Try-except-finally paradox

[Ethan Furman]

On 01/30/2014 10:12 AM, Rotwang wrote:
> On 30/01/2014 06:33, Andrew Berg wrote:
>> On 2014.01.29 23:56, Jessica Ross wrote:
>>>
>>> I found something like this in a StackOverflow discussion.
>>> --> def paradox():
>>> ...     try:
>>> ...             raise Exception("Exception raised during try")
>>> ...     except:
>>> ...             print "Except after try"
>>> ...             return True
>>> ...     finally:
>>> ...             print "Finally"
>>> ...             return False
>>> ...     return None
>>> ...
>>> --> return_val = paradox()
>>> Except after try
>>> Finally
>>> --> return_val
>>> False
>>>
>>> I understand most of this.
>>> What I don't understand is why this returns False rather than True.
>>> Does the finally short-circuit the return in the except block?
>>>
>> My guess would be that the interpreter doesn't let the finally block
>> get skipped under any circumstances, so the return value gets set to
>> True, but then it forces the finally block to be run before returning,
>> which changes the return value to False.
>
> Mine too. We can check that the interpreter gets as far as evaluating the return value in the except block:
>
> --> def paradox2():
>        try:
>            raise Exception("Raise")
>        except:
>            print("Except")
>            return [print("Return"), True][1]
>        finally:
>            print("Finally")
>            return False
>        return None
>
> --> ret = paradox2()
> Except
> Return
> Finally
> --> ret
> False

And just to be thorough, if the finally block doesn't have a return:

--> def paradox3():
         try:
             raise Exception("Raise")
         except:
             print("Except")
             return [print("Return"), True][1]
         finally:
             print("Finally")
         return None

--> print(paradox3())
Except
Return
Finally
True

--
~Ethan~