crontab

1.0.5 · active · verified Sat Apr 11

The `crontab` library (from the `josiahcarlson/parse-crontab` project) is a Python package designed to parse crontab schedule entries and determine the next execution time. It calculates delays in seconds until a scheduled event. The current version is 1.0.5, with sporadic but active development and releases.

Warnings

gotcha Potential confusion with `python-crontab` library. The `crontab` (PyPI slug) library, backed by `josiahcarlson/parse-crontab` on GitHub, focuses on parsing cron expressions and calculating next run times. Another popular library, `python-crontab` (PyPI slug), provides similar `from crontab import CronTab` imports but is designed for managing system crontab entries. Ensure you've installed and are importing the library that matches your intended use (parsing vs. system management).
Fix: Verify the library's functionality on its PyPI page or GitHub repository, and explicitly install `crontab` for parsing or `python-crontab` for system management if that's what you need.
gotcha Unsupported cron expression symbols and field limits. This library (`crontab` by josiahcarlson) explicitly states that 'W' and '#' symbols are not supported. It also only accepts 5-7 value crontab entries, which are internally 'mangled' to 7 values (seconds-year) as necessary.
Fix: Avoid using unsupported symbols. Ensure your cron expressions adhere to the 5-7 field format compatible with standard Unix cron, noting the library's specific interpretation.
gotcha Scripts executed by system cron jobs often fail due to differences in the execution environment (e.g., `PATH`, current working directory, user permissions, virtual environment activation) compared to manual execution. This is a general `cron` issue but critical for scripts scheduled based on this library's output.
Fix: Use absolute paths for scripts, commands, and any files accessed. Explicitly set `PATH` and activate virtual environments within the cron job definition. Redirect stdout/stderr to log files for debugging.

Install

pip install crontab Install stable version

Imports

CronTab
```
from crontab import CronTab
```
This library (pypi-slug: `crontab`, github: `josiahcarlson/parse-crontab`) is distinct from `python-crontab` (pypi-slug: `python-crontab`), which has similar import paths but different functionality (parsing vs. system crontab management).

Quickstart

This example demonstrates how to parse a cron string and calculate its next execution time using the `CronTab` class. It uses `datetime` and `timedelta` to convert the returned delay in seconds into an absolute datetime object.

from crontab import CronTab
from datetime import datetime

# Define a crontab entry for 25 minutes past the hour, every hour
entry = CronTab('25 * * * *')

# Get the next scheduled execution time from now
now = datetime.now()
next_run_time = now + timedelta(seconds=entry.next(now))
print(f"Current time: {now}")
print(f"Next run for '25 * * * *' will be at: {next_run_time}")

# Example with a specific start time
some_past_time = datetime(2023, 1, 1, 10, 15, 0)
next_from_past = some_past_time + timedelta(seconds=entry.next(some_past_time))
print(f"Next run from {some_past_time} would be at: {next_from_past}")

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