How to Mock Cron Schedule in Python

import re
from datetime import datetime, timedelta

class CronMock:
    def __init__(self, expression):
        self.expression = expression
        self.minutes = self._parse_field(expression.split()[0], 0, 59)
        self.hours = self._parse_field(expression.split()[1], 0, 23)
        self.days = self._parse_field(expression.split()[2], 1, 31)
        self.months = self._parse_field(expression.split()[3], 1, 12)
        self.dow = self._parse_field(expression.split()[4], 0, 6)

    def _parse_field(self, field, low, high):
        if field == '*':
            return set(range(low, high + 1))
        values = set()
        for part in field.split(','):
            if '-' in part:
                start, end = map(int, part.split('-'))
                values.update(range(start, end + 1))
            elif '/' in part:
                base, step = part.split('/')
                start = low if base == '*' else int(base)
                values.update(range(start, high + 1, int(step)))
            else:
                values.add(int(part))
        return values

    def next_run(self, start=None):
        now = start or datetime.now()
        now = now.replace(second=0, microsecond=0) + timedelta(minutes=1)
        for _ in range(525600):  # check up to a year ahead
            if (now.minute in self.minutes and now.hour in self.hours and
                now.day in self.days and now.month in self.months and
                now.weekday() in self.dow):
                return now
            now += timedelta(minutes=1)
        return None

if __name__ == "__main__":
    cron = CronMock("*/15 9-17 * * 1-5")
    print("Next run:", cron.next_run(datetime(2024, 5, 6, 10, 0)))