python 元类编码模式

元类所有的类都是由元类(type)创建的,元类负责创建类，类负责创建对象
元类方法

type(类名, 父类的元组（针对继承的情况，可以为空），包含属性的字典（名称和值）)

class Foo(object):
    bar = True
#使用type构建
Foo = type('Foo', (), {'bar':True})

 

class UpparAttrMetaClass(type):
    def __new__(cls, class_name, class_parent, class_attr):     # 重写父类的new方法
        new_attr = dict()
        for name,value in class_attr.items():
            if not name.startswith("__"):
                new_attr[name.upper()] = value

        return type.__new__(cls,class_name,class_parent,new_attr)

class Foo(metaclass=UpparAttrMetaClass):   # 你可以在写一个类的时候为其添加__metaclass__属性,定义了__metaclass__就定义了这个类的元类,类的元类可以是一个类,也可以是一个方法
    bar = "zip"

# print(hasattr(Foo, 'bar'))
# print(hasattr(Foo, 'BAR'))
if hasattr(Foo, "BAR"):
    f = Foo()
    print(f.BAR)
    # print("11111")

 

ORM框架的演变以及由来

class ModelMetaclass(type):
    def __new__(cls, name, bases, attrs):
        mappings = dict()
        # 判断是否需要保存
        for k, v in attrs.items():
            # 判断是否是指定的StringField或者IntegerField的实例对象
            if isinstance(v, tuple):
                print('Found mapping: %s ==> %s' % (k, v))
                mappings[k] = v

        # 删除这些已经在字典中存储的属性
        for k in mappings.keys():
            attrs.pop(k)

        # 将之前的uid/name/email/password以及对应的对象引用、类名字
        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(object, metaclass=ModelMetaclass):
    def __init__(self, **kwargs):
        for name, value in kwargs.items():
            setattr(self, name, value)

    def save(self):
        fields = []
        args = []
        for k, v in self.__mappings__.items():
            fields.append(v[0])
            args.append(getattr(self, k, None))

        args_temp = list()
        for temp in args:
            # 判断入如果是数字类型
            if isinstance(temp, int):
                args_temp.append(str(temp))
            elif isinstance(temp, str):
                args_temp.append("""'%s'""" % temp)
        sql = 'insert into %s (%s) values (%s)' % (self.__table__, ','.join(fields), ','.join(args_temp))
        print('SQL: %s' % sql)


class User(Model):
    uid = ('uid', "int unsigned")
    name = ('username', "varchar(30)")
    email = ('email', "varchar(30)")
    password = ('password', "varchar(30)")


u = User(uid=12345, name='Michael', email='test@orm.org', password='my-pwd')
# print(u.__dict__)
u.save()

 

详细的orm实现