Non-empty container types

Feature

It would be very helpful to have a way of specifying that a container type must be non-empty.

Pitch

As an example of why this feature would be helpful, consider the following function, average:

from typing import List


def average(numbers: List[float]) -> float:
    """Return the average of a non-empty list of numbers."""
    return sum(numbers) / len(numbers)

Rather than relying on exceptions for handling the case when the number of numbers is zero, is seems like something along the lines of the following would be more convenient and more explicit:

from typing import List, NonEmpty


def average(numbers: NonEmpty[List[float]]) -> float:
    """Return the average of a non-empty list of numbers."""
    return sum(numbers) / len(numbers)

I don't know anything about the way mypy is implemented so I'm not sure if this is a possible.

L’issue propose un type NonEmpty pour les annotations de conteneurs Python, illustré avec NonEmpty[List[float]] et la fonction average. Commencez par examiner la prise en charge existante par mypy du système de types et des constructions de typing ; l’issue ne nomme aucun fichier ni test. La réalisation nécessiterait un design convenu et une vérification de types qui préserve l’invariant de conteneur non vide.