last_sunday.py

import datetime


def get_last_sunday(dt: datetime.datetime) -> datetime.datetime:
    """
    ある時刻が与えられたとき、その時刻を前回の日曜の15時に変換する

    >>> get_last_sunday(datetime.datetime(2022, 12, 13, 2, tzinfo=datetime.timezone.utc))
    datetime.datetime(2022, 12, 11, 15, 0, tzinfo=datetime.timezone.utc)
    >>> get_last_sunday(datetime.datetime(2022, 12, 18, 14, 0, tzinfo=datetime.timezone.utc))
    datetime.datetime(2022, 12, 11, 15, 0, tzinfo=datetime.timezone.utc)
    >>> get_last_sunday(datetime.datetime(2022, 12, 18, 17, 0, tzinfo=datetime.timezone.utc))
    datetime.datetime(2022, 12, 18, 15, 0, tzinfo=datetime.timezone.utc)

    """
    # dt より前の 15:00
    if dt.time() < datetime.time(15, 0):
        dt = dt - datetime.timedelta(days=1)
    dt = datetime.datetime(*dt.timetuple()[:3], 15, 0, tzinfo=datetime.timezone.utc)

    # dt より前の日曜日
    dt = dt - datetime.timedelta(days=(dt.weekday() + 1) % 7)

    return dt


if __name__ == '__main__':
    import doctest
    doctest.testmod()