How do I get a Cron like scheduler in Python?

Question

I'm looking for a library in Python which will provide at and cron like functionality.

I'd quite like have a pure Python solution, rather than relying on tools installed on the box; this way I run on machines with no cron.

For those unfamiliar with cron: you can schedule tasks based upon an expression like:

 0 2 * * 7 /usr/bin/run-backup # run the backups at 0200 on Every Sunday
 0 9-17/2 * * 1-5 /usr/bin/purge-temps # run the purge temps command, every 2 hours between 9am and 5pm on Mondays to Fridays.

The cron time expression syntax is less important, but I would like to have something with this sort of flexibility.

If there isn't something that does this for me out-the-box, any suggestions for the building blocks to make something like this would be gratefully received.

Edit I'm not interested in launching processes, just "jobs" also written in Python - python functions. By necessity I think this would be a different thread, but not in a different process.

To this end, I'm looking for the expressivity of the cron time expression, but in Python.

Cron has been around for years, but I'm trying to be as portable as possible. I cannot rely on its presence.

Answer 1

Croniter: Parse a CRON string and get sequence of next event times

• Documentation for Croniter can be found here

None of the listed solutions take the approach of parsing a CRON schedule string. So, here is my version, using croniter. Basic gist:

import croniter
import datetime

now = datetime.datetime.now()
iterator = croniter.croniter('0 0 * * *', now)

next_datetime = iterator.get_next(datetime.datetime) # datetime not now!

remaining_seconds = next_datetime - now
remaining_seconds = remaining_seconds.total_seconds()
print(f'seconds until next event: {remaining_seconds}')

Easy!

---

More complex usecases with more detailed logic:

schedule = "*/5 * * * *" # Run every five minutes

nextRunTime = getNextCronRunTime(schedule)
while True:
     roundedDownTime = roundDownTime()
     if (roundedDownTime == nextRunTime):
         ####################################
         ### Do your periodic thing here. ###
         ####################################
         nextRunTime = getNextCronRunTime(schedule)
     elif (roundedDownTime > nextRunTime):
         # We missed an execution. Error. Re initialize.
         nextRunTime = getNextCronRunTime(schedule)
     sleepTillTopOfNextMinute()

Helper routines:

from croniter import croniter
from datetime import datetime, timedelta

# Round time down to the top of the previous minute
def roundDownTime(dt=None, dateDelta=timedelta(minutes=1)):
    roundTo = dateDelta.total_seconds()
    if dt == None : dt = datetime.now()
    seconds = (dt - dt.min).seconds
    rounding = (seconds+roundTo/2) // roundTo * roundTo
    return dt + timedelta(0,rounding-seconds,-dt.microsecond)

# Get next run time from now, based on schedule specified by cron string
def getNextCronRunTime(schedule):
    return croniter(schedule, datetime.now()).get_next(datetime)

# Sleep till the top of the next minute
def sleepTillTopOfNextMinute():
    t = datetime.utcnow()
    sleeptime = 60 - (t.second + t.microsecond/1000000.0)
    time.sleep(sleeptime)

Answer 2

If I need to check how many matches will match and I can't stay waiting in the same point, a simple off-line checker may look like:

import re
import random
import time
from datetime import datetime, timedelta
from datetime import datetime, timezone, tzinfo

class Crontab:
  ALL = '.*'
  STEP = timedelta(seconds=1)

def __init__(self, **specs):
      self.t0 = self.now()
      self.specs = specs

  def now(self):
      now = datetime.now(tz=timezone.utc).replace(microsecond=0)
      return now

  def check(self):
      "Fires all time that matches from last call"
      now = self.now()
      while self.t0 <= now:
          self.t0 += self.STEP
        for key, pattern in self.specs.items():
            value = getattr(self.t0, key, None)
            if value is not None:
                if not isinstance(value, int):
                    value = value()  # method
                if not re.match(f"{pattern}$", f"{value}"):
                    break
        else:
            # all specs (if any) matches
            return self.t0


if __name__ == "__main__":
cron = Crontab(second='0', minute='0|15|30|45')
while True:
    while not (t := cron.check()):
        time.sleep(random.randint(1, 10))
    print(f"match at: {t}")

output

match at: 2024-09-24 15:30:00+00:00

If you let pass too much time, you will get each matched timestamp every time you call t = cron.check() until eventually get None

Answer 3

I like how the pycron package solves this problem.

