Serhiy is correct.  The exact return type only needs to be ordered, and have appropriate dictionary methods such as `keys()`, `values()`, and `items()`.

The reason a mappingproxy was chosen is exactly because what you just tried is illegal and/or confusing:

- illegal because an Enum cannot be modified that way
- confusing because the dictionary returned is only a copy of the Enum class' __dict__, and successful attempts to modify it would not change the Enum class.

It is an implementation detail because the exact type of dictionary returned could change in the future.