Exception confusion

[Martin von Loewis]

Martin Sjögren <martin at strakt.com> writes:

> Doesn't this strike anybody as inconsequent?

Not at all. Just be aware that

I. catching an exception is an assignment to the exception variable.
   So if you write

      .... raise x

   except Y, z:

   then just before the except block, the assignment

   z = x

   happens.

II.Exceptions support tuple unpacking
>>> e = Exception(1,2)
>>> a,b=e
>>> a
1
>>> b
2
   [If you wonder how they do that: using __getitem__]

III. Raising an exception in the form klass,value is the same
     as raising it as klass(value)

> I wrote this little test script:
> 
> try: raise Exception, "foo"
> except Exception, arg: print type(arg)

  This really is

   arg = Exception("foo")

> try: raise Exception, "foo"
> except Exception, (arg,): print type(arg)

   (arg,) = Exception("foo")

which is tuple unpacking.

> try: raise Exception, ("foo", "bar"):
> except Exception, (arg,): print type(arg)
> 

  Tuple unpacking, namely

   (arg,) = Exception("foo", "bar")

  This fails, because the exception has two elements, but you want to
  unpack only a single one.

Hope this helps,
Martin