way to make this type safe even with custom logic. I recommend avoiding this function if you want your code to be type safe.