import pycron
import time

while True:
    if pycron.is_now('0 2 * * 0'):   # True Every Sunday at 02:00
        print('running backup')
        time.sleep(60)               # The process should take at least 60 sec
                                     # to avoid running twice in one minute
    else:
        time.sleep(15)               # Check again in 15 seconds

Answer 4

I know there are a lot of answers, but another solution could be to go with decorators. This is an example to repeat a function everyday at a specific time. The cool think about using this way is that you only need to add the Syntactic Sugar to the function you want to schedule:

@repeatEveryDay(hour=6, minutes=30)
def sayHello(name):
    print(f"Hello {name}")

sayHello("Bob") # Now this function will be invoked every day at 6.30 a.m

And the decorator will look like:

def repeatEveryDay(hour, minutes=0, seconds=0):
    """
    Decorator that will run the decorated function everyday at that hour, minutes and seconds.
    :param hour: 0-24
    :param minutes: 0-60 (Optional)
    :param seconds: 0-60 (Optional)
    """
    def decoratorRepeat(func):

        @functools.wraps(func)
        def wrapperRepeat(*args, **kwargs):

            def getLocalTime():
                return datetime.datetime.fromtimestamp(time.mktime(time.localtime()))

            # Get the datetime of the first function call
            td = datetime.timedelta(seconds=15)
            if wrapperRepeat.nextSent == None:
                now = getLocalTime()
                wrapperRepeat.nextSent = datetime.datetime(now.year, now.month, now.day, hour, minutes, seconds)
                if wrapperRepeat.nextSent < now:
                    wrapperRepeat.nextSent += td

            # Waiting till next day
            while getLocalTime() < wrapperRepeat.nextSent:
                time.sleep(1)

            # Call the function
            func(*args, **kwargs)

            # Get the datetime of the next function call
            wrapperRepeat.nextSent += td
            wrapperRepeat(*args, **kwargs)

        wrapperRepeat.nextSent = None
        return wrapperRepeat

    return decoratorRepeat

Answer 5

If you're looking for something lightweight checkout schedule:

import schedule
import time

def job():
    print("I'm working...")

schedule.every(10).minutes.do(job)
schedule.every().hour.do(job)
schedule.every().day.at("10:30").do(job)

while 1:
    schedule.run_pending()
    time.sleep(1)

Disclosure: I'm the author of that library.

Answer 6

Another trivial solution would be:

from aqcron import At
from time import sleep
from datetime import datetime

# Event scheduling
event_1 = At( second=5 )
event_2 = At( second=[0,20,40] )

while True:
    now = datetime.now()

    # Event check
    if now in event_1: print "event_1"
    if now in event_2: print "event_2"

    sleep(1)

And the class aqcron.At is:

# aqcron.py

class At(object):
    def __init__(self, year=None,    month=None,
                 day=None,     weekday=None,
                 hour=None,    minute=None,
                 second=None):
        loc = locals()
        loc.pop("self")
        self.at = dict((k, v) for k, v in loc.iteritems() if v != None)

    def __contains__(self, now):
        for k in self.at.keys():
            try:
                if not getattr(now, k) in self.at[k]: return False
            except TypeError:
                if self.at[k] != getattr(now, k): return False
        return True

Answer 7

More or less same as above but concurrent using gevent :)

"""Gevent based crontab implementation"""

from datetime import datetime, timedelta
import gevent

# Some utility classes / functions first
def conv_to_set(obj):
    """Converts to set allowing single integer to be provided"""

    if isinstance(obj, (int, long)):
        return set([obj])  # Single item
    if not isinstance(obj, set):
        obj = set(obj)
    return obj

class AllMatch(set):
    """Universal set - match everything"""
    def __contains__(self, item): 
        return True

allMatch = AllMatch()

class Event(object):
    """The Actual Event Class"""

    def __init__(self, action, minute=allMatch, hour=allMatch, 
                       day=allMatch, month=allMatch, daysofweek=allMatch, 
                       args=(), kwargs={}):
        self.mins = conv_to_set(minute)
        self.hours = conv_to_set(hour)
        self.days = conv_to_set(day)
        self.months = conv_to_set(month)
        self.daysofweek = conv_to_set(daysofweek)
        self.action = action
        self.args = args
        self.kwargs = kwargs

