What is an
Before Python 2.2 and "new-style" classes, the
metaclass provided features now found in Python itself.
Nowadays, it mainly provides three features:
Support for a class initialiser. Classes deriving from
ExtensionClass.Basecan define a method
__class_init__(self), which is called when the class is initialised (usually at module import time). Note that
selfhere is the class object, not an instance of the class.
Ensuring that any class that has
ExtensionClass.Baseas a base class.
inheritedAttributemethod, which acts a lot like
super()and is hence superfluous except for in legacy code.
The base class
protocol that is used by acquisition. It is similar to the
hook used in Python descriptors, except that
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
in new code, though when deriving from
Acquisition.Explicit, it will be included as a base class of those classes.
How does acquisition work?¶