[Python-Dev] Exception hierarchy [was Re: Another test_compiler mystery] (original) (raw)

James Y Knight foom at fuhm.net
Tue Aug 17 02:07:14 CEST 2004

On Aug 16, 2004, at 7:29 AM, Armin Rigo wrote:

Yes. Here is a patch attempting to do what I described: http://www.python.org/sf/1009929

From the patch description:

Some more thinking about [the exception hierarchy] would be welcome. Maybe AsynchronousException and a few others should not subclass Exception at all, so that "except Exception" statements don't catch them. Anyway, this patch alreaddy improves the situation, because you can catch and re-raise AsynchronousException (instead of, say, just KeyboardInterrupt).

It seems to me that something similar to what Java has would be a good idea. Namely, a new top level exception (from which all others would derive) called "Raisable", analogous to Java's Throwable. This then has two subclasses: "Exception", and "FatalError". I'm not sure FatalError is a good name, but some new name needs to be thought up for Java's "Error" class, because lots of python exceptions end in "Error" but belong under the "Exception" hierarchy (for example "KeyError").

The criteria for whether a given exception should go under "Exception" or "FatalError" is whether users' code should generally catch the exception. Thus, at least "SystemExit", "KeyboardInterrupt", and "MemoryError" should go under "FatalError". Catching those is nearly never what you wanted to do when you write "except Exception:". There's likely more -- I have not gone through all the exceptions in Python to classify them.

One issue is that creating a new category of Exceptions doesn't help people who do "except:" instead of "except Exception:". It is unlikely that person meant to catch things like MemoryError, rather, they were just being lazy. I suppose that syntax could be deprecated, at least in example code and documentation, in favor of "except Exception" for the usual case, and "except Raisable" for the cases where you do actually want to catch everything*.

James

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