[Tutor] Treating program exceptions

[Sean 'Shaleh' Perry]

> ---
> class MyProgError(Exception): pass
> class ImproperArgs(MyProgError): pass
> 
> if num_args != 6 and num_args != 4:
>     raise ImproperArgs, "Usage: %s property1 value1 property2 value2
> [property3 value3]" % (p\
> rog_name)
> ---
> 
> Using this approach, is there a way to return an error status code [like
> sys.exit()] as well as a message?
> 

the item passes along with the exception can be any python object.  So you
could do:

raise FooException, ('descriptive text', error_code)

> Are there other alternatives I should be considering? 
> 

Using an exception gives you the ability to handle similar errors in a similar
way or the same error in different ways.

try:
        if num_args != 6 and num_args != 4:
                raise ImproperArgs, (1, num_args)
        if 'badoption' in arg_list: # this would not be a static string
                raise ImproperArgs, (2, 'badoption')
        ...
catch ImproperArgs, info:
        if info[0] == 1:
                print "Either 4 or 6 options expected, %d seen" % info[1]
        elsif info[0] == 2:
                print "This option is obsolete, %s" % info[1]
        print "Usage: blah blah"
        sys.exit(info[0])

Now the usage string appears only once localized to where it will be displayed.

You could then wrap all of the initial arg checking in a function:

settings = parse_args(sys.argv)

try:
        settings = parse_args()
catch ImproperArgs:
        ...