[Python-Dev] PEP, take 2: Exception Reorganization for Python 3.0

[Ron Adam]

Nick Coghlan wrote:
> Phillip J. Eby wrote:
> 
>>+1.  The main things that need fixing, IMO, are the need for critical and 
>>control flow exceptions to be distinguished from "normal" errors.  The rest 
>>is mostly too abstract for me to care about in 2.x.
> 
> 
> I guess, before we figure out "where would we like to go?", we really need to 
> know "what's wrong with where we are right now?"
> 
> Like you, the only real problem I have with the current hierarchy is that 
> "except Exception:" is just as bad as a bare except in terms of catching 
> exceptions it shouldn't (like SystemExit). I find everything else about the 
> hierarchy is pretty workable (mainly because it *is* fairly flat - if I want 
> to catch a couple of different exception types, which is fairly rare, I can 
> just list them).

More often than not, 9 out 10 times, when ever I use "except Exception:" 
or a bare except:, what I am doing is the equivalent to:

    try:
        <statement that may fail>     # will either fail or not
    except:
        pass
    else:
        <dependent statements>

Usually I end up using "if hasattr():" or some other way to pre test the 
statement if possible as I find "except: pass" to be ugly. And putting 
both the statement that may fail together with the depending statements 
in the try:, catches too much.  Finding subtle errors hidden by a try 
block can be rather difficult at times.

Could inverse exceptions be an option?  Exceptions don't work this way 
so it would probably need to be sugar for "except <exception>:pass; else:".

Possibly?

    try:
        <statement that may fail>
    except not <a_exception>:          "except None:" as an option?
        <dependent statements>

Ok, this isn't exactly clear, and probably a -2 for several reasons.

The exception tree organization should also take into account inverse 
relationships as well.  Exceptions used for control flow are often of 
the type "if not exception: do something".

Cheers,
Ron