datetime weekday （可以返回某天是一个星期的第几天）的源码只有return 0

/* Day of week, where Monday==0, ..., Sunday==6.  1/1/1 was a Monday. */

static int

weekday(int year, int month, int day)

{

    return (ymd_to_ord(year, month, day) + 6) % 7;

}

/* year, month, day -> ordinal, considering 01-Jan-0001 as day 1. */

static int

ymd_to_ord(int year, int month, int day)

{

    return days_before_year(year) + days_before_month(year, month) + day;

}

static int

days_before_year(int year)

{

    int y = year - 1;

    /* This is incorrect if year <= 0; we really want the floor

     * here.  But so long as MINYEAR is 1, the smallest year this

     * can see is 1.

     */

    assert (year >= 1);

    return y*365 + y/4 - y/100 + y/400;

}