    def matchtime(self, t1):
        """Return True if this event should trigger at the specified datetime"""
        return ((t1.minute     in self.mins) and
                (t1.hour       in self.hours) and
                (t1.day        in self.days) and
                (t1.month      in self.months) and
                (t1.weekday()  in self.daysofweek))

    def check(self, t):
        """Check and run action if needed"""

        if self.matchtime(t):
            self.action(*self.args, **self.kwargs)

class CronTab(object):
    """The crontab implementation"""

    def __init__(self, *events):
        self.events = events

    def _check(self):
        """Check all events in separate greenlets"""

        t1 = datetime(*datetime.now().timetuple()[:5])
        for event in self.events:
            gevent.spawn(event.check, t1)

        t1 += timedelta(minutes=1)
        s1 = (t1 - datetime.now()).seconds + 1
        print "Checking again in %s seconds" % s1
        job = gevent.spawn_later(s1, self._check)

    def run(self):
        """Run the cron forever"""

        self._check()
        while True:
            gevent.sleep(60)

import os 
def test_task():
    """Just an example that sends a bell and asd to all terminals"""

    os.system('echo asd | wall')  

cron = CronTab(
  Event(test_task, 22, 1 ),
  Event(test_task, 0, range(9,18,2), daysofweek=range(0,5)),
)
cron.run()

Answer 8

You could just use normal Python argument passing syntax to specify your crontab. For example, suppose we define an Event class as below:

from datetime import datetime, timedelta
import time

# Some utility classes / functions first
class AllMatch(set):
    """Universal set - match everything"""
    def __contains__(self, item): return True

allMatch = AllMatch()

def conv_to_set(obj):  # Allow single integer to be provided
    if isinstance(obj, (int,long)):
        return set([obj])  # Single item
    if not isinstance(obj, set):
        obj = set(obj)
    return obj

# The actual Event class
class Event(object):
    def __init__(self, action, min=allMatch, hour=allMatch, 
                       day=allMatch, month=allMatch, dow=allMatch, 
                       args=(), kwargs={}):
        self.mins = conv_to_set(min)
        self.hours= conv_to_set(hour)
        self.days = conv_to_set(day)
        self.months = conv_to_set(month)
        self.dow = conv_to_set(dow)
        self.action = action
        self.args = args
        self.kwargs = kwargs

    def matchtime(self, t):
        """Return True if this event should trigger at the specified datetime"""
        return ((t.minute     in self.mins) and
                (t.hour       in self.hours) and
                (t.day        in self.days) and
                (t.month      in self.months) and
                (t.weekday()  in self.dow))

    def check(self, t):
        if self.matchtime(t):
            self.action(*self.args, **self.kwargs)

(Note: Not thoroughly tested)

Then your CronTab can be specified in normal python syntax as:

c = CronTab(
  Event(perform_backup, 0, 2, dow=6 ),
  Event(purge_temps, 0, range(9,18,2), dow=range(0,5))
)

This way you get the full power of Python's argument mechanics (mixing positional and keyword args, and can use symbolic names for names of weeks and months)

The CronTab class would be defined as simply sleeping in minute increments, and calling check() on each event. (There are probably some subtleties with daylight savings time / timezones to be wary of though). Here's a quick implementation:

class CronTab(object):
    def __init__(self, *events):
        self.events = events

    def run(self):
        t=datetime(*datetime.now().timetuple()[:5])
        while 1:
            for e in self.events:
                e.check(t)

            t += timedelta(minutes=1)
            while datetime.now() < t:
                time.sleep((t - datetime.now()).seconds)

A few things to note: Python's weekdays / months are zero indexed (unlike cron), and that range excludes the last element, hence syntax like "1-5" becomes range(0,5) - ie [0,1,2,3,4]. If you prefer cron syntax, parsing it shouldn't be too difficult however.

Answer 9

There isn't a "pure python" way to do this because some other process would have to launch python in order to run your solution. Every platform will have one or twenty different ways to launch processes and monitor their progress. On unix platforms, cron is the old standard. On Mac OS X there is also launchd, which combines cron-like launching with watchdog functionality that can keep your process alive if that's what you want. Once python is running, then you can use the sched module to schedule tasks.

Answer 10

I don't know if something like that already exists. It would be easy to write your own with time, datetime and/or calendar modules, see http://docs.python.org/library/time.html

The only concern for a python solution is that your job needs to be always running and possibly be automatically "resurrected" after a reboot, something for which you do need to rely on system dependent solutions.