pychronotab

A modern cron expression iterator with full croniter API compatibility, supporting both 5-field (standard) and 6-field (with seconds) cron expressions.

Why pychronotab?

pychronotab was created to solve namespace conflicts when using multiple task schedulers (like django-celery-beat with python-crontab and rq-scheduler with croniter) in the same project. Both python-crontab and older cron libraries expose a top-level crontab module, causing import conflicts.

pychronotab provides:

• ✅ Zero namespace conflicts - all imports under pychronotab
• ✅ Full croniter API compatibility - drop-in replacement for abandoned croniter
• ✅ 6-field cron support - includes seconds field for sub-minute scheduling
• ✅ Modern timezone handling - uses zoneinfo (Python 3.9+)
• ✅ DST-aware - handles daylight saving time transitions correctly
• ✅ Active maintenance - not abandoned

Installation

pip install pychronotab

Quick Start

Modern API

from datetime import datetime, timezone
from pychronotab import CronExpression

# 5-field cron (standard)
expr = CronExpression("*/5 * * * *", tz=timezone.utc)
now = datetime.now(timezone.utc)
next_run = expr.next(now)
print(f"Next run: {next_run}")

# 6-field cron (with seconds)
expr_seconds = CronExpression("*/30 */5 * * * *", tz=timezone.utc)
next_run = expr_seconds.next(now)
print(f"Next run (with seconds): {next_run}")

croniter-Compatible API

Drop-in replacement for croniter (just change the import):

from datetime import datetime
from pychronotab import croniter

# Standard 5-field cron
it = croniter("*/5 * * * *", datetime.now())
print(it.get_next(datetime))
print(it.get_next(datetime))

# 6-field cron with seconds
it_seconds = croniter("*/30 */5 * * * *", datetime.now())
print(it_seconds.get_next(datetime))

Cron Expression Format

5-field format (standard Unix cron):

* * * * *
│ │ │ │ │
│ │ │ │ └─── day of week (0-6, SUN-SAT)
│ │ │ └───── month (1-12, JAN-DEC)
│ │ └─────── day of month (1-31)
│ └───────── hour (0-23)
└─────────── minute (0-59)

6-field format (with seconds):

* * * * * *
│ │ │ │ │ │
│ │ │ │ │ └─── day of week (0-6, SUN-SAT)
│ │ │ │ └───── month (1-12, JAN-DEC)
│ │ │ └─────── day of month (1-31)
│ │ └───────── hour (0-23)
│ └─────────── minute (0-59)
└───────────── second (0-59)

Supported syntax:

• * - any value
• 5 - specific value
• 1-5 - range of values
• */5 - step values (every 5)
• 1,3,5 - list of values
• 1-5/2 - range with step
• JAN-DEC, SUN-SAT - month/day names

API Reference

CronExpression (Modern API)

class CronExpression:
    def __init__(self, expr: str, tz: timezone | None = None)
    def next(self, base: datetime | None = None, *, inclusive: bool = False) -> datetime
    def prev(self, base: datetime | None = None, *, inclusive: bool = False) -> datetime
    def iter(self, start: datetime, *, direction: str = "forward", inclusive: bool = False) -> Iterator[datetime]

croniter (Compatibility API)

class croniter:
    def __init__(self, expr_format: str, start_time: datetime | None = None, day_or: bool = True)
    def get_next(self, ret_type: Type = datetime) -> datetime | float
    def get_prev(self, ret_type: Type = datetime) -> datetime | float
    def get_current(self, ret_type: Type = datetime) -> datetime | float
    def all_next(self, ret_type: Type = datetime) -> Iterator[datetime | float]
    def all_prev(self, ret_type: Type = datetime) -> Iterator[datetime | float]

Migration from croniter

Simply change your import:

# Old
from croniter import croniter

# New
from pychronotab import croniter

Everything else stays the same!

Avoiding Namespace Conflicts

Unlike python-crontab (which exposes crontab module) and old croniter dependencies, pychronotab keeps everything under its own namespace:

# ✅ Safe - no conflicts
from pychronotab import CronExpression, croniter

# ❌ Never exposed - won't conflict with python-crontab
# from crontab import ...  # This won't exist in pychronotab

This means you can safely use:

• django-celery-beat (uses python-crontab)
• rq-scheduler (uses pychronotab instead of abandoned croniter)
• Any other crontab-based library

…all in the same project without import conflicts!

License

MIT License - see LICENSE file for details.

Contributing

Contributions welcome! Please open an issue or PR on GitHub.