exception handling

[Roy Smith]

Mage <mage at mage.hu> wrote:

>        Hello,
> 
> 
> def error_msg(msg):
>     sys.exit(msg)
> 
> try:
>     do_something()
>     if value != my_wish:
>        error_msg('Invalid input')
> except:
>     print "Fatal IO or Network error"
> 
> This doesn't work because sys.exit raises an exception.

Peter Otten posted a good way to do what you want, but I'm not convinced 
that it's a good idea.

You're assuming that any exception you get will be a "Fatal IO or Network 
error".  What if it's not?  What if do_something() raises FutureWarning, or 
KeyboardInterrupt, or AssertionError, or DeprecationWarning, or anything 
else which has nothing to do with IO or networks?  You would just end up 
producing a misleading error message.

If you expect do_something() will throw certain exceptions, catch them 
specifically.  It's usually a mistake to catch everything.  It's certainly 
a mistake to catch everything and assume you know what it must be.

Try running the following code and see what happens:

import traceback

def do_something():
    pass

try:
    do_something()
    if value != my_wish:
        error_msg('Invalid input')
except:
    print "Fatal IO or Network error"
    print traceback.print_exc()