'Return the time formatted according to ISO.
This is \'HH:MM:SS.mmmmmm+zz:zz\', or \'HH:MM:SS+zz:zz\' if
self.microsecond == 0.'

def isoformat(self):

    s = _format_time(self._hour, self._minute, self._second, self._microsecond)
    tz = self._tzstr()
    if tz:
        s += tz
    return s
