TL;DR
MappingProxyType is a read only proxy for mapping (e.g. dict) objects.
frozendict is an immutable dict
Answer
The proxy pattern is (quoting wikipedia):
A proxy, in its most general form, is a class functioning as an
interface to something else.
The MappingProxyType is just a simple proxy (i.e. interface) to access the real object (the real map, which on our example is dict).
the suggested frozendict object is just as set is to frozenset. a read only (immutable) object which can only be changed upon creation.
So why do we need MappingProxyType? example use case is where you want to pass a dictionary to another function but without it able to change your dictionary, it act as a read only proxy, (quoting python docs):
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.
lets see some example usage of the MappingProxyType
In [1]: from types import MappingProxyType
In [2]: d = {'a': 1, 'b': 2}
In [3]: m = MappingProxyType(d)
In [4]: m['a']
Out[4]: 1
In [5]: m['a'] = 5
TypeError: 'mappingproxy' object does not support item assignment
In [6]: d['a'] = 42
In [7]: m['a']
Out[7]: 42
In [8]: for i in m.items():
...: print(i)
('a', 42)
('b', 2)
Update:
because the PEP did not make it into python, we cannot know for sure what the implementation that would be.
by looking at the PEP we see that:
frozendict({'a': {'b': 1}})
would raise an exception as {'b': 1} is not hashable value, but on your implementation it will create the object. of-course, you can add a validation for the value as noted on the PEP.
I assume part of the PEP was memory optimization and implementation of this kind of frozendict could have benefit from the performance of dict comparison using the __hash__ implementation.