36 – Typing iterables instead of lists

Setting the type of function arguments to list when all you need is to be able to iterate over that value is a mistake:

# Why must `files` be a list?!
def create_files(files: list[Path]) -> None:
    for file in files:
        ...

Thankfully, it’s a mistake that is easy to fix: use Iterable:

from collections.abc import Iterable  # Python 3.9+

def create_files(files: Iterable[Path]) -> None: ...

(In Python 3.8, use typing.Iterable. In Python 3.9+, use collections.abc.Iterable.)

Using list is bad because it prevents you from using tuples, generators, iterables from itertools, other collections, etc.

Note: keep in mind that if you need to be able to iterate twice or more over the same iterable, you might want to use Sequence instead of Iterable because of iterators. Iterators are iterables but they can only be iterated over once.