newbie question about confusing exception handling in urllib

[Chris Angelico]

On Wed, Apr 10, 2013 at 1:05 AM, Steven D'Aprano
<steve+comp.lang.python at pearwood.info> wrote:
> One exception to this rule (no pun intended) is that sometimes you want
> to hide the details of unexpected tracebacks from your users. In that
> case, it may be acceptable to wrap your application's main function in a
> try block, catch any unexpected exceptions, log the exception, and then
> quietly exit with a short, non-threatening error message that won't scare
> the civilians

This is important to some types of security concern, too; for
instance, if I'm running a web server, I probably don't want to leak
details of exceptions and tracebacks to a potential attacker. Same
again: catch the exception, log it, return simple error message;
additionally, you can return that message as an HTTP response rather
than simply bombing the web server. But again, a bare except should
almost always be logging its exceptions.

True story, though not in Python: After taking over the code of an
ex-coworker, I was trying to fix some crazy problems. Everything I did
seemed to kinda-work, but nothing properly worked. Trying to clean up
the code to comply with "use strict" mode (which will tell you what
language this is, and it isn't Perl) was a matter of blundering about
in the dark. Turned out there was an event handler somewhere that
buried the *entire file full of code* behind a callback that caught
and suppressed everything. Gee, thanks. Web browsers these days are
pretty good at reporting exceptions - we were mainly using Chrome's
inbuilt Firebug-equivalent - but our brilliant coworker saw fit to
hide them all.

Exceptions are a huge boon.

ChrisA