[Python-Dev] GeneratorExit inheriting from Exception (original) (raw)

Michael Chermside mcherm at mcherm.com
Mon Mar 20 20:43:28 CET 2006

Barry writes:

I still believe in this, and I'm thankful for the support I've seen. It won't happen for Python 2.x, but I do plan on addressing this for Py3K.

When you do, I'd like you to consider one change to the names. You are proposing this:

Exception +- KeyboardInterrupt +- GeneratorExit +- SystemExit +- StopIteration +- Error | +- ImportError | +- (etc.) [...]

If we use that structure, I still strongly feel that what you call "Error" ought to be named "Exception" (and what you call "Exception" should be named "BaseException" or perhaps "Raisable"). The reason is this:

Most people coming to Python today are coming from languages more modern than Fortran and C, and (if they have any programming background at all) they are familiar with the idea of an exception. When chatting around the water cooler, they probably call it "an exception", rather than "a dingus", "one of those thingys that unwinds the stack until reaching a handler", or even "an error". (The term "an error" is usually used around the water cooler for any kind of mistake, whether human or programatic.) This is encouraged in Python by the fact that the keyword for handling exceptions in Python is "except".

But we don't want people using the dingus at the top of the hierarchy -- only a few experts who know what they're doing should care about that one. We want them using the one that you call "Error". So by naming that one "Exception", we take advantage of the meaning of the word, the experience from other languages, and the keywords of the language to encourage correct usage. Naming the top level dingus something slightly odd is a small price to pay (although both "BaseException" and "Raisable" seem instantly comprehensible to me).

I present Java as an example. Given it's position as the new Cobol, there are literally hundreds of thousands of not-very-skillful Java programmers out there. Yet it is rare to see them write "catch Throwable" when they should use "catch Exception". That mistake is far more common among beginning Python users. Surely we can do at least as well as Java!

-- Michael Chermside

PS: I've intentionally ignored the question Guido raised about whether Warning belongs under your "Error" or your "Exception".

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