用Metaclass实现一个精简的ORM框架

存档：

 # -*- 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):
         if name=='Model':
             return type.__new__(cls, name, bases, 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')
 # 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()

结果如下：

Found model: User
Found mapping: id ==> <IntegerField:id>
Found mapping: name ==> <StringField:username>
Found mapping: email ==> <StringField:email>
Found mapping: password ==> <StringField:password>
SQL: insert into User (id,username,email,password) values (?,?,?,?)
ARGS: [12345, 'Michael', 'test@orm.org', 'my-pwd']
[Finished in 0.2s]