Python编程：metaclass元类实现简单的ORM

# -*- coding: utf-8 -*-
# @File    : 元类实例ORM.py
# @Date    : 2018-06-01
# @Author  : Peng Shiyu
# 先定义metaclass，就可以创建类，最后创建实例
class Field(object):
    """保存数据库表的字段名和字段类型"""
    def __init__(self, name, column_type):
        self.name = name
        self.column_type = column_type
    def __str__(self):
        return "<%s %s %s>"%(self.__class__.__name__, self.name, self.column_type)
# 定义各种类型
class IntegerField(Field):
    def __init__(self, name):
        super(IntegerField, self).__init__(name, "int")
class StringField(Field):
    def __init__(self, name):
        super(StringField, self).__init__(name, "varchar(255)")
# 编写元类
"""
排除掉对Model类的修改；
在当前类（比如User）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Field属性，否则，容易造成运行时错误（实例的属性会遮盖类的同名属性）；
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("Fond Model: %s"% name)
        # 查找Field字段
        mappings = {}
        for k, v in attrs.items():
            if isinstance(v, Field):
                print("Fond Field: %s => %s"% (k, v))
                mappings[k] = v
        # 弹出Field字段
        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)
# 在Model类中，就可以定义各种操作数据库的方法，比如save()，delete()，find()，update等等
class Model(dict, metaclass=ModelMetaclass):
    def __init__(self, **kwargs):
        super(Model, self).__init__(**kwargs)
    def __getattr__(self, key):
        return self[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"% args)
class User(Model):
    # 定义类的属性到列的映射：
    id = IntegerField("id")
    name = StringField("name")
    email = StringField("email")
    password = StringField("password")
if __name__ == '__main__':
    # # 创建一个实例
    user = User(id=1, name="Tom", email="12345@qq.com", password="123456")
    # 保存到数据库
    user.save()
    # 访问元素
    print(user.name)
    print(user["name"])
    user.name = "jack"
    print(user.name)
"""
Fond Model: User
Fond Field: id => <IntegerField id int>
Fond Field: name => <StringField name varchar(255)>
Fond Field: email => <StringField email varchar(255)>
Fond Field: password => <StringField password varchar(255)>
sql: insert into User (id,name,email,password) VALUES (?,?,?,?)
args: [1, 'Tom', '12345@qq.com', '123456']
Tom
Tom
jack
"""