Python: the __getattribute__ method and descriptors

Question

according to this guide on python descriptors https://docs.python.org/howto/descriptor.html

method objects in new style classes are implemented using descriptors in

Answer 1

Actually, in CPython the default __getattribute__ implementation is not a Python method, but is instead implemented in C. It can access object slots (entries in the C structure representing Python objects) directly, without bothering to go through the pesky attribute access routine.

Just because your Python code has to do this, doesn't mean the C code has to. :-)

If you do implement a Python __getattribute__ method, just use object.__getattribute__(self, attrname), or better still, super().__getattribute__(attrname) to access attributes on self. That way you won't hit recursion either.

In the CPython implementation, the attribute access is actually handled by the tp_getattro slot in the C type object, with a fallback to the tp_getattr slot.

To be exhaustive and to fully expose what the C code does, when you use attribute access on an instance, here is the full set of functions called:

• Python translates attribute access to a call to the PyObject_GetAttr() C function. The implementation for that function looks up the tp_getattro or tp_getattr slot for your class.

• The object type has filled the tp_getattro slot with the PyObject_GenericGetAttr function, which delegates the call to _PyObject_GenericGetAttrWithDict (with the *dict pointer set to NULL and the suppress argument set to 0). This function is your object.__getattribute__ method (a special table maps between the name and the slots).

• This _PyObject_GenericGetAttrWithDict function can access the instance __dict__ object through the tp_dict slot, but for descriptors (including methods), the _PyType_Lookup function is used.

• _PyType_Lookup handles caching and delegates to find_name_in_mro on cache misses; the latter looks up attributes on the class (and superclasses). The code uses direct pointers to the tp_dict slot on each class in the MRO to reference class attributes.

• If a descriptor is found by _PyType_Lookup it is returned to _PyObject_GenericGetAttrWithDict and it calls the tp_descr_get function on that object (the __get__ hook).

When you access an attribute on the class itself, instead of _PyObject_GenericGetAttrWithDict, the type->tp_getattro slot is instead serviced by the type_getattro() function, which takes metaclasses into account too. This version calls __get__ too, but leaves the instance parameter set to None.

Nowhere does this code have to recursively call __getattribute__ to access the __dict__ attribute, as it can simply reach into the C structures directly.