python中Properties的一些小用法
阿新 • • 發佈:2018-03-30
class format height 屬性 orm 一個 one def cor
property最大的用處就是可以為一個屬性制定getter,setter,delete和doc,他的函數原型為:
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
從上邊的代碼中可以看出來,它一共接受4個參數,我們再繼續看一段代碼:
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))
通過property,我們有能力創造出一個屬性來,然後為這個屬性指定一些方法,在這裏用setter,getter的好處就是可以監聽屬性的賦值和獲取行為,表面上看上去上邊的代碼沒有問題,但是當出現繼承關系的時候,就出問題了。
class MetricRectangle(Rectangle):
def _width_get(self):
return "{} metric".format(self.x2 - self.x1)
mr = MetricRectangle(10, 10, 100, 100)
print(mr.width)
即使我們在子類中重寫了getter方法,結果卻是無效的,這說明property只對當前的類生效,於是不得不把代碼改成下邊這樣:
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,在平時的編程中,如果需要重寫屬性的話,應該重寫該類中所有的property,否則程序很很難以理解,試想一下,setter在子類,getter在父類,多麽恐怖,
另一種比較好的方案是使用裝飾器,可讀性也比較好
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))
python中Properties的一些小用法