[issue21991] The new email API should use MappingProxyType instead of returning new dicts.

[R. David Murray]

R. David Murray added the comment:

The principle example is the 'params' dictionary in headerregistry.  Currently it gets recreated every time you access that attribute.  You can *apparently* change it, but that has no real effect.  Probably the computed value should be cached the first time the attribute is accessed, and a MappingProxy over the cached value returned.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue21991>
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