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Class Decorators and Metaclasses

Function decorators turned out to be so useful that Python 2.6 and 3.0 expanded the model, allowing decorators to be applied to classes as well as functions. In short, class decorators are similar to function decorators, but they are run at the end of a class statement to rebind a class name to a callable. As such, they can be used to either manage classes just after they are created, or insert a layer of wrapper logic to manage instances when they are later created. Symbolically, the code structure:

def decorator(aClass): ...

@decorator
class C: ...

is mapped to the following equivalent:

def decorator(aClass): ...

class C: ...
C = decorator(C)

The class decorator is free to augment the class itself, or return an object that intercepts later instance construction calls. For instance, in the example in the section Counting instances per class with class methods, we could use this hook to automatically augment the classes with instance counters and any other data required:

def count(aClass):
    aClass.numInstances = 0
    return aClass                  # Return class itself, instead of a wrapper

@count
class Spam: ...                    # Same as Spam = count(Spam)

@count
class Sub(Spam): ...               # numInstances = 0 not needed here

@count
class Other(Spam): ...

Metaclasses are a similarly advanced class-based tool whose roles often intersect with those of class decorators. They provide an alternate model, which routes the creation of a class object to a subclass of the top-level type class, at the conclusion of a class statement:

class Meta(type):
    def __new__(meta, classname, supers, classdict): ...

class C(metaclass=Meta): ...

In Python 2.6, the effect is the same, but the coding differs—use a class attribute instead of a keyword argument in the class header:

class C:
    __metaclass__ = Meta
    ...

The metaclass generally redefines the __new__ or __init__ method of the type class, in order to assume control of the creation or initialization of a new class object. The net effect, as with class decorators, is to define code to be run automatically at class creation time. Both schemes are free to augment a class or return an arbitrary object to replace it—a protocol with almost limitless class-based possibilities.