Python简单使用插件结构 - python报错

def valid_password(pwd=''):
    if len(pwd)<6:
        raise ValueError('Passwords must be at least 6 characters')
    if pwd.isalnum():
        raise ValueError('Passwords must contain at least one special character')
    ...

class PluginMount(type):
    def __init__(cls,name,bases,attrs):
        if not hasattr(cls,'plugins'):
            cls.plugins=[]
        else:
            cls.plugins.append(cls)

class PasswordValidator(object):
    __metaclass__=PluginMount
    def validate(self,pwd):
        raise NotImplementedError()

class MinimumLength(PasswordValidator):
    def validate(self,pwd):
        if len(pwd)<6:
            raise ValueError('Passwords must be at least 6 characters')

class SpecialCharacters(PasswordValidator):
    def validate(self,pwd):
        if pwd.isalnum():
            raise ValueError('Passwords must contain at least one special character')

def valid_password(pwd=''):
    for plugin in PasswordValidator.plugins:
        plugin().validate(pwd)