Standard “duck types”

In typical Python code, many functions that can take a list or a dictas an argument only need their argument to be somehow “list-like” or“dict-like”. A specific meaning of “list-like” or “dict-like” (orsomething-else-like) is called a “duck type”, and several duck typesthat are common in idiomatic Python are standardized.

from typing import Mapping, MutableMapping, Sequence, Iterable
 
# Use Iterable for generic iterables (anything usable in "for"),
# and Sequence where a sequence (supporting "len" and "__getitem__") is
# required
def f(iterable_of_ints):
    # type: (Iterable[int]) -> List[str]
    return [str(x) for x in iterator_of_ints]
 
f(range(1, 3))
 
# Mapping describes a dict-like object (with "__getitem__") that we won't
# mutate, and MutableMapping one (with "__setitem__") that we might
def f(my_dict):
    # type: (Mapping[int, str]) -> List[int]
    return list(my_dict.keys())
 
f({3: 'yes', 4: 'no'})
 
def f(my_mapping):
    # type: (MutableMapping[int, str]) -> Set[str]
    my_mapping[5] = 'maybe'
    return set(my_mapping.values())
 
f({3: 'yes', 4: 'no'})