Alex Hayes added the comment:
Some background. I'm not actually trying to pickle MappingProxyType, I'm using
the schematics [1] library and recently it broke support for pickle [2] and the
cause of that is because they are using MappingProxyType [3] internally (when
they used to use dict).
The Python docs on MappingProxyType [4] provides an appropriate level of API
documentation but what it doesn't cover is when you should or should not use
MappingProxyType (understandably, it's most likely a complex answer). However,
I imagine that it's being used in schematics because at the bottom of the
Python dict documentation [5] it states;
> See also types.MappingProxyType can be used to create a read-only view of a
> dict.
To answer the question "What would be the point of pickling a MappingProxy?" I
would probably have to ask, what is the intended use of MappingProxyType?
If one was unfamiliar with the way in which pickle and MappingProxyType works I
imagine they would think there would be no reason why a "read-only view of a
dict" can't be pickled.
So I guess my questions now are;
1. Should types.MappingProxyType be pickleable?
2. Should there be some documentation about the intended use of
MappingProxyType?
I imagine the answer to the first question is "no" given how `__setstate__`
works.
[1] https://github.com/schematics/schematics
[2] https://github.com/schematics/schematics/issues/510
[3] https://github.com/schematics/schematics/pull/511
[4] https://docs.python.org/3/library/types.html#types.MappingProxyType
[5] https://docs.python.org/3/library/stdtypes.html#dict
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