[Python-Dev] Why are class dictionaries not accessible?

[Random832]

On Wed, Jun 22, 2016, at 11:11, Guido van Rossum wrote:
> This is done in order to force all mutations of the class dict to go
> through attribute assignments on the class. The latter takes care of
> updating the class struct, e.g. if you were to add an `__add__` method
> dynamically it would update tp_as_number->nb_add. If you could modify the
> dict object directly it would be more difficult to arrange for this side
> effect.

Why is this different from the fact that updating a normal object's dict
bypasses descriptors and any special logic in __setattr__? Dunder
methods are already "special" in the sense that you can't use them as
object attributes; I wouldn't be surprised by "assigning a dunder method
via the class's dict breaks things".