第926页 | Learning Python | 阅读 ‧ 电子书库

A Basic Metaclass

Perhaps the simplest metaclass you can code is simply a subclass of type with a __new__ method that creates the class object by running the default version in type. A metaclass __new__ like this is run by the __call__ method inherited from type; it typically performs whatever customization is required and calls the type superclass’s __new__ method to create and return the new class object:

class Meta(type):
    def __new__(meta, classname, supers, classdict):
        # Run by inherited type.__call__
        return type.__new__(meta, classname, supers, classdict)

This metaclass doesn’t really do anything (we might as well let the default type class create the class), but it demonstrates the way a metaclass taps into the metaclass hook to customize—because the metaclass is called at the end of a class statement, and because the type object’s __call__ dispatches to the __new__ and __init__ methods, code we provide in these methods can manage all the classes created from the metaclass.

Here’s our example in action again, with prints added to the metaclass and the file at large to trace:

class MetaOne(type):
    def __new__(meta, classname, supers, classdict):
        print('In MetaOne.new:', classname, supers, classdict, sep='\n...')
        return type.__new__(meta, classname, supers, classdict)

class Eggs:
    pass

print('making class')
class Spam(Eggs, metaclass=MetaOne):      # Inherits from Eggs, instance of Meta
    data = 1                              # Class data attribute
    def meth(self, arg):                  # Class method attribute
        pass

print('making instance')
X = Spam()
print('data:', X.data)

Here, Spam inherits from Eggs and is an instance of MetaOne, but X is an instance of and inherits from Spam. When this code is run with Python 3.0, notice how the metaclass is invoked at the end of the class statement, before we ever make an instance—metaclasses are for processing classes, and classes are for processing instances:

making class
In MetaOne.new:
...Spam
...(<class '__main__.Eggs'>,)
...{'__module__': '__main__', 'data': 1, 'meth': <function meth at 0x02AEBA08>}
making instance
data: 1