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On Thu, Jun 23, 2016 at 8:01 AM, Random832 <random832@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.