描述符(__get__和__set__和__delete__)

一、描述符

• 描述符是什么：描述符本质就是一个新式类，在这个新式类中，至少实现了__get__()，set()，delete()中的一个，这也被称为描述符协议
  • __get__()：调用一个属性时，触发
  • __set__()：为一个属性赋值时，触发
  • __delete__()：采用del删除属性时，触发
• 定义一个描述符

class Foo:  # 在python3中Foo是新式类，它实现了__get__()，__set__()，__delete__()中的一个三种方法的一个，这个类就被称作一个描述符

    def __get__(self, instance, owner):

        pass

    def __set__(self, instance, value):

        pass

    def __delete__(self, instance):

        pass

二、描述符的作用

• 描述符是干什么的：描述符的作用是用来代理另外一个类的属性的，必须把描述符定义成这个类的类属性，不能定义到构造函数中

class Foo:

    def __get__(self, instance, owner):

        print('触发get')

    def __set__(self, instance, value):

        print('触发set')

    def __delete__(self, instance):

        print('触发delete')

f1 = Foo()

• 包含这三个方法的新式类称为描述符，由这个类产生的实例进行属性的调用/赋值/删除，并不会触发这三个方法

f1.name = 'randy'

f1.name

del f1.name

2.1 何时，何地，会触发这三个方法的执行

class Str:

    """描述符Str"""

    def __get__(self, instance, owner):

        print('Str调用')

    def __set__(self, instance, value):

        print('Str设置...')

    def __delete__(self, instance):

        print('Str删除...')

class Int:

    """描述符Int"""

    def __get__(self, instance, owner):

        print('Int调用')

    def __set__(self, instance, value):

        print('Int设置...')

    def __delete__(self, instance):

        print('Int删除...')

class People:

    name = Str()

    age = Int()

    def __init__(self, name, age):  # name被Str类代理，age被Int类代理

        self.name = name

        self.age = age

# 何地？：定义成另外一个类的类属性

# 何时？：且看下列演示

p1 = People('alex', 18)

Str设置...

Int设置...

• 描述符Str的使用

p1.name

p1.name = 'randy'

del p1.name

Str调用

Str设置...

Str删除...

• 描述符Int的使用

p1.age

p1.age = 18

del p1.age

Int调用

Int设置...

Int删除...

• 我们来瞅瞅到底发生了什么

print(p1.__dict__)

print(People.__dict__)

{}

{'__module__': '__main__', 'name': <__main__.Str object at 0x107a86940>, 'age': <__main__.Int object at 0x107a863c8>, '__init__': <function People.__init__ at 0x107ba2ae8>, '__dict__': <attribute '__dict__' of 'People' objects>, '__weakref__': <attribute '__weakref__' of 'People' objects>, '__doc__': None}

• 补充

print(type(p1) == People)  # type(obj)其实是查看obj是由哪个类实例化来的

print(type(p1).__dict__ == People.__dict__)

True

True

三、两种描述符

3.1 数据描述符

• 至少实现了__get__()和__set__()

class Foo:

    def __set__(self, instance, value):

        print('set')

    def __get__(self, instance, owner):

        print('get')

3.2 非数据描述符

• 没有实现__set__()

class Foo:

    def __get__(self, instance, owner):

        print('get')

四、描述符注意事项

1. 描述符本身应该定义成新式类，被代理的类也应该是新式类

2. 必须把描述符定义成这个类的类属性，不能为定义到构造函数中

3. 要严格遵循该优先级，优先级由高到底分别是

   1.类属性

   2.数据描述符

   3.实例属性

   4.非数据描述符

   5.找不到的属性触发__getattr__()

五、使用描述符

• 众所周知，python是弱类型语言，即参数的赋值没有类型限制，下面我们通过描述符机制来实现类型限制功能

5.1 牛刀小试

class Str:

    def __init__(self, name):

        self.name = name

    def __get__(self, instance, owner):

        print('get--->', instance, owner)

        return instance.__dict__[self.name]

    def __set__(self, instance, value):

        print('set--->', instance, value)

        instance.__dict__[self.name] = value

    def __delete__(self, instance):

        print('delete--->', instance)

        instance.__dict__.pop(self.name)

class People:

    name = Str('name')

    def __init__(self, name, age, salary):

        self.name = name # 触发__set__

        self.age = age

        self.salary = salary

p1 = People('randy', 18, 3231.3)

set---> <__main__.People object at 0x107a86198> randy

• 调用

print(p1.__dict__)

print(p1.name)

{'name': 'randy', 'age': 18, 'salary': 3231.3}

get---> <__main__.People object at 0x107a86198> <class '__main__.People'>

randy

• 赋值

print(p1.__dict__)

p1.name = 'randysun'

print(p1.__dict__)

