[issue34586] collections.ChainMap should have a get_where method

[Zahari Dim]

Zahari Dim <zaharid at gmail.com> added the comment:

>
> I've discussed this with other core devs and spent a good deal of time evaluating this proposal.  I'm going to pass on the this one but do think it was a inspired suggestion.  Thank you for the proposal.

Thank you for taking the time to consider it. I understand that there
are many proposals.

>
> ----------
>
> Note, the original get_where() recipe has an issue.  Upon successful lookup, it returns a 2-tuple but on failure it calls __missing__ which typically returns a scalar (if it doesn't raise an exception).

FWIW this was intended to work when `__missing__` was subclassed to
raise a more specific exception. The case where it is made to return a
value clearly doesn't play well with the proposed method, and would
likely need to be subclassed as well. I consider this an acceptable
trade off because I find this use case rather esoteric: the same
functionality could be achieved in an arguably clearer way by passing
a mapping with the desired missing semantics as the outermost scope.

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<https://bugs.python.org/issue34586>
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