[Python-Dev] Pre-PEP: Exception Reorganization for Python 3.0

[Nick Coghlan]

Phillip J. Eby wrote:
> I like this a lot, and a good bit of it could actually be done in 2.5, 
> apart from the Exception/StandardError move, assuming also that the 
> renamed errors were also available under their old names.  We could 
> probably go so far as to add Raisable to the hierarchy, but I don't 
> think we could actually get quite to your proposed structure without 
> breaking any programs.  On the other hand, if we introduce 
> CriticalException and ControlFlowException in 2.5 (inheriting from 
> Exception), and create a new Error base for StandardError, then promote 
> subclassing and catching it instead of Exception, then there will be 
> less to do in 3.0.  So, my thoughts for the 2.x series are:
> 
> 
>    Exception
>       CriticalException
>       ControlFlowException
>       Error
>          StandardError

If we leave Exception at the top of the hierarchy for Py3k, and use Error to 
replace StandardError, the hierarchy could look like this:

  Exception
  +-- ControlFlowException (new)
       +-- GeneratorExit
       +-- KeyboardInterrupt
       +-- StopIteration
       +-- SystemExit
  +-- CriticalError (new)
       +-- MemoryError
       +-- SystemError
  +-- Error (formerly StandardError)
       +-- AssertionError
       +-- AttributeError
       +-- ImportError
       +-- TypeError
       +-- WeakReferenceError (formerly ReferenceError)

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.

So the question is whether or not the Raisable/Exception combination is liked 
enough that we want to dance through the requisite hoops to get there.

Notice that I've classified KeyboardInterrupt as user-initiated control flow 
and put it under ControlFlowException above. This means that everything under 
CriticalError and Error actually ends with the word 'Error'.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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