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.as_string())
          else:
              # Store new value
              self._value = coord.Angle(value, unit=self.unit)
              if self.checker:
                  self.checker(*dummy)
docs: This gets called on any attempt to change the value