<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def isoformat(self, sep='T'):
 """Return the time formatted according to ISO. This is 'YYYY-MM-DD HH:MM:SS.mmmmmm', or 'YYYY-MM-DD HH:MM:SS' if self.microsecond == 0. If self.tzinfo is not None, the UTC offset is also attached, giving 'YYYY-MM-DD HH:MM:SS.mmmmmm+HH:MM' or 'YYYY-MM-DD HH:MM:SS+HH:MM'. Optional argument sep specifies the separator between date and time, default 'T'. """

        s = ("%04d-%02d-%02d%c" % (self._year, self._month, self._day,
                                  sep) +
                _format_time(self._hour, self._minute, self._second,
                             self._microsecond))
        off = self.utcoffset()
        if off is not None:
            if off.days < 0:
                sign = "-"
                off = -off
            else:
                sign = "+"
            hh, mm = divmod(off, timedelta(hours=1))
            assert not mm % timedelta(minutes=1), "whole minute"
            mm //= timedelta(minutes=1)
            s += "%s%02d:%02d" % (sign, hh, mm)
        return s