HKUDS/DeepTutor · error · ValueError

invalid cron expression {schedule.expr!r}: {exc}

Error message

invalid cron expression {schedule.expr!r}: {exc}

What it means

compute_next_run wraps croniter evaluation in a broad except and re-raises any parsing/evaluation failure as ValueError including the offending expression and underlying cause. Invalid or unsupportable cron strings fail fast at validation time rather than at fire time.

Source

Thrown at deeptutor/services/cron/service.py:146

        return now_ms + schedule.every_seconds * 1000

    if schedule.kind == "cron" and schedule.expr:
        try:
            from zoneinfo import ZoneInfo

            from croniter import croniter

            tz = ZoneInfo(schedule.tz) if schedule.tz else datetime.now().astimezone().tzinfo
            base = datetime.fromtimestamp(now_ms / 1000, tz=tz)
            next_dt = croniter(schedule.expr, base).get_next(datetime)
            return int(next_dt.timestamp() * 1000)
        except ImportError:
            raise ValueError(
                "cron expressions need the 'croniter' package — "
                "use an 'every' or 'at' schedule instead"
            ) from None
        except Exception as exc:
            raise ValueError(f"invalid cron expression {schedule.expr!r}: {exc}") from None

    return None


def validate_schedule(schedule: CronSchedule) -> None:
    """Reject schedules that could never run (raises ValueError)."""
    if schedule.kind == "at":
        if not schedule.at_ms:
            raise ValueError("'at' schedules need a time")
        if schedule.at_ms <= _now_ms():
            raise ValueError("'at' time is in the past")
        return
    if schedule.kind == "every":
        if not schedule.every_seconds or schedule.every_seconds < 30:
            raise ValueError("'every' interval must be at least 30 seconds")
        return
    if schedule.kind == "cron":
        if schedule.tz:

View on GitHub (pinned to 3e82f13042)

Solutions

1. Fix the expression to standard 5-field cron syntax (minute hour day month weekday)
2. Validate user input with croniter.croniter(expr, datetime.now()) before calling add_job
3. Log schedule.expr on failure to identify which job config is broken

Example fix

# before
expr = "0 9 * * *" + user_input  # concatenated garbage
job = svc.add_job(CronSchedule(kind="cron", expr=expr), ...)
# after
from croniter import croniter
try:
    croniter(user_input, datetime.now())
    expr = user_input
except Exception:
    expr = "0 9 * * *"
job = svc.add_job(CronSchedule(kind="cron", expr=expr), ...)

from datetime import datetime
from croniter import croniter

def valid_cron(expr: str) -> bool:
    try:
        croniter(expr, datetime.now())
        return True
    except Exception:
        return False

assert valid_cron(schedule.expr), f"bad cron: {schedule.expr!r}"

try:
    svc.add_job(schedule, ...)
except ValueError as e:
    if "invalid cron expression" in str(e):
        log.warning("rejecting cron %r", schedule.expr)
        return error_response(str(e))
    raise

When it happens

Trigger: Passing a malformed expression like CronSchedule(kind="cron", expr="99 * * *") or "* * *" (wrong field count) to add_job or validate_schedule.

Common situations: Typos in hand-written cron strings; converting 5-field to 6-field/Quartz syntax; unsanitized user-supplied cron input from forms or chat commands.

Related errors

• 'every' interval must be at least 30 seconds
• cron expressions need the 'croniter' package — use an 'every
• 'at' schedules need a time
• 'at' time is in the past
• unknown timezone {schedule.tz!r}

AI-assisted analysis of HKUDS/DeepTutor@3e82f13042 (2026-08-27). Data as JSON: /api/errors/bec021ed12de62c9. Report an issue: GitHub.