title: setter method of property decorator not being called
body: <p>I am trying to use a property method to set the status of a class instance, with the following class definition:</p>
  
  <pre><code>class Result:
      def __init__(self,x=None,y=None):
          self.x = float(x)
          self.y = float(y)
          self._visible = False
          self._status = "You can't see me"
  
      @property
      def visible(self):
          return self._visible
  
      @visible.setter
      def visible(self,value):
          if value == True:
              if self.x is not None and self.y is not None:
                  self._visible = True
                  self._status = "You can see me!"
              else:
                  self._visible = False
                  raise ValueError("Can't show marker without x and y coordinates.")
          else:
              self._visible = False
              self._status = "You can't see me"
  
      def currentStatus(self):
          return self._status
  </code></pre>
  
  <p>From the results though, it seems that the setter method is not being executed, although the internal variable is being changed:</p>
  
  <pre><code>&gt;&gt;&gt; res = Result(5,6)
  &gt;&gt;&gt; res.visible
  False
  &gt;&gt;&gt; res.currentStatus()
  "You can't see me"
  &gt;&gt;&gt; res.visible = True
  &gt;&gt;&gt; res.visible
  True
  &gt;&gt;&gt; res.currentStatus()
  "You can't see me"
  </code></pre>
  
  <p>What am I doing wrong?</p>