property

一、property用法

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

二、__get__, __set__（对于属性的方法）

class RevealAccess(object):
    """A data descriptor that sets and returns values
       normally and prints a message logging their access.
    """
 
    def __init__(self, initval=None, name='var'):
        self.val = initval
        self.name = name
 
    def __get__(self, obj, objtype):
        print('Retrieving', self.name)
        return self.val
 
    def __set__(self, obj, val):
        print('Updating', self.name)
        self.val = val
 
>>> class MyClass(object):
    x = RevealAccess(10, 'var "x"')
    y = 5
 
>>> m = MyClass()
>>> m.x
Retrieving var "x"
10
>>> m.x = 20
Updating var "x"
>>> m.x
Retrieving var "x"
20
>>> m.y
5

三、property模拟实现

class Property(object):
    "Emulate PyProperty_Type() in Objects/descrobject.c"
 
    def __init__(self, fget=None, fset=None, fdel=None, doc=None):
        self.fget = fget
        self.fset = fset
        self.fdel = fdel
        if doc is None and fget is not None:
            doc = fget.__doc__
        self.__doc__ = doc
 
    def __get__(self, obj, objtype=None):
        if obj is None:
            return self
        if self.fget is None:
            raise AttributeError("unreadable attribute")
        return self.fget(obj)
 
    def __set__(self, obj, value):
        if self.fset is None:
            raise AttributeError("can't set attribute")
        self.fset(obj, value)
 
    def __delete__(self, obj):
        if self.fdel is None:
            raise AttributeError("can't delete attribute")
        self.fdel(obj)
 
    def getter(self, fget):
        return type(self)(fget, self.fset, self.fdel, self.__doc__)
 
    def setter(self, fset):
        return type(self)(self.fget, fset, self.fdel, self.__doc__)
 
    def deleter(self, fdel):
        return type(self)(self.fget, self.fset, fdel, self.__doc__)