python - 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'>

Answer 1

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.