python描述符的应用

使用描述符为python实现类型检测

class Typed:
    def __get__(self, instance, owner):
        print(instance)
        print(owner)

    def __set__(self, instance, value):
        pass

class Girl:
    name = Typed()

    def __init__(self, name, age):
        self.name = name  # 我希望传入的name是str
        self.age = age   # 我希望传入的age是int

g = Girl("satori", 18)
g.name
'''
<__main__.Girl object at 0x03D28430>
<class '__main__.Girl'>
'''
# 可以看到当我们获取被代理的值，会触发get方法，instance就是Girl的实例对象
# owner则是Girl这个类

　　

class Typed:
    def __get__(self, instance, owner):
        pass

    def __set__(self, instance, value):
        print(instance)
        print(value)

class Girl:
    name = Typed()

    def __init__(self, name, age):
        self.name = name  # 我希望传入的name是str
        self.age = age   # 我希望传入的age是int

g = Girl("satori", 18)
'''
<__main__.Girl object at 0x03D28430>
satori

'''
# 可以看到当我们给被代理的值赋值的时候，会触发set方法，instance就是Girl的实例对象
# value则是我们赋的值

　　

class Typed:
    def __init__(self, key):
        self.key = key

    def __get__(self, instance, owner):
        return instance.__dict__[self.key]

    def __set__(self, instance, value):
        instance.__dict__[self.key] = value

    def __delete__(self, instance):
        instance.__dict__.pop(self.key)

class Girl:
    name = Typed("name")
    age = Typed("age")

    def __init__(self, name, age):
        self.name = name
        self.age = age

g = Girl("satori", 18)
print(g.__dict__)
g.name = "mashiro"
print(g.__dict__)
del g.name
print(g.__dict__)

'''
{'name': 'satori', 'age': 18}
{'name': 'mashiro', 'age': 18}
{'age': 18}
'''

# 等于说  我饶了一圈，并没有直接将属性添加的字典里面，而是向上找了代理，让代理帮我把属性添加的字典里面去

　　那么，接下来就可以实现类型检测了

class Typed:
    def __init__(self, key, except_type):
        self.key = key
        self.except_type = except_type

    def __get__(self, instance, owner):
        return instance.__dict__[self.key]

    def __set__(self, instance, value):
        if isinstance(value, self.except_type):
            instance.__dict__[self.key] = value
        else:
            raise Exception(f"{self.key} must be type {self.except_type}")

    def __delete__(self, instance):
        instance.__dict__.pop(self.key)

class Girl:
    name = Typed("name", str)
    age = Typed("age", int)

    def __init__(self, name, age):
        self.name = name
        self.age = age

g = Girl("satori", 18)
print(g.__dict__)  # {'name': 'satori', 'age': 18}

try:
    import traceback
    g = Girl("satori", "18")

except Exception:
    print(traceback.format_exc())

'''
Exception: age must be type <class 'int'>
'''

# 于是我们便手动实现了python的类型检测

　　

类的装饰器

# 装饰器不仅可以给函数用，还可以给类用

def deco(obj):
    print("======")
    return obj

@deco
class Girl:
    def __init__(self, name, age):
        self.name = name
        self.age = age

g = Girl("satori", 18)
print(g.__dict__)

'''
======
{'name': 'satori', 'age': 18}
'''

　　

描述符实现property

class LazeProperty:

   def __init__(self, func):
       self.func = func

   def __get__(self, instance, owner):

       # 如果是类调用，那么instance为空
       if instance is None:
           return self
       res = self.func(instance)
       setattr(instance, self.func.__name__, res)  # 将值设进去，如果有从字典里面找，没有再计算
       return res

class Girl:

    def __init__(self, name, age):
        self.name = name
        self.age = age

    @LazeProperty  # info = LazeProperty(info)
    def info(self):
        return f"my name is {self.name},age is {self.age}"

g = Girl("satori", 18)
print(g.info)  # my name is satori,age is 18

print(Girl.info)  # <__main__.LazeProperty object at 0x04EF3910>