Reverse Date Generators for Python

While working on a data collection project, I needed a way to generate previous month and week datetime objects. For example, to be able to filter all data generated within the past week or the past month. Python's datetime module has fairly robust date and time functionality, but nothing that would fit my requirements. This code is released in the public domain.

Previous Month

from datetime import datetime

def reverse_month():
	dt = datetime.now()
	dt = datetime(month=dt.month, day=1, year=dt.year)
	while True:
		yield dt
		m = dt.month
		y = dt.year
		if m == 1:
			m = 12
			y = y - 1
		else:
			m = m - 1
		dt = datetime(month=m, day=1, year=y)

Previous Week

from datetime import datetime, timedelta

def reverse_week():
	dt = datetime.date(datetime.now())
	while True:
		if dt.weekday() != 6:
			dt = dt - timedelta(days=dt.weekday() + 1)
		else:
			dt = dt - timedelta(days=7)
		yield dt

The previous week follows the American convention of Sunday being the first day of the week and Saturday being the last day. Line 6 can be changed to seed the start of the week on any other day.

Sample

rmon = reverse_month()
rweek = reverse_week()

monthseed = datetime.date(datetime.now()) + timedelta(days=1)
weekseed = datetime.date(datetime.now()) + timedelta(days=1)

while True:
	tempmon = rmon.next()
	tempweek = rweek.next()
	print "%s <= date < %s" % (tempmon, monthseed)
	print "%s <= date < %s" % (tempweek, weekseed)
	monthseed = tempmon
	weekseed = tempweek
	raw_input()

Output

2010-02-01 00:00:00 <= date < 2010-02-21
2010-02-14 <= date < 2010-02-21

2010-01-01 00:00:00 <= date < 2010-02-01 00:00:00
2010-02-07 <= date < 2010-02-14

2009-12-01 00:00:00 <= date < 2010-01-01 00:00:00
2010-01-31 <= date < 2010-02-07

2009-11-01 00:00:00 <= date < 2009-12-01 00:00:00
2010-01-24 <= date < 2010-01-31

2009-10-01 00:00:00 <= date < 2009-11-01 00:00:00
2010-01-17 <= date < 2010-01-24