Why is a class __dict__ a mappingproxy?

Question

I wonder why a class __dict__ is a mappingproxy, but an instance __dict__ is just a plain dict

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Answer 1

Since Python 3.3 mappingproxy type was renamed from dictproxy. There was an interesting discussion on this topic.

It's a little bit hard to find the documentation for this type, but the documentation for vars method describes this perfectly (though it wasn't documented for a while):

Objects such as modules and instances have an updateable __dict__ attribute; however, other objects may have write restrictions on their __dict__ attributes (for example, classes use a types.MappingProxyType to prevent direct dictionary updates).

If you need to assign a new class attribute you could use setattr. It worth to note that mappingproxy is not JSON serializable, check out the issue to understand why.

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Also the history of this type is a quite interesting:

• Python 2.7: type(A.__dict__) returns as type(dict()), and it's possible to assign new attributes through __dict__, e.g. A.__dict__['foo'] = 'bar'.

• Python 3.0 - 3.2: type(A.__dict__) returns , the difference now a little bit more clear. Trying to assign a new attribte gives TypeError. There was an attempt to add dictproxy as a public builtin type.

• Python 3.3: adds type, that described above.