[issue11297] Make ChainMap() public in the collections module.

[Alex]

Alex <alex.gaynor at gmail.com> added the comment:

An important distinction with Django's push/pop is that they mutate the Context (ChainMap) rather than return a fresh instance.

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nosy: +alex

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue11297>
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