Add Day_3: 面向对象×手写一个小ORM
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# -*- coding: utf-8 -*-
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"""
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Day_3 参考解答
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=============
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用纯 Python 复刻 Django Model 的机制。
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建议：先自己写，写不出来再看；看完后关掉文件自己默写。
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"""
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import time
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# ============================================================
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# 题目一：设计一个 User 类
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# ============================================================
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class User:
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all_users = [] # 类属性：模拟数据库表
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def __init__(self, name, email, age):
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self.name = name
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self.email = email
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self.age = age
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User.all_users.append(self) # 把新用户"存表"
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def get_full_name(self):
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return f"用户：{self.name}（{self.email}）"
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def __str__(self):
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# 给"人"看的：print(user) 时显示，Django admin 列表里显示的就是它
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return self.name
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def __repr__(self):
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# 给"开发者调试"看的：要能还原出原对象，eval(repr(u)) 应得到相同信息
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return f"User(name={self.name!r}, email={self.email!r}, age={self.age!r})"
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if __name__ == '__main__':
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u1 = User("zhangsan", "zs@163.com", 20)
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u2 = User("lisi", "ls@163.com", 21)
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u3 = User("wangwu", "ww@163.com", 19)
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for u in User.all_users:
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print(u, "->", repr(u))
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# zhangsan -> User(name='zhangsan', email='zs@163.com', age=20)
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# lisi -> User(name='lisi', email='ls@163.com', age=21)
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# wangwu -> User(name='wangwu', email='ww@163.com', age=19)
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restored = eval(repr(u1)) # __repr__ 的用途：可还原
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print(restored.name, restored.email) # zhangsan zs@163.com
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# 思考题答案：
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# __str__ 面向普通用户/展示，__repr__ 面向开发者/调试，且要求无歧义可还原；
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# 只实现 __repr__ 时，print() 会退而使用 __repr__；反之不会。
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# ============================================================
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# 题目二：继承 —— 复刻 models.Model
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# ============================================================
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class BaseModel:
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objects = [] # 类属性：模拟数据库表（所有子类共用）
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def save(self):
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BaseModel.objects.append(self)
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print(f"[DB] 已保存 {type(self).__name__}: {self}")
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def delete(self):
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BaseModel.objects.remove(self)
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print(f"[DB] 已删除 {self}")
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def to_dict(self):
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return dict(self.__dict__) # 实例的所有属性 => 字典
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def __str__(self):
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return f"<{type(self).__name__}#{id(self)}>"
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class Article(BaseModel):
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def __init__(self, title, content, author):
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self.title = title
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self.content = content
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self.author = author
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self.created_at = time.strftime("%Y-%m-%d %H:%M:%S")
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# 进阶：重写 save，先打日志，再调用父类逻辑
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def save(self):
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print(f"[LOG] 正在保存文章《{self.title}》...")
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super().save() # 父类的 save 负责真正"入库"
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def __str__(self):
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return f"<Article: {self.title}>"
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class Comment(BaseModel):
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def __init__(self, content, author):
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self.content = content
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self.author = author
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def __str__(self):
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return f"<Comment: {self.content[:10]}...>"
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if __name__ == '__main__':
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a1 = Article("Django 入门", "从 Hello World 开始", "zhangsan")
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a1.save() # 触发重写后的 save（带日志 + super）
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c1 = Comment("写得太好了！", "lisi")
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c1.save()
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for obj in BaseModel.objects:
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print(obj.to_dict())
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print(issubclass(Article, BaseModel)) # True
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print(isinstance(a1, BaseModel)) # True
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a1.delete()
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print(len(BaseModel.objects)) # 1（只剩评论）
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# ============================================================
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# 题目三（进阶）：descriptor —— 复刻 models.CharField
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# ============================================================
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_NOT_SET = object() # 哨兵值：表示"字段还没赋过值"
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class CharField:
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def __init__(self, max_length=255):
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self.max_length = max_length
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self.name: str | None = None # 字段对应的属性名，由 ModelBase.__init_subclass__ 填入
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def __get__(self, instance, owner):
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if instance is None:
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return self # 通过类访问（Article.title）：返回描述符本身
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return instance.__dict__.get(self.name, _NOT_SET)
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def __set__(self, instance, value):
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if not isinstance(value, str):
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raise ValueError(f"{self.name} 必须是字符串")
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if len(value) > self.max_length:
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raise ValueError(f"{self.name} 长度不能超过 {self.max_length}")
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instance.__dict__[self.name] = value # 存到实例自己的字典里
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class EmailField(CharField):
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def __set__(self, instance, value):
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if "@" not in value or "." not in value:
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raise ValueError(f"{self.name} 不是合法的邮箱格式")
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super().__set__(instance, value) # 通过校验后再走父类存储
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class IntegerField:
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def __init__(self, min_value=None, max_value=None):
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self.min_value = min_value
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self.max_value = max_value
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self.name: str | None = None
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def __get__(self, instance, owner):
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if instance is None:
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return self
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return instance.__dict__.get(self.name, _NOT_SET)
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def __set__(self, instance, value):
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# bool 是 int 的子类，所以要显式排除 True/False
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if not isinstance(value, int) or isinstance(value, bool):
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raise ValueError(f"{self.name} 必须是整数")
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if self.min_value is not None and value < self.min_value:
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raise ValueError(f"{self.name} 不能小于 {self.min_value}")
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if self.max_value is not None and value > self.max_value:
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raise ValueError(f"{self.name} 不能大于 {self.max_value}")
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instance.__dict__[self.name] = value
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class ModelBase:
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# 子类定义完成后自动调用：把每个 Field 对象和它的属性名绑定
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def __init_subclass__(cls, **kwargs):
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super().__init_subclass__(**kwargs)
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for attr_name, attr_value in cls.__dict__.items():
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if isinstance(attr_value, (CharField, IntegerField)):
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attr_value.name = attr_name
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# 为避免与本文件上方题目二的 Article 类重名，这里命名为 Post，
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# 题目描述里想定义哪个类都可以，机制完全相同。
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class Post(ModelBase):
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title = CharField(max_length=50)
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content = CharField(max_length=1000)
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author_email = EmailField(max_length=100)
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views = IntegerField(min_value=0)
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if __name__ == '__main__':
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a = Post()
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a.title = "Django 入门"
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a.content = "跟着 Day_3 慢慢理解框架"
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a.author_email = "zs@163.com"
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a.views = 10
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print(a.title, a.views, a.author_email) # Django 入门 10 zs@163.com
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print(Post.title) # 类访问 => CharField 对象本身
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# 以下三条都应该抛 ValueError，逐行取消注释验证
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# a.title = "x" * 100
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# a.views = -5
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# a.author_email = "not-an-email"
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# 尚未赋值的字段返回哨兵（不是属性不存在，也不是默认值）
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print(a.__dict__.get("views")) # 已被赋值为 10
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