[Python-Dev] RE: PEP-317

[Guido van Rossum]

> While we're at weird exception stuff... while aping bits of the
> exception handling code for pypy, I noticed that this should "work",
> and indeed it does:
> 
> >>> raise ((((Exception,), 'anything'), 'can', 'go'), 'in here'), 'arg'
> Traceback (most recent call last):
>   File "<stdin>", line 1, in ?
> Exception: arg
> 
> Any takers for an explanation?

When there were only string exceptions, I noticed the need for naming
a group of exceptions, e.g.

    io_related = (IOError, OSError, socket.SocketError)

    try:
	<some I/O intense stuff>
    except io_related, e:
	print "Some kind of I/O related error occurred:", e

So I implemented this.  Then (still back in the string exceptions
days) I realized that once something like that is named, it pretty
much behaves like a single exception, so I figured that it should be
possible to raise it as well.  This allows "upgrading" an exception
declared in a module to a group of exceptions without breaking code
that raises one of these.

I think the reasoning still holds, but:

(1) The syntax to catch multiple exceptions causes common mistakes
    like this:

    try:
        <some indexing operation>
    except KeyError, IndexError:
        <error handling>

(2) For upgrading exceptions, multiple inheritance makes more sense.

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