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

[Jan Kaliszewski]

18-07-2009, 02:59 Steven D'Aprano <steve at 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 at chopin.edu.pl>