python中Properties的一些小用法，,property最大

python中Properties的一些小用法，,property最大

property最大的用处就是可以为一个属性制定getter，setter，delete和doc，他的函数原型为：

    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

从上边的代码中可以看出来，它一共接受4个参数，我们再继续看一段代码：

class Rectangle(object):    def __init__(self, x1, y1, x2, y2):        self.x1, self.y1 = x1, y1        self.x2, self.y2 = x2, y2    def _width_get(self):        return self.x2 - self.x1    def _width_set(self, value):        self.x2 = self.x1 + value    def _height_get(self):        return self.y2 - self.y1    def _height_set(self, value):        self.y2 = self.y1 + value    width = property(_width_get, _width_set, doc="rectangle width measured from left")    height = property(_height_get, _height_set, doc="rectangle height measured from top")    def __repr__(self):        return "{}({}, {}, {}, {})".format(self.__class__.__name__,                                           self.x1,                                           self.y1,                                           self.x2,                                           self.y2)rectangle = Rectangle(10, 10, 30, 15)print(rectangle.width, rectangle.height)rectangle.width = 50print(rectangle)rectangle.height = 50print(rectangle)print(help(rectangle))

通过property，我们有能力创造出一个属性来，然后为这个属性指定一些方法，在这里用setter，getter的好处就是可以监听属性的赋值和获取行为，表面上看上去上边的代码没有问题，但是当出现继承关系的时候，就出问题了。

class MetricRectangle(Rectangle):    def _width_get(self):        return "{} metric".format(self.x2 - self.x1)mr = MetricRectangle(10, 10, 100, 100)print(mr.width)

即使我们在子类中重写了getter方法，结果却是无效的，这说明property只对当前的类生效，于是不得不把代码改成下边这样：

class MetricRectangle(Rectangle):    def _width_get(self):        return "{} metric".format(self.x2 - self.x1)    width = property(_width_get, Rectangle.width.fset)mr = MetricRectangle(10, 10, 100, 100)print(mr.width)

因此，在平时的编程中，如果需要重写属性的话，应该重写该类中所有的property，否则程序很很难以理解，试想一下，setter在子类，getter在父类，多么恐怖，

另一种比较好的方案是使用装饰器，可读性也比较好

class Rectangle(object):    def __init__(self, x1, y1, x2, y2):        self.x1, self.y1 = x1, y1        self.x2, self.y2 = x2, y2    @property    def width(self):        """rectangle width measured from left"""        return self.x2 - self.x1    @width.setter    def width(self, value):        self.x2 = self.x1 + value    @property    def height(self):        return self.y2 - self.y1    @height.setter    def height(self, value):        self.y2 = self.y1 + value    def __repr__(self):        return "{}({}, {}, {}, {})".format(self.__class__.__name__,                                           self.x1,                                           self.y1,                                           self.x2,                                           self.y2)rectangle = Rectangle(10, 10, 30, 15)print(rectangle.width, rectangle.height)rectangle.width = 50print(rectangle)rectangle.height = 50print(rectangle)print(help(rectangle))

python中Properties的一些小用法