第923页 | Learning Python | 阅读 ‧ 电子书库

Class Statement Protocol

Subclassing the type class to customize it is really only half of the magic behind metaclasses. We still need to somehow route a class’s creation to the metaclass, instead of the default type. To fully understand how this is arranged, we also need to know how class statements do their business.

We’ve already learned that when Python reaches a class statement, it runs its nested block of code to create its attributes—all the names assigned at the top level of the nested code block generate attributes in the resulting class object. These names are usually method functions created by nested defs, but they can also be arbitrary attributes assigned to create class data shared by all instances.

Technically speaking, Python follows a standard protocol to make this happen: at the end of a class statement, and after running all its nested code in a namespace dictionary, it calls the type object to create the class object:

class = type(classname, superclasses, attributedict)

The type object in turn defines a __call__ operator overloading method that runs two other methods when the type object is called:

type.__new__(typeclass, classname, superclasses, attributedict)
type.__init__(class, classname, superclasses, attributedict)

The __new__ method creates and returns the new class object, and then the __init__ method initializes the newly created object. As we’ll see in a moment, these are the hooks that metaclass subclasses of type generally use to customize classes.

For example, given a class definition like the following:

class Spam(Eggs):                # Inherits from Eggs
    data = 1                     # Class data attribute
    def meth(self, arg):         # Class method attribute
        pass

Python will internally run the nested code block to create two attributes of the class (data and meth), and then call the type object to generate the class object at the end of the class statement:

Spam = type('Spam', (Eggs,), {'data': 1, 'meth': meth, '__module__': '__main__'})

Because this call is made at the end of the class statement, it’s an ideal hook for augmenting or otherwise processing a class. The trick lies in replacing type with a custom subclass that will intercept this call. The next section shows how.