<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _set_tz(values, tz, preserve_UTC=False, coerce=False):
 """ coerce the values to a DatetimeIndex if tz is set preserve the input shape if possible Parameters values : ndarray tz : string/pickled tz object preserve_UTC : boolean, preserve the UTC of the result coerce : if we do not have a passed timezone, coerce to M8[ns] ndarray """

    if tz is not None:
        name = getattr(values, 'name', None)
        values = values.ravel()
        tz = timezones.get_timezone(_ensure_decoded(tz))
        values = DatetimeIndex(values, name=name)
        if values.tz is None:
            values = values.tz_localize('UTC').tz_convert(tz)
        if preserve_UTC:
            if tz == 'UTC':
                values = list(values)
    elif coerce:
        values = np.asarray(values, dtype='M8[ns]')

    return values