def _is_new(self, identifier):
        """Returns True if identifier hasn't been seen before."""
        if identifier in self.tracker:
            return False
        else:
            self.tracker.append(identifier)
            self.tracker.pop(0)
            return True
def _is_new ( self , identifier ) : if identifier in self . tracker : return False else : self . tracker . append ( identifier ) self . tracker . pop ( 0 ) return True
Returns True if identifier hasn't been seen before.