Exceptions' Behaviour

[Tom Good]

Martin Sj?ren <martin at strakt.com> wrote in message news:<mailman.995459309.19355.python-list at python.org>...
> I've been fooling around with exceptions a bit today, and found some
> things that I don't quite understand.
> 
> class A (Exception): pass
> class B (Exception): pass
> 
>         try:
>             raise A, B
>         except A, var:
>             print "A,", var
>         except (A, B), var:
>             print "(A,B),", var
> 
> This prints 'A,   main  .B'
> 
>         try:
>             raise A, B
>         except (A, B), var:
>             print "(A,B),", var
>         except A, var:
>             print "A,", var
> 
> This prints '(A, B),   main  .B'
> 
> 
> That's rather predictiable, here comes the funky part:
> 
>         try:
>             raise (A, B)
>         except A, var:
>             print "A,", var
>         except (A, B), var:
>             print "(A,B),", var
> 
> This prints 'A, '
> 
>         try:
>             raise (A, B)
>         except (A, B), var:
>             print "(A,B),", var
>         except A, var:
>             print "A,", var
> 
> This prints '(A,B), '
> 
> !!!
> What's going on here? If (A, B) is raised like A, B, why don't I get
> anything in my 'var' variable? If not, how come (A, B) is matched against
> A?
> 
> I'm really confused here...
> 
> Martin Sjögren

Consider this:

PythonWin 2.1 (#15, Jun 15 2001, 14:13:47) [MSC 32 bit (Intel)] on
win32.
Portions Copyright 1994-2001 Mark Hammond (MarkH at ActiveState.com) -
see 'Help/About PythonWin' for further copyright information.
>>> # raise A,B raises A with B as its argument
>>> raise "a", "b"
Traceback (most recent call last):
  File "<interactive input>", line 1, in ?
a: b
>>> #raise (A, B) raises A and ignores B
>>> raise ("a", "b")
Traceback (most recent call last):
  File "<interactive input>", line 1, in ?
a



Keep in mind also, that a statement like:

  except (A, B):

means to catch either A or B.  That is, it does NOT mean to catch the
tuple (A,B).




Tom