[Python] How to use MaxLen of typing.Annotation of python 3.9?

I'm aware there's this new typing format Annotated where you can specify some metadata to the entry variables of a function. From the docs, you could specify the maximum length of an incoming list such as:

• Annotated can be used with nested and generic aliases:

​

T = TypeVar('T')
Vec = Annotated[list[tuple[T, T]], MaxLen(10)]
V = Vec[int]

V == Annotated[list[tuple[int, int]], MaxLen(10)]


But I cannot fully comprehend what MaxLen is. Are you supposed to import a class from somewhere else? I've tried importing typing.MaxLen but it doesn't seem to exist (I'm using Python 3.9.6, which I think it should exist here...?).

Example code of what I imagined should have worked:

from typing import List, Annotated, MaxLen

def function(foo: Annotated[List[int], MaxLen(10)]):
# ...
return True


Where can one find MaxLen?

EDIT:

It seems like MaxLen is some sort of class you have to create. The problem is that I cannot see how you should do it. Are there public examples? How can someone implement this function?

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