Question

Is it reasonable in Python to catch a generic exception, then use isinstance() to detect the specific type of exception in order to handle it appropriately?

I’m playing around with the dnspython toolkit at the moment, which has a range of exceptions for things like a timeout, an NXDOMAIN response, etc. These exceptions are subclasses of dns.exception.DNSException, so I am wondering if it’s reasonable, or pythonic, to catch DNSException then check for a specific exception with isinstance().

e.g.

try:
    answers = dns.resolver.query(args.host)
except dns.exception.DNSException as e:
    if isinstance(e, dns.resolver.NXDOMAIN):
        print "No such domain %s" % args.host
    elif isinstance(e, dns.resolver.Timeout):
        print "Timed out while resolving %s" % args.host
    else:
        print "Unhandled exception"

I’m new to Python so be gentle!

Answer 1

That’s what multiple except clauses are for:

try:
    answers = dns.resolver.query(args.host)
except dns.resolver.NXDOMAIN:
    print "No such domain %s" % args.host
except dns.resolver.Timeout:
    print "Timed out while resolving %s" % args.host
except dns.exception.DNSException:
    print "Unhandled exception"

Be careful about the order of the clauses: The first matching clause will be taken, so move the check for the superclass to the end.