How to add custom exception for non-integer input

Hi Daniel,

One way to approach this would be to create a Custom Exception. This could be as simple as:

class NumberTooLargeException(Exception):
    pass

Then replace this line from Craig's code:

    raise ValueError("There are only {} tickets remaining".format(tickets_remaining))

With:

    raise NumberToolargeException(num_tickets, tickets_remaining)

You can then treat the except ValueError clause as dealing only with values that can't be expressed as ints (e.g., 'blue') and rewrite the corresponding print statement accordingly; and add a new except NumberToolargeException as err clause to provide specific guidance for this error:

except NumberTooLargeException as err:
    requested, available = err.args
    print(f'You requested {requested} tickets but there are only {available} tickets remaining')

Since you mentioned overriding ValueError, this is possible (at least, subclassing ValueError is possible), but not recommended. It's dangerous to subclass any error that you are likely to check for, since the except clause for the parent class will also catch errors for the subclass. For example, suppose we created NumberTooLargeException but as a subclass of ValueError instead of Exception and our try/except structure looked like this:

...
try:
    num_tickets = int(num_tickets)
    if num_tickets > tickets_remaining:
        raise NumberTooLargeException(num_tickets, tickets_remaining)
    except ValueError:
        # only strings that can't be turned into ints should end up here
        print("That input was not a number")
    except NumberTooLargeException as err:
        # numbers that are too big should end up here
        requested, available = err.args
        print(f'You requested {requested} tickets but there are only {available} tickets remaining')
    else:
        ...

At a glance, the above code looks fine. But try asking for more tickets than there are available and, because NumberTooLargeExceptions are ValueErrors, you'll get an error message telling you that it wasn't a number.

Hope that was of some use to you.

Cheers

Alex