元类
- 类就是一组用来描述如何生成一个对象的代码段
- 类也是对象，你可以在运行时动态的创建它们，就像其他任何对象一样
- type还有一种完全不同的功能，动态的创建类
type可以接收一个类的描述作为参数，然后返回一个类
type(类名，由父类名称组成的元组（针对继承的情况，可以为空)，包含属性的字典（名称和值））
- 元类就是用来创建类的"东西" ; 元类就是类的类
- Python中所有的东西，注意我是指所有的东西--都是对象。这包括整数、字符串、函数以及类
-
metaclass属性
Python会通过 __metaclass__创建一个名字为Foo的类（对象）
ORM
- ORM是Python编程语言后端web框架Django的核心思想，"Object Relational Mapping"，即对象-关系映射，简称ORM
#ORM实现原理
class ModelMetaclass(type):
def __new__(cls, name, bases, attrs):
# name="User"
# bases=(object,)
# attrs = {
# "uid": ('uid', "int unsigned"),
# "name": ('username', "varchar(30)"),
# "email": ('email', "varchar(30)"),
# "password": ('password', "varchar(30)")
# "save": xxxxxx,
# "__init__": xxxxxx2
# }
mappings = dict()
# mappings = {
# "uid": ('uid', "int unsigned"),
# "name": ('username', "varchar(30)"),
# "email": ('email', "varchar(30)"),
# "password": ('password', "varchar(30)")
# }
# 判断是否需要保存
for k, v in attrs.items():
# 判断是否是元组类型
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) # del attrs[k]
# attrs = {
# "save": xxxxxx,
# "__init__": xxxxxx2，
# "__mappings__": {
# "uid": ('uid', "int unsigned"),
# "name": ('username', "varchar(30)"),
# "email": ('email', "varchar(30)"),
# "password": ('password', "varchar(30)")
# },
# "__table__": "User"
# }
# 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 User(metaclass=ModelMetaclass):
uid = ('uid', "int unsigned")
name = ('username', "varchar(30)")
email = ('email', "varchar(30)")
password = ('password', "varchar(30)")
# 当指定元类之后，以上的类属性将不在类中，而是在__mappings__属性指定的字典中存储
# 以上User类中有
# __mappings__ = {
# "uid": ('uid', "int unsigned")
# "name": ('username', "varchar(30)")
# "email": ('email', "varchar(30)")
# "password": ('password', "varchar(30)")
# }
# __table__ = "User"
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))
sql = 'insert into %s (%s) values (%s)' % (self.__table__, ','.join(fields), ','.join([str(i) for i in args]))
print('SQL: %s' % sql)
u = User(uid=12345, name='Michael', email='test@orm.org', password='my-pwd')
# print(u.__dict__)
u.save()
