How to except the unexpected?
peter at engcorp.com
Sat Mar 4 02:01:19 CET 2006
Rene Pijlman wrote:
> One of the things I dislike about Java is the need to declare exceptions
> as part of an interface or class definition. But perhaps Java got this
> I've writen an application that uses urllib2, urlparse, robotparser and
> some other modules in the battery pack. One day my app failed with an
> urllib2.HTTPError. So I catch that. But then I get a urllib2.URLError, so
> I catch that too. The next day, it encounters a urllib2.HTTPError, then a
> IOError, a socket.timeout, httplib.InvalidURL,...
> How do you program robustly with these modules throwing all those
> different (and sometimes undocumented) exceptions at you?
I do it by not micromanaging things. Presumably if you plan to catch an
exception, you have a specific procedure in mind for handling the
problem. Maybe a retry, maybe an alternate way of attempting the same
thing? Look to the code that you are putting in those except:
statements (or that you think you want to put in them) to decide what to
do about this situation. If each type of exception will be handled in a
different manner, then you definitely want to identify each type by
looking at the source or the docs, or doing it empirically.
Most of the time there isn't a whole lot of real "handling" going on in
an exception handler, but merely something like logging and/or reporting
it onscreen in a cleaner fashion than a traceback, then failing anyway.
This is one reason Java does get it wrong: 95% of exceptions don't
need and shouldn't have special handling anyway.
Good code should probably have a very small set of real exception
handling cases, and one or two catchalls at a higher level to avoid
barfing a traceback at the user.
> A catchall seems like a bad idea, since it also catches AttributeErrors
> and other bugs in the program.
Generally speaking this won't be a problem if you have your catchalls at
a fairly high level and have proper unit tests for the lower level code
which is getting called. You are doing unit testing, aren't you? ;-)
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