python meta type abc

python-metaclass.py

handlers = {}


class CustomMetaclass(type):
    def __new__(mcs, name, bases, attrs):
        cls = type.__new__(mcs, name, bases, attrs)
        for ext in attrs["formats"]:
            handlers[ext] = cls
        return cls


class Handler(metaclass=CustomMetaclass):
    formats = []
    # common stuff for all kinds of file format handlers


class ImageHandler(Handler):
    formats = ["jpeg", "png"]


class AudioHandler(Handler):
    formats = ["mp3", "wav"]
#
print(handlers)
# {'mp3': __main__.AudioHandler,
#  'jpeg': __main__.ImageHandler,
#  'png': __main__.ImageHandler,
#  'wav': __main__.AudioHandler}


python解释器扫描模块，实例化类；
实例化类是利用元类实例化；
利用type可以实例化一个类；
可以干预一个类的创建过程，修改他的元类即可；
元类有一个__new__方法，

mcs, name, bases, attrs
类，类名，父类，类的属性（__dict__）