[Python-Dev] Pre-PEP: Exception Reorganization for Python 3.0 (original) (raw)

Phillip J. Eby pje at telecommunity.com
Sun Jul 31 18:49:32 CEST 2005

At 12:12 PM 7/31/2005 -0400, James Y Knight wrote:

On Jul 30, 2005, at 8:57 PM, Nick Coghlan wrote: > I wouldn't mind using Exception/Error instead of Raisable/Exception > - and it > seriously reduces the pain of making this transition. Indeed, most > of it > becomes doable within the 2.x series - the only tricky parts are > semantic > changes involved with moving the KeyboardInterrupt, MemoryError and > SystemError out from under StandardError, and moving EOFError under > IOError.

It seems to me that it increases the pain of transition. I do not see any reason why adding a new class above the current hierarchy causes any problems. However, changing the recommended base class for user-defined exceptions from "Exception" to "Error" will cause problems for every library.

I think you're ignoring the part where most exception handlers are already broken. At least adding CriticalException and ControlFlowException makes it possible to add this:

 try:
     ...
 except (CriticalException,ControlFlowException):
     raise
 except:
     ...

This isn't great, I admit, but at least it would actually work.

I also don't see how changing the recommended base class from Exception to Error causes problems for every library. Sure, it forces them to move (eventually!), but it's a trivial change, and makes it possible to do the right thing with exceptions (e.g. except Error:) as soon as all the libraries you depend on have moved to using Error.

Moving KeyboardInterrupt, MemoryError, and SystemError out from under Exception will be a backwards compatibility issue, but that's the case in all proposals.

Actually, in my tweak of Nick's proposal they're still Exception subclasses, so nothing breaks.

Additionally, I predict the pain from doing that will be minor. In cases where apps want to catch all exceptions, they must already use "except:" instead of "except Exception:", as not all exceptions derive from Exception. And catching the above three in any circumstance other than when explicitly mentioned and when wanting to catch every exception is probably wrong. So moving them will probably not cause many programs to malfunction.

In which case, maybe we should just implement Nick's full proposal, since the only thing that it would break is code that uses "except Exception:" to catch SystemExit or StopIteration.

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