[Python-Dev] Re: Dangerous exceptions (was Re: Another test_compilermystery) (original) (raw)

Tim Peters tim.peters at gmail.com
Wed Sep 8 05:14:49 CEST 2004

After a bit more thought (and it's hard to measure how little), I'd like to see "bare except" deprecated. That doesn't mean no way to catch all exceptions, it means being explicit about intent. Only a few of the bare excepts I've seen in my Python life did what was actually intended, and there's something off in the design when the easiest thing to say usually does a wrong thing.

I think Java has a saner model in this particular respect:

Throwable Exception Error

Java's distinction between "checked" and "unchecked" exceptions is a distinct layer of complication on top of that. All exceptions derive from Throwable. A "catch" clause requires specifying a class (there's no "bare except"). "An Error is a subclass of Throwable that indicates serious problems that a reasonable application should not try to catch". That includes AssertionError and VirtualMachineError. Those are exceptions that should never occur. It also includes ThreadDeath, which is expected to occur, but

The class ThreadDeath is specifically a subclass of Error rather

than Exception, even though it is a "normal occurrence", because many applications catch all occurrences of Exception and then discard the exception.

and it's necessary for ThreadDeath to reach the top level else the thread never really dies.

In that respect, it's interesting that SystemExit and KeyboardInterrupt are intended to "reach the top level" too, but can't be relied on to do so because of ubiquitous bare excepts and even pseudo-careful "except Exception:"s now. If people changed those to "except StandardError:", SystemExit would make it to the top but KeyboardInterrupt still wouldn't.

Raisable Exception Stubborn ControlFlow KeyboardInterrupt StopIteration SystemExit MemoryError

introduces a class of stubborn exceptions, those that wouldn't be caught by "except Exception:", and with the intent that there's no way you should get the effect of "except Raisable" without explictly saying just that (once bare except's deprecation is complete).

Oh well. We should elect a benevolent dictator for Python!

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