[Python-ideas] Introduce collections.Reiterable

[Stephen J. Turnbull]

Neil Girdhar writes:

 > If infinite sequences are so common, it might be better to add a
 > collections.abc for them.

I suspect this falls under the "not every 3-line function" clause,
because it would really require a PEP to get right (changes to
builtins like list and dict would be needed, IIUC).

Just inherit from Sequence and add a __len__ which returns a unique
object (probably could be None, actually), and check for that private
protocol yourself.

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