[issue23401] Add pickle support of Mapping views

[Guido van Rossum]

Guido van Rossum added the comment:

I find it extremely suspicious that if you pickle the keys of a large object it also pickles that object. These are views, not copied data, for a reason. I also take back that this is okay for concrete dict.

As for your reason (http://bugs.python.org/issue23401#msg267317), that doesn't explain the use case for pickling keys views. If you forbade pickling them would any user code break? I seriously doubt it. Who on earth would want to pickle a keys view on an OrderedDict? It doesn't contain any information other than the underlying object. These views exist as alternate APIs to dicts, not as objects by themselves.

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<http://bugs.python.org/issue23401>
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