metaclass 常用方式

一个类作为metaclass的时候，我们需要重写它的__new__方法，这个方法的参数包括要创建class object的 metaclass,类名，父类集合，类成员

class MyMetaclass(type):
	def __new__(metaclz,name,bases,attrs):
		print("create new class "，metaclz,name)
		return type.__new__(metaclz, name, bases, attrs)

　　这样在调用__new__的时候，就给了我们一些发挥的空间了，一般我们可以修改bases、attrs中的成员，或则根据这自己定义的一些规则做一些特别的事情。

     比如，可以利用这个来写测试框架、代码规则检测，装饰类的方法，写ORM框架等等。

 　 

    ORM框架大致demo:

    

# -*- coding:utf-8 -*- 

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):
    	#一般的ORM框架这里应该通过bases来处理
        if name=='Model':
            return type.__new__(cls, name, bases, attrs)
        mappings = dict()

        #取出 数据库中的字段
        for k, v in attrs.iteritems():
            if isinstance(v, Field):
                print('Found mapping: %s==>%s' % (k, v))
                mappings[k] = v
        #删除相关的类成员
        for k in mappings.iterkeys():
            attrs.pop(k)
        attrs['__table__'] = name # 假设表名和类名一致
        attrs['__mappings__'] = mappings # 保存属性和列的映射关系
        return type.__new__(cls, name, bases, attrs)

class Model(dict):
    __metaclass__ = ModelMetaclass

    def __init__(self, **kw):
    	print("kw:",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__.iteritems():
            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',zz='4555')
u.save()