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Thu Jun 23 11:19:44 EDT 2016 · https://mail.python.org/pipermail/python-dev/2016-June/145394.html

On Thu, Jun 23, 2016 at 8:01 AM, Random832 <random832 at fastmail.com> wrote:

> 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".
>

It was a long time when I wrote this, but IIRC the breakage could express
itself as a segfault or other C-level crash due to some internal state
invariant of the type object being violated, not just an exception. The
existence of ctypes notwithstanding, we take C-level crashes very seriously.

-- 
--Guido van Rossum (python.org/~guido)
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[Python-Dev] Why are class dictionaries not accessible?