{'name': 'randy', 'age': 18, 'salary': 3231.3}

set---> <__main__.People object at 0x107a86198> randysun

{'name': 'randysun', 'age': 18, 'salary': 3231.3}

• 删除

print(p1.__dict__)

del p1.name

print(p1.__dict__)

{'name': 'randysun', 'age': 18, 'salary': 3231.3}

delete---> <__main__.People object at 0x107a86198>

{'age': 18, 'salary': 3231.3}

5.2 拔刀相助

class Str:

    def __init__(self, name):

        self.name = name

    def __get__(self, instance, owner):

        print('get--->', instance, owner)

        return instance.__dict__[self.name]

    def __set__(self, instance, value):

        print('set--->', instance, value)

        instance.__dict__[self.name] = value

    def __delete__(self, instance):

        print('delete--->', instance)

        instance.__dict__.pop(self.name)

class People:

    name = Str('name')

    def __init__(self, name, age, salary):

        self.name = name

        self.age = age

        self.salary = salary

# 疑问：如果我用类名去操作属性呢

try:

    People.name  # 报错，错误的根源在于类去操作属性时，会把None传给instance

except Exception as e:

    print(e)

get---> None <class '__main__.People'>

'NoneType' object has no attribute '__dict__'

• 修订__get__方法

class Str:

    def __init__(self, name):

        self.name = name

    def __get__(self, instance, owner):

        print('get--->', instance, owner)

        if instance is None:

            return self

        return instance.__dict__[self.name]

    def __set__(self, instance, value):

        print('set--->', instance, value)

        instance.__dict__[self.name] = value

    def __delete__(self, instance):

        print('delete--->', instance)

        instance.__dict__.pop(self.name)

class People:

    name = Str('name')

    def __init__(self, name, age, salary):

        self.name = name

        self.age = age

        self.salary = salary

print(People.name)  # 完美，解决

get---> None <class '__main__.People'>

<__main__.Str object at 0x107a86da0>

5.3 磨刀霍霍

class Str:

    def __init__(self, name, expected_type):

        self.name = name

        self.expected_type = expected_type

    def __get__(self, instance, owner):

        print('get--->', instance, owner)

        if instance is None:

            return self

        return instance.__dict__[self.name]

    def __set__(self, instance, value):

        print('set--->', instance, value)

        if not isinstance(value, self.expected_type):  # 如果不是期望的类型，则抛出异常

            raise TypeError('Expected %s' % str(self.expected_type))

        instance.__dict__[self.name] = value

    def __delete__(self, instance):

        print('delete--->', instance)

        instance.__dict__.pop(self.name)

class People:

    name = Str('name', str)  # 新增类型限制str

    def __init__(self, name, age, salary):

        self.name = name

        self.age = age

        self.salary = salary

try:

    p1 = People(123, 18, 3333.3)  # 传入的name因不是字符串类型而抛出异常

except Exception as e:

    print(e)

set---> <__main__.People object at 0x1084cd940> 123

Expected <class 'str'>

5.4 大刀阔斧

class Typed:

    def __init__(self, name, expected_type):

        self.name = name

        self.expected_type = expected_type

    def __get__(self, instance, owner):

        print('get--->', instance, owner)

        if instance is None:

            return self

        return instance.__dict__[self.name]

    def __set__(self, instance, value):

        print('set--->', instance, value)

        if not isinstance(value, self.expected_type):

            raise TypeError('Expected %s' % str(self.expected_type))

        instance.__dict__[self.name] = value

    def __delete__(self, instance):

        print('delete--->', instance)

        instance.__dict__.pop(self.name)

class People:

    name = Typed('name', str)

    age = Typed('name', int)

    salary = Typed('name', float)

    def __init__(self, name, age, salary):

        self.name = name

        self.age = age

        self.salary = salary

try:

    p1 = People(123, 18, 3333.3)

except Exception as e:

    print(e)

set---> <__main__.People object at 0x1082c7908> 123

Expected <class 'str'>

try:

    p1 = People('randy', '18', 3333.3)

except Exception as e:

    print(e)

set---> <__main__.People object at 0x1078dd438> randy

set---> <__main__.People object at 0x1078dd438> 18

Expected <class 'int'>

p1 = People('randy', 18, 3333.3)

set---> <__main__.People object at 0x1081b3da0> randy

set---> <__main__.People object at 0x1081b3da0> 18

set---> <__main__.People object at 0x1081b3da0> 3333.3

• 大刀阔斧之后我们已然能实现功能了，但是问题是，如果我们的类有很多属性，你仍然采用在定义一堆类属性的方式去实现，low，这时候我需要教你一招：独孤九剑

5.4.1 类的装饰器:无参

def decorate(cls):

    print('类的装饰器开始运行啦------>')

    return cls

@decorate  # 无参：People = decorate(People)

class People:

    def __init__(self, name, age, salary):

        self.name = name

        self.age = age

        self.salary = salary

p1 = People('randy', 18, 3333.3)

