I found the official docs on how python special methods are looked up.

https://docs.python.org/3/reference/datamodel.html#special-l...

Apparently the runtime is even more picky than I showed. The method has to be defined on the object's type, not in the object's instance dictionary. So, really the lookup is something like:

  if hasattr(type(a), '__add__'):

The link I provided explains the rationale for bypassing the instance dictionary and `__getattribute__` method.