用Metaclass实现一个精简的ORM框架

 1 # -*- coding: utf-8 -*-
 2 class Field(object):
 3 
 4     def __init__(self, name, column_type):
 5         self.name = name
 6         self.column_type = column_type
 7 
 8     def __str__(self):
 9         return ‘<%s:%s>‘ % (self.__class__.__name__, self.name)
10 class StringField(Field):
11 
12     def __init__(self, name):
13         super(StringField, self).__init__(name, ‘varchar(100)‘)
14 
15 class IntegerField(Field):
16 
17     def __init__(self, name):
18         super(IntegerField, self).__init__(name, ‘bigint‘)
19 class ModelMetaclass(type):
20 
21     def __new__(cls, name, bases, attrs):
22         if name==‘Model‘:
23             return type.__new__(cls, name, bases, attrs)
24         print(‘Found model: %s‘ % name)
25         mappings = dict()
26         for k, v in attrs.items():
27             if isinstance(v, Field):
28                 print(‘Found mapping: %s ==> %s‘ % (k, v))
29                 mappings[k] = v
30         for k in mappings.keys():
31             attrs.pop(k)
32         attrs[‘__mappings__‘] = mappings # 保存属性和列的映射关系
33         attrs[‘__table__‘] = name # 假设表名和类名一致
34         return type.__new__(cls, name, bases, attrs)
35 class Model(dict, metaclass=ModelMetaclass):
36 
37     def __init__(self, **kw):
38         super(Model, self).__init__(**kw)
39 
40     def __getattr__(self, key):
41         try:
42             return self[key]
43         except KeyError:
44             raise AttributeError(r"‘Model‘ object has no attribute ‘%s‘" % key)
45 
46     def __setattr__(self, key, value):
47         self[key] = value
48 
49     def save(self):
50         fields = []
51         params = []
52         args = []
53         for k, v in self.__mappings__.items():
54             fields.append(v.name)
55             params.append(‘?‘)
56             args.append(getattr(self, k, None))
57         sql = ‘insert into %s (%s) values (%s)‘ % (self.__table__, ‘,‘.join(fields), ‘,‘.join(params))
58         print(‘SQL: %s‘ % sql)
59         print(‘ARGS: %s‘ % str(args))
60 class User(Model):
61     # 定义类的属性到列的映射：
62     id = IntegerField(‘id‘)
63     name = StringField(‘username‘)
64     email = StringField(‘email‘)
65     password = StringField(‘password‘)
66 # 创建一个实例：
67 u = User(id=12345, name=‘Michael‘, email=‘test@orm.org‘, password=‘my-pwd‘)
68 # 保存到数据库：
69 u.save()