Question

In Python, given a date, how do I find the preceding weekday? (Weekdays are Mon to Fri. I don’t care about holidays)

Answer 1

Simply subtract a day from the given date, then check if the date is a weekday. If not, subtract another, until you do have a weekday:

from datetime import date, timedelta
def prev_weekday(adate):
    adate -= timedelta(days=1)
    while adate.weekday() > 4: # Mon-Fri are 0-4
        adate -= timedelta(days=1)
    return adate

Demo:

>>> prev_weekday(date.today())
datetime.date(2012, 8, 20)
>>> prev_weekday(date(2012, 8, 20))
datetime.date(2012, 8, 17)

Alternatively, use an offset table; no need to make this a mapping, a tuple will do just fine:

_offsets = (3, 1, 1, 1, 1, 1, 2)
def prev_weekday(adate):
    return adate - timedelta(days=_offsets[adate.weekday()])