[Python-ideas] Revisiting Immutable Mappings

[Philip Martin]

Hi, I first want to thank everyone in the community for the contributions
over the years. I know the idea of a frozendict has been proposed before
and rejected. I have a use case for a frozendict implementation that to my
knowledge was not discussed during previous debates. My reasoning for a
frozendict class stems from patterns I typically see arise when performing
ETL or data integrations and conversions. I generally have used
MappingProxyType as a way to set default mapping to a function or to set an
empty mapping to a function. I've created a gist with an example use case:

https://gist.github.com/pmart123/493edf84d9aa61691ca7321325ebb6ab

I've included an example of what code typically looks like when using
MappingProxyType and what it could look like with a
frozendict implementation. I believe this use case may also be
under-reported in open source code as it often crops up when integrating
third-party data sources, which at times can't be open sourced due to
licensing issues. I would love to hear if anyone has used MappingProxyType
in a similar manner, or if this use case could help warrant a frozendict in
the standard library.
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