#coding:utf-8
#一、首先来定义Field类,它负责保存数据库表的字段名和字段类型:
class Field:
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')
#二、定义元类,控制Model对象的创建
class ModelMetaclass(type):
'''定义元类
定义:创建类的类
优点: 对子类操作,功能像装饰器一样可以动态定制和修改被装饰的类
缺点:得程序代码的维护变得困难
实际应用:创建API
前景:
- Python中的类可以生成一个对象(类的能力)(只要使用关键词class,Python解释器在执行的时候就会创建一个对象)
- 同时python的类也是一个对象
- 可以增加属性
- 赋值给一个变量
- 等
动态的创建类
MyShinyClass = type('MyShinyClass', (), {}) # 返回一个类对象
print MyShinyClass
'''
def __new__(cls, name, bases, attrs):
if name=='Model':
return super(ModelMetaclass,cls).__new__(cls, name, bases, attrs)
mappings = dict()
# for k, v in attrs.iteritems():
for k, v in attrs.items():
# 保存类属性和列的映射关系到mappings字典
if isinstance(v, Field):
print('Found mapping: %s==>%s' % (k, v))
mappings[k] = v
# for k in mappings.iterkeys():
for k in mappings.keys():
#将类属性移除,使定义的类字段不污染User类属性,只在实例中可以访问这些key
attrs.pop(k)
attrs['__table__'] = name.lower() # 假设表名和为类名的小写,创建类时添加一个__table__类属性
attrs['__mappings__'] = mappings # 保存属性和列的映射关系,创建类时添加一个__mappings__类属性
return super(ModelMetaclass,cls).__new__(cls, name, bases, attrs)
#三、编写Model基类
class Model(dict,metaclass=ModelMetaclass):
"""
定义了__metaclass__就定义了这个类的元类
类反射的四个基本函数
hasattr
getattr
setattr
delattr
反射动态获取模块的方法名
_getattr_
_delattr_
_setattr_
"""
# __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):
print("开始执行 setattr -- ")
# self[key] = value
self.__dict__[key] = value
print("已经设置了属性")
def save(self):
fields = []
params = []
args = []
# for k, v in self.__mappings__.iteritems():
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))
#最后,我们使用定义好的ORM接口,使用起来非常的简单。
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')
# 创建一个实例:
u = User(id=12345, name='Michael', email='test@orm.org', password='my-pwd')
# 保存到数据库:
u.save()
#输出
# Found mapping: email==><StringField:email>
# Found mapping: password==><StringField:password>
# Found mapping: id==><IntegerField:id>
# Found mapping: name==><StringField:username>
# SQL: insert into User (password,email,username,id) values (?,?,?,?)
# ARGS: ['my-pwd', 'test@orm.org', 'Michael', 12345]