Why is a class __dict__ a mappingproxy?

A mappingproxy is simply a dict with no __setattr__ method.

You can check out and refer to this code.

from types import MappingProxyType
d={'key': "value"}
m = MappingProxyType(d)
print(type(m)) # <class 'mappingproxy'>

m['key']='new' #TypeError: 'mappingproxy' object does not support item assignment

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mappingproxy is since Python 3.3. The following code shows dict types:

class C:pass
ci=C()
print(type(C.__dict__)) #<class 'mappingproxy'>
print(type(ci.__dict__)) #<class 'dict'>

This helps the interpreter assure that the keys for class-level attributes and methods can only be strings.

Elsewhere, Python is a "consenting adults language", meaning that dicts for objects are exposed and mutable by the user. However, in the case of class-level attributes and methods for classes, if we can guarantee that the keys are strings, we can simplify and speed-up the common case code for attribute and method lookup at the class-level. In particular, the __mro__ search logic for new-style classes is simplified and sped-up by assuming the class dict keys are strings.

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 <type 'dict'> 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 <class 'dict_proxy'>, the difference is introduced. Trying to assign a new attribte gives TypeError. There was an attempt to add dictproxy as a public built-in type.
• Python 3.3: adds the <class 'mappingproxy'> type described above.