simple cron implementation with useful logging

cron.py

import datetime
import logging
import subprocess
import sys
import time

MINUTE = datetime.timedelta(seconds=60)

logging.basicConfig(
    stream=sys.stderr,
    level=logging.INFO,
    format='%(asctime)s %(levelname)-8s %(message)s',
)


def parse_crontab(path):
    entries = []
    with open(path) as fh:
        for line in fh:
            if line.startswith('#') or not line.strip():
                continue
            else:
                entries.append(line.rstrip().split('\t', 5))
    return entries


def is_match(entry, now):
    return (
        (entry[0] == '*' or int(entry[0]) == now.minute)
        and (entry[1] == '*' or int(entry[1]) == now.hour)
        and (entry[2] == '*' or int(entry[2]) == now.day)
        and (entry[3] == '*' or int(entry[3]) == now.month)
        and (entry[4] == '*' or int(entry[4]) == now.weekday())
    )


def log_errors(processes):
    processes_keep = []
    for cmd, p in processes:
        if p.poll() is None:
            processes_keep.append((cmd, p))
        elif p.returncode != 0:
            logging.error('%s exited with %i' % (cmd, p.returncode))
    return processes_keep


if __name__ == '__main__':
    crontab = parse_crontab(sys.argv[1])
    processes = []
    now = datetime.datetime.now()
    while True:
        # NOTE: as a side effect, this mechanism correctly handles
        # daylight saving time: ``now`` always increases monotonously.
        # If ``datetime.now()`` moves back, ``now``, waits for it to
        # catch up. If ``datetime.now()`` jumps forward, ``now``
        # immediately moves through the complete hour.
        while now <= datetime.datetime.now():
            for entry in crontab:
                if is_match(entry, now):
                    cmd = entry[-1]
                    logging.info('Running %s' % cmd)
                    p = subprocess.Popen(cmd, shell=True)
                    processes.append((cmd, p))

            processes = log_errors(processes)
            now += MINUTE

        time.sleep(60)