R: [Python-Dev] Deprecating string exceptions

[Guido van Rossum]

> The question is, whether, given
> 
> class Base:pass
> class MyExc(Base, Exception):pass
> 
> it would be valid to write
> 
> try:
>   foo() # raises MyExc
> except Base:
>   pass

The "except Base:" class would of course be illegal because Base
doesn't derive from Exception.  (Although the error might only be
detected when MyExc is raised -- but PyChecker could do better.)  But
the statement "raise MyExc" would be perfectly legal, and could be
caught either with "except MyExc" or "except Exception".

> If that ought to be allowed, I fail to see the rationale to require
> that exceptions be derived from Exception: The rationale could be "all
> exceptions are primarily instances of Exception" - yet in this case,
> Base is the primary class, and Exception is just a mix-in that plays
> no further role in this case of exception handling.

I have never heard of the concept "primarily an instance of".  I also
don't understand why that should be the rule.

> If you want to allow Base in the except clause, but do allow Base as
                 ^^^^^
Did you mean "disallow"?

> the base class of MyExc, I'd be curious what run-time semantics you'd
> associate with this example.

See above.

Before we waste more time on this, let me explain why I like the rule
that all exception classes be derived from Exception.  It's only a
vague liking, and maybe it's not worth making it a rule.  I like it
because including Exception (or one of its well-known subclasses) in
the base class is a clue to the reader that a particular class
declaration is used as an exception.  AFAIK this is the only reason
why Java has a similar rule; C++ does not, and lets you throw any
class.

I can't see any implementation benefits from the requirement.  It
sounds like you can't either -- or else you would have pointed them
out by now.  So maybe we shouldn't bother with this rule, and then we
should take the recommendation out of the documentation.  But I still
kind of like it, for the reason I explained in the previous paragraph.

--Guido van Rossum (home page: http://www.python.org/~guido/)