89 lines
3.3 KiB
Python
89 lines
3.3 KiB
Python
# -*- coding: utf-8 -*-
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"""
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Day_6 参考解答
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=============
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主题：面向对象（三）—— descriptor 复刻 models.CharField（进阶）。
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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一遍。
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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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# 题目描述里想定义哪个类都可以（Article/Post 皆可），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_6 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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