[Tutor] for loops and exceptions

[Kent Johnson]

Matthew White wrote:
> Hello,
> 
>>From a general style and/or programmatic perspective, which is a "better"
> way to write this bit of code?

Hmm, neither?
> 
> try:
>     (dn, attrs) = conn.search_s(search_base, search_scope, search_filter, search_attrs):
> except Exception, e:
>         warn_the_user(e)
>         do_something_useful()
> 
> for (name, data) in (dn, attrs):
>         print '%s' % (name)
>         for key, value in data.iteritems():
>                 print '%s => %s' % (key, value)

This will run the for loop even if you get an exception. I think the for 
loop is buggy, too, it is different from the one below.
> 
> OR
> 
> try:
>     for dn, attrs in conn.search_s(search_base, search_scope, search_filter, search_attrs):
>         print dn
>         for key, value in attrs.iteritems():
>             print '\t%s => %s' % (key, value)
>         print
> except Exception, e:
>      warn_the_user(e)
>      do_something_useful

This might be OK. It will catch exceptions in the for loop, which you 
may or may not want.

If this code is at a low level of your program, it's best to only catch 
expected exceptions, and let unexpected exceptions propagate to a higher 
level. At the higher level, you can have a generic except block that 
reports the error and moves on. This is often in a top-level loop or 
event handler. Your code has elements of both - code that does real work 
with a generic except block that reports errors.

Anyway here is a construction that is very useful for catching just the 
exceptions you expect to see from a bit of code, while running the rest 
of the code outside of the try block.

try:
     (dn, attrs) = conn.search_s(search_base, search_scope, 
search_filter, search_attrs):
except ConnectionError, e:
         warn_the_user(e)
         do_something_useful()
else:
         print dn
         for key, value in attrs.iteritems():
             print '\t%s => %s' % (key, value)
         print

The idea is to put as little code as possible in the scope of the try, 
and to catch as few exceptions as possible. If no exception is raised, 
the else: block will run. If there is an exception in that code, it will 
propagate to a higher level.

Kent