251 – Hashable protocol

The module collections.abc provides a protocol Hashable that is suitable for when you want to specify that certain objects must be hashable (so they can be used as dictionary keys, for example).

The function count_unique, shown below, uses Hashable to make sure the built-in set can be used:

from collections.abc import Hashable

def count_unique(values: list[Hashable]) -> int:
    return len(set(values))

By using Hashable, a function call like the one shown below type checks because integers are hashable:

count_unique([1, 2, 3, 2, 1, 2])  # 3

The return value is 3, naturally.

On the other hand, a function call with a list of lists doesn’t type check (and fails at runtime) because lists are not hashable:

count_unique([[1, 2], [2, 3], [1, 2]])