trick(八)、类的Properties
trick(八)、类的Properties
狼啸风云
发布于 2022-09-30 11:18:58
发布于 2022-09-30 11:18:58
property最大的用处就是可以为一个属性制定getter，setter，delete和doc，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 = 50
print(rectangle)
rectangle.height = 50
print(rectangle)
print(help(rectangle))
Output:
-----------------------------------------------------------------------------------------
20 5
Rectangle(10, 10, 60, 15)
Rectangle(10, 10, 60, 60)
Help on Rectangle in module __main__ object:
class Rectangle(builtins.object)
| Methods defined here:
|
| __init__(self, x1, y1, x2, y2)
| Initialize self. See help(type(self)) for accurate signature.
|
| __repr__(self)
| Return repr(self).
|
| ----------------------------------------------------------------------
| Data descriptors defined here:
|
| __dict__
| dictionary for instance variables (if defined)
|
| __weakref__
| list of weak references to the object (if defined)
|
| height
| rectangle height measured from top
|
| width
| rectangle width measured from left
None
-----------------------------------------------------------------------------------------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)
Output:
---
90
---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)
Output:
----------
90 metric
----------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 = 50
print(rectangle)
rectangle.height = 50
print(rectangle)
print(help(rectangle))
Output:
-----------------------------------------------------------------------------------------
20 5
Rectangle(10, 10, 60, 15)
Rectangle(10, 10, 60, 60)
Help on Rectangle in module __main__ object:
class Rectangle(builtins.object)
| Methods defined here:
|
| __init__(self, x1, y1, x2, y2)
| Initialize self. See help(type(self)) for accurate signature.
|
| __repr__(self)
| Return repr(self).
|
| ----------------------------------------------------------------------
| Data descriptors defined here:
|
| __dict__
| dictionary for instance variables (if defined)
|
| __weakref__
| list of weak references to the object (if defined)
|
| height
|
| width
| rectangle width measured from left
None
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原始发表：2022-09-08，如有侵权请联系 cloudcommunity@tencent.com 删除
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