Python类@property:使用setter但是避开getter？

Question

在python类中,@ property是一个很好的装饰器,可以避免使用显式的setter和getter函数.然而,它的成本是"经典"类功能的2-5倍.在我的情况下,在设置属性的情况下这是非常好的,与设置时需要完成的处理相比,开销是微不足道的.

但是,在获得房产时我不需要处理.它总是只是"回归自我.属性".是否有一种优雅的方式来使用setter但不使用getter,而不需要使用不同的内部变量？

为了说明,下面的类具有属性"var",它指的是内部变量"_var".它需要更长的时间称为"VAR"比"_var"但它会是很好,如果开发者和用户都可以只使用"无功",而无需守得的"_var"的轨道.

class MyClass(object):
  def __init__(self):
    self._var = None

  # the property "var". First the getter, then the setter
  @property
  def var(self):
    return self._var
  @var.setter
  def var(self, newValue):
    self._var = newValue
    #... and a lot of other stuff here

  # Use "var" a lot! How to avoid the overhead of the getter and not to call self._var!
  def useAttribute(self):
    for i in xrange(100000):
      self.var == 'something'

对于那些感兴趣的人,在我的电脑上调用"var"平均需要204 ns,而调用"_var"平均需要44 ns.

Answer 1

property在这种情况下不要使用.一个property对象是一个数据描述符,这意味着任何访问instance.var将调用该描述符和Python将永远不会寻找在实例本身的属性.

您有两个选择:使用.__setattr__()钩子或构建仅实现的描述符.__set__.

使用.__setattr__()钩子

class MyClass(object):
    var = 'foo'

    def __setattr__(self, name, value):
        if name == 'var':
            print "Setting var!"
            # do something with `value` here, like you would in a
            # setter.
            value = 'Set to ' + value
        super(MyClass, self).__setattr__(name, value)

现在,当正常的属性查询用来阅读 .var,但分配给当.var该__setattr__方法被调用来代替,让你拦截value,并根据需要进行调整.

演示:

>>> mc = MyClass()
>>> mc.var
'foo'
>>> mc.var = 'bar'
Setting var!
>>> mc.var
'Set to bar'

一个setter描述符

setter描述符只会拦截变量赋值:

class SetterProperty(object):
    def __init__(self, func, doc=None):
        self.func = func
        self.__doc__ = doc if doc is not None else func.__doc__
    def __set__(self, obj, value):
        return self.func(obj, value)

class Foo(object):
    @SetterProperty
    def var(self, value):
        print 'Setting var!'
        self.__dict__['var'] = value

注意我们需要如何分配实例.__dict__属性以防止再次调用setter.

演示:

>>> f = Foo()
>>> f.var = 'spam'
Setting var!
>>> f.var = 'ham'
Setting var!
>>> f.var
'ham'
>>> f.var = 'biggles'
Setting var!
>>> f.var
'biggles'

Answer 2

propertypython docs:https://docs.python.org/2/howto/descriptor.html#properties

class MyClass(object):
    def __init__(self):
        self._var = None

    # only setter
    def var(self, newValue):
        self._var = newValue

    var = property(None, var)


c = MyClass()
c.var = 3
print ('ok')
print (c.var)

输出:

ok
Traceback (most recent call last):
  File "Untitled.py", line 15, in <module>
    print c.var
AttributeError: unreadable attribute

Answer 3

@WeizhongTu 的回答

class MyClass(object):
    def __init__(self):
        self._var = None

    # only setter
    def var(self, newValue):
        self._var = newValue

    var = property(None, var)


c = MyClass()
c.var = 3
print ('ok')
print (c.var)

很好，除了使变量不可获取的事实......

类似的解决方案但保留吸气剂是

var = property(lambda self: self._var, var)

代替

var = property(None, var)