Python 3 __getattribute__ vs dot access behaviour

Question

I read a bit on python\'s object attribute lookup (here: https://blog.ionelmc.ro/2015/02/09/understanding-python-metaclasses/#object-attribute-lookup).

Seems pretty stra

Answer 1

It's easy to get the impression that __getattribute__ is responsible for more than it really is. thing.attr doesn't directly translate to thing.__getattribute__('attr'), and __getattribute__ is not responsible for calling __getattr__.

The fallback to __getattr__ happens in the part of the attribute access machinery that lies outside __getattribute__. The attribute lookup process works like this:

• Find the __getattribute__ method through a direct search of the object's type's MRO, bypassing the regular attribute lookup process.
• Try __getattribute__.
  • If __getattribute__ returned something, the attribute lookup process is complete, and that's the attribute value.
  • If __getattribute__ raised a non-AttributeError, the attribute lookup process is complete, and the exception propagates out of the lookup.
  • Otherwise, __getattribute__ raised an AttributeError. The lookup continues.
• Find the __getattr__ method the same way we found __getattribute__.
  • If there is no __getattr__, the attribute lookup process is complete, and the AttributeError from __getattribute__ propagates.
• Try __getattr__, and return or raise whatever __getattr__ returns or raises.

At least, in terms of the language semantics, it works like that. In terms of the low-level implementation, some of these steps may be optimized out in cases where they're unnecessary, and there are C hooks like tp_getattro that I haven't described. You don't need to worry about that kind of thing unless you want to dive into the CPython interpreter source code.