code: def _callback(self, *dummy):
          
          # retrieve the value from the Entry
          value = self._variable.get()
  
          # run the validation. Returns None if no good
          newvalue = self.validate(value)
  
          if newvalue is None:
              # Invalid: restores previously stored value
              # no checker run.
              self._variable.set(self._value)
  
          elif newvalue != value:
              # If the value is different update appropriately
              # Store new value.
              self._value = newvalue
              self._variable.set(self.newvalue)
          else:
              # Store new value
              self._value = value
docs: This gets called on any attempt to change the value