code: def valuefrompostdata(self, postdata):
          
          if self.id in postdata and postdata[self.id] != '':
              return float(postdata[self.id])
          else:
              return None
docs: This parameter method searches the POST data and retrieves the values it needs. It does not set the value yet though, but simply returns it. Needs to be explicitly passed to parameter.set()