example-code-2e/15-more-types/mysum.py

import functools
import operator
from collections.abc import Iterable
from typing import overload, Union, TypeVar

T = TypeVar('T')
S = TypeVar('S')  # <1>

@overload
def sum(it: Iterable[T]) -> Union[T, int]: ...  # <2>
@overload
def sum(it: Iterable[T], /, start: S) -> Union[T, S]: ...  # <3>
def sum(it, /, start=0):  # <4>
    return functools.reduce(operator.add, it, start)