Descriptors - The Magic behind python - Raymond Hettinger
What is a descriptor
It is like a magic dot…
Reading the dot, invokes its magic.
http://tinyurl.com/d63d has all the gory details.
Technically
An object that defines __get__, __set__ and __delete__.
Must be put into the class dictionary, not in the instance dictionary. If you put it in the instance dictionary, it will not be invoked. It is only invoked when it is put into the class dictionary.
Sample
desc.py
__get__is invoked when you read the__set__when something is assigned to the object.__del__when the object is deleted?
Notes
Property is a descriptor.
It is trivially easy to write your own variants.
The following two __getattribute__ methods are not the same:
A.x translates to
type.__getattribute__(A, 'x')a.x translates to
object.__getattribute__(a, 'x')
If we override __getattribute__ you can create your own new types
of magic for dotted access.
Every time you see a . think __getattribute__.
Super provides it’s own __getattribute__. It’s special trick is
to search the __mro__ during dotted access.
Functions
Functions are descriptors, running dir(f) shows that functions have
a __get__ methods.
If you put a method in a class dictionary, the __get__ method will
activate upon dotted access.
Slots
When a class is created, space is pre-allocated for each slot.
How python works
Dotted attribute access like A.x or a.x calls the __getattribute__
method.
Let’s make something new
Useful Methods (nothing to do with the talk)
vars(d)
How to describe __
double under
under, under
dunder