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

[Jan Kaliszewski]

Dnia 18-07-2009 o 14:07:06 Terry Jones <terry at jon.es> napisał(a):

> I agree that having finally subsume returns and exceptions in the try  
> block will at first be surprising to people, and that it can lead to  
> easy-to-create but difficult-to-find bugs.

IMHO it is enough to treat it as a bug.

> But I think the current behavior is definitely for the best:
[snip]
> - If finally were to automatically propagate any exception from the try
>   clause, how would you turn that off when you needed/wanted to? You'd  
>   soon wind up wanting another keyword, perhaps seriouslyfinally, that
>   was guaranteed to be run at the very end.

No, simply:

try:
     try:
        ...
     finally:
        ...
except Foo:
     ...

>   If finally propagated exceptions, what should it do if there's an
>   exception raised in the finally block?  It begins to get complicated.
>   The primitive blanket solution is better, IMO.

But normally finally propagates exceptions. The problem is that 'return'
breakes that rule. A rule that is both logicall and intuitive.

> - Besides, if you do want a finally clause to propagate an exception from
>   its try clause, that's simple as you can just store the exception
>   somewhere in an except clause and then let the finally detect it and
>   re-raise it, if appropriate.

Do you think about:

try:
     exc = None
     ....
except BaseException as exc:
     pass
finally
     if exc:
        raise exc
     else:
        return foobar

...?

Easy, this example is a joke. The workaround is: not to use return in
finally.
Never. (After all if you need catch an exception, you should use except).

> By keeping finally's semantics simple and resisting the temptation to  
> make it more sophisticated (e.g., propagating exceptions or return  
> values) you wind up with a better tool for the programmer who does
> know what they're doing.

IMHO rather the current behaviour is sophisticated, or rather surprising
and weird.

Regards,

zuo