Codecov Report

Merging #619 into master will increase coverage by 0.12%.
The diff coverage is 42.18%.

@@            Coverage Diff             @@
##           master     #619      +/-   ##
==========================================
+ Coverage      41%   41.12%   +0.12%     
==========================================
  Files          76       76              
  Lines       17184    17207      +23     
  Branches     4514     4508       -6     
==========================================
+ Hits         7046     7077      +31     
- Misses       8140     8177      +37     
+ Partials     1998     1953      -45

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Are member descriptor and data descriptor actually different things or were member descriptors just missing the set (and hence not working correctly in this case)?

They are different, member descriptors shouldn't have __set__ as the presence of it affects the lookup.

In CPython:

>>> type(object.__dict__['__class__'])
<class 'getset_descriptor'>
>>> type(type.__dict__['__mro__'])
<class 'member_descriptor'>

I'm been thinking of a nicer way to do this in rust, instead of having different new_xxx_descriptor methods, instead a builder, which would statically work out the python type needed.

vm.new_property()
     .add_getter(...)
     .add_setter(...)
     .build()

Great.

The builder sounds nice, probably reasonable to add that later and merge this as is.