深夜问道 Python 题，实在没想明白。求指点!

下面是廖雪峰 python 教程中的编写一个简易 ORM 框架的例子，附上源代码

#!/usr/bin/env python3
# -*- coding: utf-8 -*-

' Simple ORM using metaclass '

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))

# testing code:

class User(Model):
    id = IntegerField('id')
    name = StringField('username')
    email = StringField('email')
    password = StringField('password')

u = User(id=12345, name='Michael', email='[email protected]', password='my-pwd')
u.save()

其中编写元类时，下面这句不太理解：

for k in mappings.keys():
	attrs.pop(k)

为什么要在找到符合条件的 key-value 对并将其放入新的字典 mapping 后，还要将这些 key-value 对从 attrs 字典中去除？ 我试验了一下，如果不这么做，最后的 ARGS 打印结果是

ARGS: [<__main__.IntegerField object at 0x10cf63630>, <__main__.StringField object at 0x10cf636d8>, <__main__.StringField object at 0x10cf63668>, <__main__.StringField object at 0x10cf636a0>]

而不是创建实例 u 是传入的参数的值，即

ARGS: ['my-pwd', '[email protected]', 'Michael', 12345]

按我的理解，就算不执行 pop 操作，在创建实例 u 后，当执行 u.save()到

args.append(getattr(self, k, None))

这一步时，实例 u 的属性应该会自动覆盖类 User 的属性，按这个道理， getattr(self,k,None)返回的应该是传入的参数的值，即 my-pwd', '[email protected]', 'Michael', 12345 这些。 所以，实在没想通，到底为什么要执行元类中的 pop 操作。