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

Michael Chermside mcherm at mcherm.com
Wed Sep 8 19:19:29 CEST 2004

[various discussion about how to NOT catch exceptions by default]

I've been working in the Java world for a while now, and they seem to have solved this problem quite handily -- no one ever seems to get confused about it. Ignoring Java's "checked exceptions" (which are a different problem), they've done the following:

Throwable | +-- Error | | | +-- OutOfMemorError (and others like it) | +-- Exception | +-- IndexOutOfBoundsException (and others like it) | +--

As I see it, there are several common cases:

(1) You want to catch a particular (normal) exception. (eg: to handle the problem -- this is the normal thing!) (2) You want to catch any normal exception. (eg: to log and ignore after calls to some subsystem) (3) You want to catch a particular special exception (eg: to deal with KeyboardInterrupt, or MemoryError) (4) You want to catch ANYTHING, then re-throw it afterward (eg: to cleanup a DB connection)

We currently make (1) easy (of course!) by writing "except Foo:", and we make (4) easy by writing "except:" (the bare except). But most users only want to use (1) and (2)... only experts use (3) and (4). So I certainly agree that bare except is used by people who want (2) and should be using (4) instead.

I would think that both the end goal (python 3000) AND the transition plan are straightforward. For now, a bare except can't be changed because of backward compatibility. So create a top-level class which is NOT named "Exception" ("Raisable" anyone?). Our hierarchy would look like this:

Raisable | +-- MemoryError +-- SystemExit +-- KeyboardInterrupt +-- oddballs like ZODB's ConflictError | +-- Exception | +-- most everything else +-- user defined exceptions

That's not quite as flat as today's hierarchy, but it works pretty well. When non-experts want to catch all exceptions, if bare excepts are deprecated, they will write "except Exception:" (that's just psychology), so most users will wind up with (2) when that's what they want. Experts can easily do (3) by catching the exception by name, and experts can do (4) by catching Raisable. When Python 3000 is released and backward compatibility is not required, we can either remove the bare except, or change it to mean the same as "except Exception" (Python 3000 will, of course, forbid raising strings or anything not decended from Raisable).

I don't need anyone to buy into this approach, but since it seems so straightforward to me I thought I should write it out anyhow.

-- Michael Chermside

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