def _assertField(self, name):
        """Raise AttributeError when PacketHistory has no field with the given
        name.
        """
        if name not in self._names:
            msg    = 'PacketHistory "%s" has no field "%s"'
            values = self._defn.name, name
            raise AttributeError(msg % values)
Raise AttributeError when PacketHistory has no field with the given
        name.