Overview

expiration is designed to manage complex expiration rules. My specific use case is for backups, where I want to keep yearly backups forever, monthly backups for 2 years, weekly backups for 3 months, daily backups for 2 weeks, and hourly backups for 1 week. with expiration, you’d implement the rules describing this as:

>>> from __future__ import print_function
>>> import expiration
>>> from datetime import timedelta
>>> expiration_rules = expiration.RetentionRules(timedelta(weeks=1),   # default value
...                                              [expiration.RetentionCriterion(expiration.date.year_of,
...                                                                             None),
...                                               expiration.RetentionCriterion(expiration.date.month_of,
...                                                                             timedelta(days=2*365)),
...                                               expiration.RetentionCriterion(expiration.date.week_of,
...                                                                             timedelta(days=30*3)),
...                                               expiration.RetentionCriterion(expiration.date.day_of,
...                                                                             timedelta(weeks=2)),
...                                               ])

Next, you need some items that you want to check for expiration. These objects can be any type at all: expiration does not inspect them, except to extract a timestamp from them. And you provide that timestamp access function, called time_key.

For demonstration, I’ll create a bunch of objects with expiration dates:

>>> from collections import namedtuple
>>> from datetime import datetime
>>> Item = namedtuple('Item', 'name expiration')
>>> items = [Item(1, datetime(2012, 1, 1)),
...          Item(2, datetime(2014, 1, 3)),
...          Item(3, datetime(2014, 2, 1)),
...          Item(4, datetime(2014, 2, 2)),
...          Item(4, datetime(2014, 7, 1)),
...          Item(5, datetime(2014, 7, 25, 8, 0)),
...          Item(6, datetime(2014, 7, 25, 9, 0)),
...          Item(7, datetime(2014, 7, 25, 10, 0)),
...         ]

And now, we find which ones have expired:

>>> from operator import attrgetter
>>> for item, criterion in expiration.find_expired_items(items, attrgetter('expiration'),
...                                                             expiration_rules,
...                                                             asof_timestamp=datetime(2014, 8, 1)):
...    print(item)
Item(name=4, expiration=datetime.datetime(2014, 2, 2, 0, 0))

This shows that under these rules, item 4 is the only one that would be deleted. It’s not the first one in a month and it’s more than 3 months old, so it’s due to be expired.

If, instead, we ask what items have expired as of January 1, 2020:

>>> for item, criterion in expiration.find_expired_items(items, attrgetter('expiration'),
...                                                      expiration_rules,
...                                                      asof_timestamp=datetime(2020, 1, 1)):
...    print(item)
Item(name=3, expiration=datetime.datetime(2014, 2, 1, 0, 0))
Item(name=4, expiration=datetime.datetime(2014, 2, 2, 0, 0))
Item(name=4, expiration=datetime.datetime(2014, 7, 1, 0, 0))
Item(name=5, expiration=datetime.datetime(2014, 7, 25, 8, 0))
Item(name=6, expiration=datetime.datetime(2014, 7, 25, 9, 0))
Item(name=7, expiration=datetime.datetime(2014, 7, 25, 10, 0))

The only items that would be kept are 1 and 2, since they’re the first items at the start of each year, which has an infinite expiration time.

Limitations

Because find_expired_items only compares one item at a time to the rules, it can effectively only implement “first of” rules. That is, apply a criterion to the first backup of the year, or the first one of the month, etc. It cannot currently implement rules like “keep the second backup of the month”. If you did delete the first backup and kept the second one, then the next time find_expired_items ran, it would see what used to be the second item as now being the first one.

Change log