Python編程：metaclass元類實(shí)現(xiàn)簡(jiǎn)單的ORM
發(fā)布時(shí)間：2021-11-23 點(diǎn)擊數(shù)：579
ORM全稱“Object Relational Mapping”，即對(duì)象-關(guān)系映射
把關(guān)系數(shù)據(jù)庫(kù)的一行映射為一個(gè)對(duì)象，一個(gè)類對(duì)應(yīng)一個(gè)表
這樣，寫代碼更簡(jiǎn)單，不用直接操作SQL語(yǔ)句。
# -*- coding: utf-8 -*- # @File : 元類實(shí)例ORM.py # @Date : 2018-06-01 # @Author : Peng Shiyu # 先定義metaclass，就可以創(chuàng)建類，最后創(chuàng)建實(shí)例 class Field(object): """保存數(shù)據(jù)庫(kù)表的字段名和字段類型""" 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) # 定義各種類型 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)") # 編寫元類 """ 排除掉對(duì)Model類的修改； 在當(dāng)前類（比如User）中查找定義的類的所有屬性，如果找到一個(gè)Field屬性，就把它保存到一個(gè)__mappings__的dict中， 同時(shí)從類屬性中刪除該Field屬性，否則，容易造成運(yùn)行時(shí)錯(cuò)誤（實(shí)例的屬性會(huì)遮蓋類的同名屬性）； 把表名保存到__table__中，這里簡(jiǎn)化為表名默認(rèn)為類名。 """ 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 # 彈出Field字段 for k in mappings.keys(): attrs.pop(k) attrs["__mappings__"] = mappings # 保存屬性和列的映射關(guān)系 attrs["__table__"] = name # 假設(shè)表名和類名一致 return type.__new__(cls, name, bases, attrs) # 在Model類中，就可以定義各種操作數(shù)據(jù)庫(kù)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): # 定義類的屬性到列的映射： id = IntegerField("id") name = StringField("name") email = StringField("email") password = StringField("password") if __name__ == '__main__': # # 創(chuàng)建一個(gè)實(shí)例 user = User(id=1, name="Tom", email="12345@qq.com", password="123456") # 保存到數(shù)據(jù)庫(kù) user.save() # 訪問(wèn)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 """