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='[email protected]', 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', '[email protected]', 'my-pwd']
[Finished in 0.2s]