Conflict between "Day of Month" & "Day of Week"

[CooledCoffee]

Cron expressions such as "0 0 1-7 * 4" (first thursday of every month) is not supported by your library.
The error is caused by these code:

    if not matchers.day.any:
        _assert(matchers.weekday.any,
            "missing a wildcard specifier for weekday")
    if not matchers.weekday.any:
        _assert(matchers.day.any,
            "missing a wildcard specifier for day")

in the CronTab._make_matchers method.

[grahambell]

Wouldn't "0 0 1-7 * 4" mean days 1-7 and also any Thursdays? The wikipedia page given in the parse-crontab README instead mentions the # symbol for expressions such as "the first Thursday".

The crontab(5) manual on my system says:

Note: The day of a command's execution can be specified in the following two fields — 'day of month', and 'day of week'. If both fields are restricted (i.e., do not contain the "*" character), the command will be run when either field matches the current time. For example, "30 4 1,15 * 5" would cause a command to be run at 4:30 am on the 1st and 15th of each month, plus every Friday.

[josiahcarlson]

As @grahambell mentioned, the '#' specifier is supposed to handle that behavior, but this module explicitly excludes that functionality. And as @grahambell quoted, when both day of month and week are restricted, cron expressions don't work the way you would expect.

To address this limitation, I would suggest using the "0 0 1-7 * *" variant, then verifying upon trigger that the date/time is on the day of week that you want. I do a similar thing to handle timezones and DST changes in my own code that uses this package.