types.MappingProxyType states that proxy[key] will throw a KeyError, but that is not always true

Documentation

In the documentation, it states proxy[key] will:

Return the item of the underlying mapping with key key. Raises a KeyError if key is not in the underlying mapping.

https://docs.python.org/3/library/types.html#types.MappingProxyType

However, it is possible to get an AttributeError instead, and in fact, unit tests for the dataclasses module require this to be true (metadata in dataclasses.field is wrapped in a mapping proxy):

https://github.com/python/cpython/blob/5f4c7cf3f49629bcf890c305e0a90ffd708110a2/Lib/test/test_dataclasses/__init__.py#L2065-L2066

The unit tests above are from Python3.12.

Looks like the CPython source code only forwards the getitem call to the inner mapping:

https://github.com/python/cpython/blob/5f4c7cf3f49629bcf890c305e0a90ffd708110a2/Objects/descrobject.c#L1042-L1046

Inizia dalla documentazione di MappingProxyType nella pagina collegata della documentazione Python, quindi confrontane la formulazione con i test dataclasses indicati e con l’implementazione in Objects/descrobject.c. Il lavoro è concluso quando la documentazione descrive accuratamente il comportamento delle eccezioni dimostrato dai test e dall’implementazione.