类的装饰器开始运行啦------>

5.4.2 类的装饰器:有参

def typeassert(**kwargs):

    def decorate(cls):

        print('类的装饰器开始运行啦------>', kwargs)

        return cls

    return decorate

@typeassert(

    name=str, age=int, salary=float

)  # 有参：1.运行typeassert(...)返回结果是decorate，此时参数都传给kwargs 2.People=decorate(People)

class People:

    def __init__(self, name, age, salary):

        self.name = name

        self.age = age

        self.salary = salary

p1 = People('randy', 18, 3333.3)

类的装饰器开始运行啦------> {'name': <class 'str'>, 'age': <class 'int'>, 'salary': <class 'float'>}

5.5 刀光剑影

class Typed:

    def __init__(self, name, expected_type):

        self.name = name

        self.expected_type = expected_type

    def __get__(self, instance, owner):

        print('get--->', instance, owner)

        if instance is None:

            return self

        return instance.__dict__[self.name]

    def __set__(self, instance, value):

        print('set--->', instance, value)

        if not isinstance(value, self.expected_type):

            raise TypeError('Expected %s' % str(self.expected_type))

        instance.__dict__[self.name] = value

    def __delete__(self, instance):

        print('delete--->', instance)

        instance.__dict__.pop(self.name)

def typeassert(**kwargs):

    def decorate(cls):

        print('类的装饰器开始运行啦------>', kwargs)

        for name, expected_type in kwargs.items():

            setattr(cls, name, Typed(name, expected_type))

        return cls

    return decorate

@typeassert(

    name=str, age=int, salary=float

)  # 有参：1.运行typeassert(...)返回结果是decorate，此时参数都传给kwargs 2.People=decorate(People)

class People:

    def __init__(self, name, age, salary):

        self.name = name

        self.age = age

        self.salary = salary

print(People.__dict__)

p1 = People('randy', 18, 3333.3)

类的装饰器开始运行啦------> {'name': <class 'str'>, 'age': <class 'int'>, 'salary': <class 'float'>}

{'__module__': '__main__', '__init__': <function People.__init__ at 0x10797a400>, '__dict__': <attribute '__dict__' of 'People' objects>, '__weakref__': <attribute '__weakref__' of 'People' objects>, '__doc__': None, 'name': <__main__.Typed object at 0x1080b2a58>, 'age': <__main__.Typed object at 0x1080b2ef0>, 'salary': <__main__.Typed object at 0x1080b2c18>}

set---> <__main__.People object at 0x1080b22e8> randy

set---> <__main__.People object at 0x1080b22e8> 18

set---> <__main__.People object at 0x1080b22e8> 3333.3

六、描述符总结

• 描述符是可以实现大部分python类特性中的底层魔法，包括@classmethod，@staticmethd，@property甚至是__slots__属性
• 描述父是很多高级库和框架的重要工具之一，描述符通常是使用到装饰器或者元类的大型框架中的一个组件.

七、自定制@property

• 利用描述符原理完成一个自定制@property，实现延迟计算（本质就是把一个函数属性利用装饰器原理做成一个描述符：类的属性字典中函数名为key，value为描述符类产生的对象）

7.1 property回顾

class Room:

    def __init__(self, name, width, length):

        self.name = name

        self.width = width

        self.length = length

    @property

    def area(self):

        return self.width * self.length

r1 = Room('alex', 1, 1)

print(r1.area)

1

7.2 自定制property

class Lazyproperty:

    def __init__(self, func):

        self.func = func

    def __get__(self, instance, owner):

        print('这是我们自己定制的静态属性，r1.area实际是要执行r1.area()')

        if instance is None:

            return self

        return self.func(instance)  # 此时你应该明白，到底是谁在为你做自动传递self的事情

class Room:

    def __init__(self, name, width, length):

        self.name = name

        self.width = width

        self.length = length

    @Lazyproperty  # area=Lazyproperty(area) 相当于定义了一个类属性,即描述符

    def area(self):

        return self.width * self.length

r1 = Room('alex', 1, 1)

print(r1.area)

这是我们自己定制的静态属性，r1.area实际是要执行r1.area()

1

7.3 实现延迟计算功能

class Lazyproperty:

    def __init__(self, func):

        self.func = func

    def __get__(self, instance, owner):

        print('这是我们自己定制的静态属性，r1.area实际是要执行r1.area()')

        if instance is None:

            return self

        else:

            print('--->')

            value = self.func(instance)

            setattr(instance, self.func.__name__, value)  # 计算一次就缓存到实例的属性字典中

            return value

class Room:

    def __init__(self, name, width, length):

        self.name = name

        self.width = width

        self.length = length

    @Lazyproperty  # area=Lazyproperty(area) 相当于'定义了一个类属性,即描述符'

    def area(self):

        return self.width * self.length

r1 = Room('alex', 1, 1)

print(r1.area)  # 先从自己的属性字典找,没有再去类的中找,然后出发了area的__get__方法

这是我们自己定制的静态属性，r1.area实际是要执行r1.area()

