Python metaclass

Python metaclass

简介

python中的类可以由type来动态的创建

type(name,bases,dct)

• name: 将要被创建的class的名字
• bases: 指定将要被创建类的夫类
• dct: 类中的属性

metaclass 为python中的语法糖, 目的在于让python能够对于创建类进行操作和修改, 可以写出非常漂亮简洁的代码

1. 修改子类中的属性

class InterfaceMeta(type):
    def __new__(cls, name, parents, dct):
        # create a class_id if it's not specified
        if 'class_id' not in dct:
            dct['class_id'] = name.lower()

        # open the specified file for writing
        if 'file' in dct:
            filename = dct['file']
            dct['file'] = open(filename, 'w')

        # we need to call type.__new__ to complete the initialization
        return super(InterfaceMeta, cls).__new__(cls, name, parents, dct)

2. 注册所有基础自基类的子类

假设: 现在给定一个与数据库相关接口的基类, 它的子类实现了自己与数据库相关的接口, 当定义子类之后需要自动的注册在基类的注册中心中.

元类

class DBInterfaceMeta(type):
    # we use __init__ rather than __new__ here because we want
    # to modify attributes of the class *after* they have been
    # created
    def __init__(cls, name, bases, dct):
        if not hasattr(cls, 'registry'):
            # this is the base class.  Create an empty registry
            cls.registry = {}
        else:
            # this is a derived class.  Add cls to the registry
            interface_id = name.lower()
            cls.registry[interface_id] = cls

        super(DBInterfaceMeta, cls).__init__(name, bases, dct)

基类

class DBInterface(object):
    __metaclass__ = DBInterfaceMeta

print(DBInterface.registry)

子类

class FirstInterface(DBInterface):
    pass

class SecondInterface(DBInterface):
    pass

class SecondInterfaceModified(SecondInterface):
    pass

print(DBInterface.registry)

Reference

primer on python metaclass