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.

El issue propone un tipo NonEmpty para las anotaciones de contenedores de Python, ilustrado con NonEmpty[List[float]] y la función average. Empieza revisando el soporte existente de mypy para el sistema de tipos y las construcciones de typing; el issue no menciona archivos ni pruebas. Para darlo por terminado se requerirían un diseño acordado y una comprobación de tipos que preserve la invariante de que el contenedor no está vacío.