CONSTRUCT - Adding Functionality to the Overall System

Georg Brandl g.brandl-nospam at
Tue Sep 19 02:22:41 EDT 2006

Damjan wrote:
>>> I understand that I can use __metaclass__ to create a class which
>>> modifies the behaviour of another class.
>>> How can I add this metaclass to *all* classes in the system?
>>> (In ruby I would alter the "Class" class)
>> You'd have to set
>> __metaclass__ = whatever
>> at the top of each module whose classes are to get the new behavior.
> I think '__metaclass__ = whatever' affects only the creation of classes that
> would otherwise be old-style classes?


If you set __metaclass__ = type, every class in that module will be new-style.

If you set __metaclass__ = MyClass, and MyClass inherits from <type>, every
class in that module will be new-style and have MyClass as a metaclass.

The usual way to create new-style classes, inheriting from object or another
new-style class, works because if no __metaclass__ is defined, the first
base class's class is taken as the metaclass.


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