backup pruning script inspired by Time Machine

#!/usr/bin/env python

import datetime
import glob
import logging
import os
import shutil
import sys

def datefrompath(path):
    name = os.path.basename(path)
    year = int(name[0:4])
    month = int(name[4:6])
    day = int(name[6:8])
    hour = int(name[9:11])
    minute = int(name[11:13])
    second = int(name[13:15])
    return datetime.datetime(year, month, day, hour, minute, second)

def keep(folder):
    logging.info('keep %s', folder)

def pruneerror(function, path, excinfo):
    logging.error('unable to prune %s', path)

def prune(folder):
    logging.info('prune %s', folder)
    shutil.rmtree(folder, False, pruneerror)

# keep oldest backup per interval from now through period back in time
def prunefolders(folders, now, interval, period):
    then = now - period;
    maybeprune = []

    while now > then:
        now -= interval

        while folders:
            folder = folders.pop()
            folderdate = datefrompath(folder)

            if folderdate > now:
                maybeprune.append(folder)
            else:
                folders.append(folder)
                break;

        if maybeprune:
            keep(maybeprune.pop())
            while maybeprune:
                prune(maybeprune.pop())


repository = os.path.abspath(sys.argv[1])

logging.basicConfig(
    filename = os.path.join(repository, 'prune.log'),
    format = '%(asctime)s:%(levelname)s: %(message)s',
    level = logging.INFO
    )

logging.info('pruning %s', repository)

folders = glob.glob(os.path.join(repository, '20*'))
folders.sort()

oneday = datetime.timedelta(days = 1)
oneweek = datetime.timedelta(weeks = 1)
onemonth = datetime.timedelta(weeks = 4)
oneyear = datetime.timedelta(weeks = 52)

now = datetime.datetime.now()
then = now - oneday

# keep all backups for last 24 hours
logging.info('hourly')
while folders:
    folder = folders.pop()
    folderdate = datefrompath(folder)

    if folderdate > then:
        keep(folder)
    else:
        folders.append(folder)
        break

midnight = datetime.datetime(now.year, now.month, now.day)

# keep oldest backup daily for one month
logging.info('daily')
prunefolders(folders, midnight, oneday, onemonth)

# keep oldest backup weekly for one year
logging.info('weekly')
prunefolders(folders, midnight, oneweek, oneyear)

# discard all backups older than one year
logging.info('yearly')
while folders:
    prune(folders.pop())

logging.shutdown()