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.

该 issue 提议为 Python 容器注解引入 NonEmpty 类型，并以 NonEmpty[List[float]] 和 average 函数作为示例。首先检查 mypy 现有的类型系统和 typing 构造支持；该 issue 没有指定文件或测试。完成这项工作需要达成一致的设计，并实现能够保持容器非空不变量的类型检查。