python笔记57-@property源码解读与使用

前言

Python 内置的 @property 装饰器的作用是把方法变成属性，调用的时候不需要加括号

property源码

property() 方法的传四个参数

• fget --获取属性值的函数
• fset --设置属性值的函数
• fdel --删除属性值函数
• doc --属性描述信息

class property(object):
    """
    property(fget=None, fset=None, fdel=None, doc=None) -> property attribute
    
    fget is a function to be used for getting an attribute value, and likewise
    fset is a function for setting, and fdel a function for del'ing, an
    attribute.  Typical use is to define a managed attribute x:
    
    class C(object):
        def getx(self): return self._x
        def setx(self, value): self._x = value
        def delx(self): del self._x
        x = property(getx, setx, delx, "I'm the 'x' property.")
    
    Decorators make defining new properties or modifying existing ones easy:
    
    class C(object):
        @property
        def x(self):
            "I am the 'x' property."
            return self._x
        @x.setter
        def x(self, value):
            self._x = value
        @x.deleter
        def x(self):
            del self._x
    """
    def deleter(self, *args, **kwargs): # real signature unknown
        """ Descriptor to change the deleter on a property. """
        pass

    def getter(self, *args, **kwargs): # real signature unknown
        """ Descriptor to change the getter on a property. """
        pass

    def setter(self, *args, **kwargs): # real signature unknown
        """ Descriptor to change the setter on a property. """
        pass

    def __delete__(self, *args, **kwargs): # real signature unknown
        """ Delete an attribute of instance. """
        pass

    def __getattribute__(self, *args, **kwargs): # real signature unknown
        """ Return getattr(self, name). """
        pass

    def __get__(self, *args, **kwargs): # real signature unknown
        """ Return an attribute of instance, which is of type owner. """
        pass

    def __init__(self, fget=None, fset=None, fdel=None, doc=None): # known special case of property.__init__
        """
        property(fget=None, fset=None, fdel=None, doc=None) -> property attribute
        
        fget is a function to be used for getting an attribute value, and likewise
        fset is a function for setting, and fdel a function for del'ing, an
        attribute.  Typical use is to define a managed attribute x:
        
        class C(object):
            def getx(self): return self._x
            def setx(self, value): self._x = value
            def delx(self): del self._x
            x = property(getx, setx, delx, "I'm the 'x' property.")
        
        Decorators make defining new properties or modifying existing ones easy:
        
        class C(object):
            @property
            def x(self):
                "I am the 'x' property."
                return self._x
            @x.setter
            def x(self, value):
                self._x = value
            @x.deleter
            def x(self):
                del self._x
        
        # (copied from class doc)
        """
        pass

    @staticmethod # known case of __new__
    def __new__(*args, **kwargs): # real signature unknown
        """ Create and return a new object.  See help(type) for accurate signature. """
        pass

    def __set__(self, *args, **kwargs): # real signature unknown
        """ Set an attribute of instance to value. """
        pass

    fdel = property(lambda self: object(), lambda self, v: None, lambda self: None)  # default

    fget = property(lambda self: object(), lambda self, v: None, lambda self: None)  # default

    fset = property(lambda self: object(), lambda self, v: None, lambda self: None)  # default

    __isabstractmethod__ = property(lambda self: object(), lambda self, v: None, lambda self: None)  # default

源码解读

先看源码里面第一个示例

class C(object):

    def getx(self):
        return self._x

    def setx(self, value):
        self._x = value

    def delx(self):
        del self._x

    x = property(getx, setx, delx, "I'm the 'x' property.")

实例化后调用x属性

c = C()
c.x = 20     # c.x = value 将触发 setter
print(c.x)   # c.x 将触发 getter
del c.x      # del c.x 触发 deleter

c 是 C 的实例化对象, c.x 将触发 getter,c.x = value 将触发 setter ， del c.x 触发 deleter。
如果给定 doc 参数，其将成为这个属性值的 docstring，否则 property 函数就会复制 fget 函数的 docstring（如果有的话）

装饰器使定义新属性或修改现有属性变得容易，property 的 getter, setter 和 deleter 方法同样可以用作装饰器：

class C(object):
    def __init__(self):
        self._x = None
 
    @property
    def x(self):
        """I'm the 'x' property."""
        return self._x
 
    @x.setter
    def x(self, value):
        self._x = value
 
    @x.deleter
    def x(self):
        del self._x

这个代码和第一个例子完全相同，但要注意这些额外函数的名字和 property 下的一样，例如这里的 x。

@property 只读属性

我们知道类的属性是可以动态赋值的

class People(object):

    def __init__(self):
        self.name = "yoyo"
        self.age = 20

people = People()
print(people.age)   # 20
# 属性动态赋值
people.age = 22
print(people.age)   # 22

如果我不想让age属性改变，可以使用 @property 可以设置只读属性的 getter 方法

class People(object):

    def __init__(self):
        self.name = "yoyo"
        self.age = 20

    @property
    def get_age(self):
        return 20


people = People()
print(people.get_age)   # 20

如果使用这种赋值代码people.get_age = 22，会抛出异常AttributeError: can't set attribute

使用示例

设置 my_age 属性可读可写

class Person(object):
    def __init__(self, name, age):
        self.__name = name
        self.__age = age

    @property
    def my_age(self):
        return self.__age

    @my_age.setter
    def my_age(self, value):
        """设置my_age属性，并校验值"""
        if not isinstance(value, int):
            raise ValueError('年龄必须是数字!')
        if value < 0 or value > 100:
            raise ValueError('年龄必须是0-100')
        self.__age = value


person = Person("yoyo", 20)
print(person.my_age)  # 20
person.my_age = 22
print(person.my_age)  # 22
person.my_age = "hh"  # ValueError: 年龄必须是数字!

如果你想设置my_age属性可以删除，需用到property 的 deleter 装饰器