Python：15.元类

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

'a test module'

__author__ = 'Alex Xu'

# 元类
'''
1. 元类：metaclass。
2. 先定义metaclass，就可以创建类，最后创建实例
'''

# ORM（Object Relational Mapping）对象-关系映射
# 定义Field类，负责保存数据库表的字段名和字段类型
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)

# 基于Field派生
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')

# 编写ModelMetaclass
'''
1. 排除掉对Model类的修改
2. 在当前类中查找定义的类的所有属性，
如果找到一个Field属性，就把它保存到一个__mappings__的dict中，
同时从类属性中删除该属性，否则容易造成运行时错误（实例属性会遮盖类的同名属性）
3. 把表名保存到__table__中，这里简化为表名默认为类名
'''
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)

# 基类model
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))

# 调用
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')

u = User(id = 12345, name = 'Michael', email = 'test@gmail.com', password = 'my-pwd')
u.save()