[Python-Dev] Exceptions *must*? be old-style classes?

[Robey Pointer]

Guido van Rossum wrote:

>>The base of the Exception hierarchy happens to be a classic class.
>>But why are they "required" to be classic?
>>
>>More to the point, is this a bug, a missing feature, or just a bug in
>>the documentation for not mentioning the restriction?
>>    
>>
>
>It's an unfortunate feature; it should be mentioned in the docs; it
>should also be fixed, but fixing it isn't easy (believe me, or it
>would have been fixed in Python 2.2).
>
>To be honest, I don't recall the exact reasons why this wasn't fixed
>in 2.2; I believe it has something to do with the problem of
>distinguishing between string and class exception, and between the
>various forms of raise statements.
>
>I think the main ambiguity is raise "abc", which could be considered
>short for raise str, "abc", but that would be incompatible with except
>"abc". I also think that the right way out of there is to simply
>hardcode a check that says that raise "abc" raises a string exception
>and raising any other instance raises a class exception. But there's a
>lot of code that has to be changed.
>
>It's been suggested that all exceptions should inherit from Exception,
>but this would break tons of existing code, so we shouldn't enforce
>that until 3.0. (Is there a PEP for this? I think there should be.)
>  
>

There's actually a bug open on the fact that exceptions can't be 
new-style classes:

https://sourceforge.net/tracker/?func=detail&atid=105470&aid=518846&group_id=5470

I added some comments to try to stir it up but there ended up being a 
lot of confusion and I don't think I helped much.  The problem is that 
people want to solve the larger issues (raising strings, wanting to 
force all exceptions to be new-style, etc) but those all have long-term 
solutions, while the current bug just languishes.

robey