首页 > 代码库 > 关于廖雪峰提到的元类的应用实例的解释

关于廖雪峰提到的元类的应用实例的解释

class Field(object):

    def __init__(self, name, column_type):
        self.name = name
        self.column_type = column_type

    def __str__(self):
        return <%s:%s % (self.__class__.__name__, self.name)

class StringField(Field):

    def __init__(self, name):
        super(StringField, self).__init__(name, varchar(100))

class IntegerField(Field):

    def __init__(self, name):
        super(IntegerField, self).__init__(name, bigint)

class ModelMetaclass(type):

    def __new__(cls, name, bases, attrs):   #传入的是类的属性
        if name==Model:
           return type.__new__(cls, name, bases, attrs)
        print(attrs)
        print(Found model: %s % name)
        mappings = dict()

        for k, v in attrs.items():
            if isinstance(v, Field):
                print(Found mapping: %s ==> %s % (k, v))
                mappings[k] = v
        for k in mappings.keys():
            attrs.pop(k)
        attrs[__mappings__] = mappings # 保存属性和列的映射关系
        attrs[__table__] = name # 假设表名和类名一致
        return type.__new__(cls, name, bases, attrs)

class Model(dict, metaclass=ModelMetaclass):

    def __init__(self, **kw):
        super(Model, self).__init__(**kw)

    def __getattr__(self, key):
        try:
            return self[key]
        except KeyError:
            raise AttributeError(r"‘Model‘ object has no attribute ‘%s‘" % key)

    def __setattr__(self, key, value):
        self[key] = value

    def save(self):
        fields = []
        params = []
        args = []
        for k, v in self.__mappings__.items():
            fields.append(v.name)
            params.append(?)
            args.append(getattr(self, k, None))
        sql = insert into %s (%s) values (%s) % (self.__table__, ,.join(fields), ,.join(params))
        print(SQL: %s % sql)
        print(ARGS: %s % str(args))

class User(Model):
    # 定义类的属性到列的映射:
    id = IntegerField(id)
    name = StringField(username)
    email = StringField(email)
    password = StringField(password)


u = User(id=12345, name=Michael, email=test@orm.org, password=my-pwd,sdfjojf=sdfe)
u.save()

当创建实例时,需要调用User类,首先会检查有没有__metaclass__属性,找到了,通过metalclass创建类(基于User类,传入的name为User,父类为Model,属性为User类的属性)。之后在实例化过程中,需要调用创建的新的User类中的方法,如果没有,需要到父类(metalclass创建新类会传承User类的父类)中查找,比如说之后会调用__init__方法,之后又调用save方法。

程序来源:

http://www.liaoxuefeng.com/wiki/0014316089557264a6b348958f449949df42a6d3a2e542c000/0014319106919344c4ef8b1e04c48778bb45796e0335839000

关于廖雪峰提到的元类的应用实例的解释