dates.py 文件源码

def get_next_timezone_transition(zone=None, dt=None):
    """Given a timezone it will return a :class:`TimezoneTransition` object
    that holds the information about the next timezone transition that's going
    to happen.  For instance this can be used to detect when the next DST
    change is going to happen and how it looks like.

    The transition is calculated relative to the given datetime object.  The
    next transition that follows the date is used.  If a transition cannot
    be found the return value will be `None`.

    Transition information can only be provided for timezones returned by
    the :func:`get_timezone` function.

    :param zone: the timezone for which the transition should be looked up.
                 If not provided the local timezone is used.
    :param dt: the date after which the next transition should be found.
               If not given the current time is assumed.
    """
    zone = get_timezone(zone)
    if dt is None:
        dt = datetime.utcnow()
    else:
        dt = dt.replace(tzinfo=None)

    if not hasattr(zone, '_utc_transition_times'):
        raise TypeError('Given timezone does not have UTC transition '
                        'times.  This can happen because the operating '
                        'system fallback local timezone is used or a '
                        'custom timezone object')

    try:
        idx = max(0, bisect_right(zone._utc_transition_times, dt))
        old_trans = zone._transition_info[idx - 1]
        new_trans = zone._transition_info[idx]
        old_tz = zone._tzinfos[old_trans]
        new_tz = zone._tzinfos[new_trans]
    except (LookupError, ValueError):
        return None

    return TimezoneTransition(
        activates=zone._utc_transition_times[idx],
        from_tzinfo=old_tz,
        to_tzinfo=new_tz,
        reference_date=dt
    )