larzcron

Cron expressions and an in-process scheduler. Pure Python, zero dependencies.

Parse and match standard cron expressions, compute the next/previous fire time, and run jobs on cron schedules or fixed intervals — all in-process, with no daemon, no service, and nothing to install.

from larzcron import CronExpression
from datetime import datetime

c = CronExpression("*/15 9-17 * * mon-fri")     # every 15 min, 9-5, weekdays
c.matches(datetime(2026, 7, 27, 9, 30))         # True
c.next(datetime(2026, 7, 27, 9, 31))            # -> 2026-07-27 09:45

Why

• Zero dependencies. No croniter, no APScheduler, no service to run — just the standard library.
• Full everyday cron syntax. *, ranges (1-5), lists (1,3,5), steps (*/15, 0-30/10), three-letter names (mon, jan), Sunday as 0 or 7, and macros (@daily, @hourly, @weekly, @monthly, @yearly). It even honours the Vixie-cron rule where a restricted day-of-month or day-of-week matches.
• Answers the real questions. matches(dt), next(after), previous(before) — parse a schedule and know exactly when it fires.
• A scheduler when you want one. Register cron and interval jobs and either run a background thread or drive it deterministically with run_pending(now) (perfect for tests and cron/lambda-style ticks).

Install

pip install larzcron

Expressions

CronExpression("0 * * * *")        # top of every hour
CronExpression("*/15 * * * *")     # every 15 minutes
CronExpression("0 9-17 * * mon-fri")
CronExpression("0 0 1,15 * *")     # 1st and 15th at midnight
CronExpression("@daily")           # macros

c.matches(dt)          # bool
c.next(after)          # next datetime strictly after `after`
c.previous(before)     # previous matching datetime

Scheduler

from larzcron import Scheduler

sched = Scheduler()

@sched.cron("0 * * * *")     # hourly
def rollup():
    ...

@sched.every(30)             # every 30 seconds
def heartbeat():
    ...

sched.start()                # background daemon thread; sched.stop() to end

For tests and cron-driven environments, tick it yourself — deterministic, no threads:

sched.run_pending(now=some_datetime)   # fires whatever is due at `now`

Add an on_error=lambda job, exc: ... handler to catch job exceptions without stopping the scheduler.

Tests

python -m unittest discover -s tests -v      # 27 tests, zero deps

The Larz stack

Pure-Python, zero-dependency building blocks: larz · larzchain · larzmoney · larzcrypt · larzdb · larzagent · larzchart · larzmark · larztask · larzvault · larzvm · larzcache · larzvalidate · larzid · larzrpc · larzstate · larzhttp · larzconf · larzcron

License

MIT © larz-scripter