The ExtensionClass

Description

What is ExtensionClass.Base used for?

What is an ExtensionClass?

Before Python 2.2 and "new-style" classes, the ExtensionClass.ExtensionClass metaclass provided features now found in Python itself. Nowadays, it mainly provides three features:

  • Support for a class initialiser. Classes deriving from ExtensionClass.Base can define a method __class_init__(self), which is called when the class is initialised (usually at module import time). Note that self here is the class object, not an instance of the class.
  • Ensuring that any class that has ExtensionClass as a __metaclass__ implicitly gets ExtensionClass.Base as a base class.
  • Providing an inheritedAttribute method, which acts a lot like super() and is hence superfluous except for in legacy code.

The base class ExtensionClass.Base provides the __of__ protocol that is used by acquisition. It is similar to the __get__ hook used in Python descriptors, except that __of__ is called when an implementor is retrieved from an instance as well as from a class. Here is an example: 

>>> from ExtensionClass import Base
>>> class Container(Base):
...     pass

>>> class Item(Base):
...     def __init__(self):
...         self.visited = []
...     def __of__(self, parent):
...         self.visited.append(parent)
...         return self

>>> container = Container()
>>> item = Item()
>>> item.visited
[]
>>> container.item1 = item
>>> item.visited
[]
>>> container.item1
<__main__.O object at 0x10cc0ddd0>
>>> item.visited
[<__main__.C object at 0x10cc0dc90>]

>>> container.item1 # again
<__main__.O object at 0x10cc0ddd0>
>>> item.visited
[<__main__.C object at 0x10cc0dc90>, <__main__.C object at 0x10cc0dc90>]

There is probably little reason to use ExtensionClass.Base in new code, though when deriving from Acquisition.Implicit or Acquisition.Explicit, it will be included as a base class of those classes.