--->

1

print(r1.area)  # 先从自己的属性字典找,找到了,是上次计算的结果,这样就不用每执行一次都去计算

1

八、打破延迟计算

• 一个小的改动，延迟计算的美梦就破碎了

class Lazyproperty:

    def __init__(self, func):

        self.func = func

    def __get__(self, instance, owner):

        print('这是我们自己定制的静态属性，r1.area实际是要执行r1.area()')

        if instance is None:

            return self

        else:

            value = self.func(instance)

            instance.__dict__[self.func.__name__] = value

            return value

        # return self.func(instance) # 此时你应该明白,到底是谁在为你做自动传递self的事情

    def __set__(self, instance, value):

        print('hahahahahah')

class Room:

    def __init__(self, name, width, length):

        self.name = name

        self.width = width

        self.length = length

    @Lazyproperty  # area=Lazyproperty(area) 相当于定义了一个类属性,即描述符

    def area(self):

        return self.width * self.length

print(Room.__dict__)

{'__module__': '__main__', '__init__': <function Room.__init__ at 0x107d53620>, 'area': <__main__.Lazyproperty object at 0x107ba3860>, '__dict__': <attribute '__dict__' of 'Room' objects>, '__weakref__': <attribute '__weakref__' of 'Room' objects>, '__doc__': None}

r1 = Room('alex', 1, 1)

print(r1.area)

print(r1.area)

print(r1.area)

这是我们自己定制的静态属性，r1.area实际是要执行r1.area()

1

这是我们自己定制的静态属性，r1.area实际是要执行r1.area()

1

这是我们自己定制的静态属性，r1.area实际是要执行r1.area()

1

print(

    r1.area

)  #缓存功能失效,每次都去找描述符了,为何,因为描述符实现了set方法,它由非数据描述符变成了数据描述符,数据描述符比实例属性有更高的优先级,因而所有的属性操作都去找描述符了

这是我们自己定制的静态属性，r1.area实际是要执行r1.area()

1

九、自定制@classmethod

class ClassMethod:

    def __init__(self, func):

        self.func = func

    def __get__(

            self, instance,

            owner):  #类来调用,instance为None,owner为类本身,实例来调用,instance为实例,owner为类本身,

        def feedback():

            print('在这里可以加功能啊...')

            return self.func(owner)

        return feedback

class People:

    name = 'randy'

    @ClassMethod  # say_hi=ClassMethod(say_hi)

    def say_hi(cls):

        print('你好啊,帅哥 %s' % cls.name)

People.say_hi()

p1 = People()

在这里可以加功能啊...

你好啊,帅哥 randy

p1.say_hi()

在这里可以加功能啊...

你好啊,帅哥 randy

• 疑问,类方法如果有参数呢,好说,好说

class ClassMethod:

    def __init__(self, func):

        self.func = func

    def __get__(self, instance, owner

                ):  # 类来调用,instance为None,owner为类本身,实例来调用,instance为实例,owner为类本身,

        def feedback(*args, **kwargs):

            print('在这里可以加功能啊...')

            return self.func(owner, *args, **kwargs)

        return feedback

class People:

    name = 'randy'

    @ClassMethod  # say_hi=ClassMethod(say_hi)

    def say_hi(cls, msg):

        print('你好啊,帅哥 %s %s' % (cls.name, msg))

People.say_hi('你是那偷心的贼')

p1 = People()

在这里可以加功能啊...

你好啊,帅哥 randy 你是那偷心的贼

p1.say_hi('你是那偷心的贼')

在这里可以加功能啊...

你好啊,帅哥 randy 你是那偷心的贼

一十、自定制@staticmethod

class StaticMethod:

    def __init__(self, func):

        self.func = func

    def __get__(

            self, instance,

            owner):  # 类来调用，instance为None，owner为类本身，实例来调用，instance为实例，owner为类本身

        def feedback(*args, **kwargs):

            print('在这里可以加功能啊...')

            return self.func(*args, **kwargs)

        return feedback

class People:

    @StaticMethod  # say_hi = StaticMethod(say_hi)

    def say_hi(x, y, z):

        print('------>', x, y, z)

People.say_hi(1, 2, 3)

p1 = People()

在这里可以加功能啊...

------> 1 2 3

p1.say_hi(4, 5, 6)

在这里可以加功能啊...

------> 4 5 6

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