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Python编程:metaclass元类实现简单的ORM

发布时间:2021-11-23 点击数:126
# -*- 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 """