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

>>> class A:
...     pass

>>> a = A()
>>> type(a.__dict__)
<class 'dict'>
>>> type(A.__dict__)
<class 'mappingproxy'>
Asked By: José Luis

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Source

Answers:

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.

Answered By: Raymond Hettinger

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

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'>
Answered By: prosti

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.

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.
Answered By: funnydman