Type annotation for itertools

[dineshbvadhia]

This convoluted example produces pairs of integers. The first integer is from a 'number' list and the second is an odd integer using itertools.slice. Once the pairs are produced, the 'number' and matching 'odd' integer lists are extracted using zip(,). The code works but mypy throws the errors shown below:

import itertools

def get_pair(number: list[int], mixed: list[int], odd: list[int]) -> tuple[list[int], list[int]]:

    pair: tuple[int, int] = itertools.islice(((number[e], r) for e, r in enumerate(mixed) if r in odd), 4)

    a: list[int]
    b: list[int]
    a, b = list(zip(*pair))

    return a, b

def main():
    number: list[int] = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17]
    print('number:', type(number), number)

    mixed: list[int] = [3, 7, 11, 15]
    print('mixed:', type(mixed), mixed)

    odd: list[int] = [3, 5, 9, 13, 15]
    print('odd:', type(odd), odd)

    a: list[int]
    b: list[int]
    a, b = get_pair(number, mixed, odd)

    print('a:', a)
    print('b:', b)

if __name__ == '__main__':
    main()

> python bad_slice.py
number: <class 'list'> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17]
mixed: <class 'list'> [3, 7, 11, 15]
odd: <class 'list'> [3, 5, 9, 13, 15]
a: (1, 4)
b: (3, 15)

> mypy bad_slice.py --no-incremental
bad_slice.py:5: error: Incompatible types in assignment (expression has type "islice[Tuple[int, int]]", variable has type "Tuple[int, int]")
bad_slice.py:9: error: No overload variant of "zip" matches argument type "Tuple[int, int]"
bad_slice.py:9: note: Possible overload variants:
bad_slice.py:9: note:     def [_T_co, _T1] __new__(cls, Iterable[_T1]) -> zip[Tuple[_T1]]
bad_slice.py:9: note:     def [_T_co, _T1, _T2] __new__(cls, Iterable[_T1], Iterable[_T2]) -> zip[Tuple[_T1, _T2]]
bad_slice.py:9: note:     def [_T_co, _T1, _T2, _T3] __new__(cls, Iterable[_T1], Iterable[_T2], Iterable[_T3]) -> zip[Tuple[_T1, _T2, _T3]]
bad_slice.py:9: note:     def [_T_co, _T1, _T2, _T3, _T4] __new__(cls, Iterable[_T1], Iterable[_T2], Iterable[_T3], Iterable[_T4]) -> zip[Tuple[_T1, _T2, _T3, _T4]]
bad_slice.py:9: note:     def [_T_co, _T1, _T2, _T3, _T4, _T5] __new__(cls, Iterable[_T1], Iterable[_T2], Iterable[_T3], Iterable[_T4], Iterable[_T5]) -> zip[Tuple[_T1, _T2, _T3, _T4, _T5]]
bad_slice.py:9: note:     def [_T_co] __new__(cls, Iterable[Any], Iterable[Any], Iterable[Any], Iterable[Any], Iterable[Any], Iterable[Any], *iterables: Iterable[Any]) -> zip[Tuple[Any, ...]]
Found 2 errors in 1 file (checked 1 source file)

[erictraut]

You are attempting to assign a value of type isslice[tuple[int, int]] to a variable that is declared to be of type tuple[int, int]. Those are not compatible. So mypy is correct in flagging this as an error.

[dineshbvadhia]

Using reveal_locals() in get_pair() gives:

a: builtins.list[builtins.int]
b: builtins.list[builtins.int]
mixed: builtins.list[builtins.int]
number: builtins.list[builtins.int]
odd: builtins.list[builtins.int]
pair: itertools.islice[Tuple[builtins.int, builtins.int]]

builtins.list[builtins.int] is list[int], but what is the translation of 'itertools.islice[Tuple[builtins.int, builtins.int]] for pair?

[JelleZijlstra]

See here for context: https://github.com/python/typeshed/blob/43a9ab08d77af669652720e7579bc97956eb31aa/stdlib/itertools.pyi#L96

It's essentially Iterator[tuple[int, int]], though islice has a few more methods.

[dineshbvadhia]

Ok, got a mypy Success with:

def get_pair(number: list[int], mixed: list[int], odd: list[int]) -> tuple[tuple[int, ...], tuple[int, ...]]:
    pair: Iterator[tuple[int, int]] = itertools.islice(((number[e], r) for e, r in enumerate(mixed) if r in odd), 4)
    a: tuple[int, ...]
    b: tuple[int, ...]
    a, b = list(zip(*pair))
    return a, b

Thank-you.

What is the significance of the '...' in tuple[int, ...] ?

[JelleZijlstra]

tuple[int, ...] means a tuple containing an arbitrary number of ints. tuple[int, int] is a tuple of exactly two ints, tuple[int] is a tuple of exactly one int.