Parse cron expression and compute next run datetime in Python

from datetime import datetime, timedelta
import re

def parse_cron_and_next_run(cron_expr, base_time=None):
    """Parse a cron expression and compute the next run time."""
    if base_time is None:
        base_time = datetime.now().replace(second=0, microsecond=0)

    fields = cron_expr.split()
    if len(fields) != 5:
        raise ValueError("Cron expression must have exactly 5 fields: minute hour day month weekday")

    minute, hour, day, month = [int(f) for f in fields[:4]]
    weekday = int(fields[4])

    candidate = base_time + timedelta(minutes=1)
    for _ in range(100000):  # safety limit
        if candidate.month == month and \
           candidate.day == day and \
           candidate.hour == hour and \
           candidate.minute == minute and \
           candidate.weekday() == weekday:
            return candidate
        candidate += timedelta(minutes=1)
    raise ValueError("No matching time found within search limit")

if __name__ == "__main__":
    # Example: run at 9:15 AM on Mondays
    cron = "15 9 * * 1"
    base = datetime(2024, 1, 1, 8, 30)  # Monday, Jan 1, 2024 8:30 AM
    next_run = parse_cron_and_next_run(cron, base)
    print(f"Cron: '{cron}'")
    print(f"Base time: {base}")
    print(f"Next run: {next_run}")