[docs] [issue37145] collections.abc.MappingView mixins rely on undocumented _mapping

[Geoffrey Sneddon]

Geoffrey Sneddon <geoffers+pythonbugs at gmail.com> added the comment:

How do you use ItemsView without:

 * relying on __init__ and _mapping, which are both private implementation details, or
 * reimplementing __len__, __contains__, and __iter__?

Given you can't use the mixin implementations of __len__, __contains__, and __iter__ without relying on private implementation details, should they actually be considered mixin implementations? Shouldn't they just be abstract methods given you can't actually use them?

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