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.

この issue では、NonEmpty[List[float]] と average 関数を例として、Python のコンテナーアノテーション向けの NonEmpty 型を提案しています。まず、mypy に既存の型システムおよび typing 構成要素のサポートを確認してください。issue ではファイルもテストも指定されていません。完了には、空でないコンテナーの不変条件を維持する、合意済みの設計と型チェックが必要です。