Re: [libvirt] [jenkins-ci PATCH] lcitool: Raise Error instead of Exception

On Mon, Mar 11, 2019 at 07:11:49PM +0100, Andrea Bolognani wrote: > On Mon, 2019-03-11 at 17:55 +0000, Daniel P. Berrangé wrote: > > I'm curious why the "Error" class exists at all ? It doesn't seem > > to add anything that the normal "Exception" class can't do, and > > leads to bugs like the one here. > > I seem to understand you're not supposed to use Exception directly, > but rather define your own exception types: > > https://docs.python.org/3/tutorial/errors.html#user-defined-exceptions > > I remember reading more about this, but I can't find the source > right now. I think it largely depends on the scope / scale of the application you are writing. If the code is complex enough that you're going to want to be catching exceptions in specific scenarios, then creating subclasses makes sense as you can avoid mistakenly catching too much exceptions. In the lcitool case though, we're not trying to do anything very clever with catching specific exceptions as our app is quite small & self contained. We just want to catch everything to hide the ugly stack traces by default. IMHO in that scenario it is fine to just use the base exception class directly. BTW, there's no need for the "message" field, even with the Error class, as the default Exception class will stringify to report just the message it received: eg this is equiv: @@ -703,5 +700,5 @@ if __name__ == "__main__": try: Application().run() except Error as err: - sys.stderr.write("{}: {}\n".format(sys.argv[0], err.message)) + sys.stderr.write("{}: {}\n".format(sys.argv[0], err)) sys.exit(1) anyway, I think we can just simplify our life & delete the Error class. It would probably be worth registering a "--debug" flag to the CLI to disable the global exception catch, so that we can see the full stack trace when needed.