Re: [Python-ideas] Make return inside a finally a SyntaxError

July 18, 2009

      18-07-2009, 02:59 Steven D'Aprano <steve@pearwood.info> wrote:
...
On Sat, 18 Jul 2009 09:41:10 am Michael wrote:
...
Here are two examples of why allowing return inside a finally block
is a bad idea:
def f():
   try:
     return 3
   finally:
     return 4
Given that the finally block is guaranteed to run after exiting the try
block, I don't see anything objectionable about that. Surprising,
perhaps, possibly even an artifact of the CPython implementation, but
why is it a "bad idea" that needs to be protected against?
...
def f():
   try:
     raise Exception()
   finally:
     return 4
That's no weirder than:
def f():
    try:
        raise Exception()
    except Exception:
        return 4
or:
def f():
    try:
        raise Exception()
    except Exception:
        pass
    return 4
The problem is that finally clause is not for catching exceptions
but for doing clean-up actions. And here finally-clause *stops*
propagation of any exception! (also from lower levels!)

def f()
     try:
         raise KeyError
     except KeyError:
         raise Exception
     finally:
         return 4

...returns 4 without obstacles!

It's weird and missleading. I thing it's definitely a bug.

-- 
Jan Kaliszewski <zuo@chopin.edu.pl>