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gh-152636: Add a note about frozendict to the types.MappingProxyType #152637

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gh-152636: Add a note about frozendict to the types.MappingProxyType #152637

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gh-152636: Add a note that `frozendict` is preferable than `MappingPr…
sobolevn Jun 29, 2026
abd4665
Use Raymonds wording for the doc
sobolevn Jun 30, 2026
4da78f7
Update types.rst
sobolevn Jul 5, 2026
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12 changes: 12 additions & 0 deletions Doc/library/types.rst
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Expand Up @@ -390,6 +390,18 @@ Standard names are defined for the following types:
signifying (respectively) the types of the underlying mapping's keys and
values.

.. note::

There are important differences between ``MappingProxyType``
and :class:`frozendict`.

``MappingProxyType`` is a wrapper around a mapping that renders
the mapping immutable to the viewer of the proxy
without affecting the underlying container.
``MappingProxyType`` can expose its internal mutable mapping in some rare cases.

On the other hand, ``frozendict`` is a concrete immutable mapping.

@picnixz picnixz Jul 5, 2026 •
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AFAIK both MappingProxyType and frozendict are (immutable) mappings if we consider collections.abc.Mapping. So... I don't know whether there is really a need for that note in the end. "concrete immutable mapping" applies to both of them. I'm sorry for being nitpicky though but I agree with Inada here that this note could be more confusing depending on who reads it.

We already say

   Read-only proxy of a mapping. It provides a dynamic view on the mapping's
   entries, which means that when the mapping changes, the view reflects these
   changes.

I would just say (if I were to change the docs, but that's not even sure)

   Read-only proxy of a mapping. It provides a dynamic view on the mapping's
   entries, which means that when the mapping changes, the view reflects these
   changes.

   Use :class:`frozendict` if you want an immutable shallow copy of a mapping.


.. versionadded:: 3.3

.. versionchanged:: 3.9
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