Try-except-finally paradox
Ethan Furman
ethan at stoneleaf.us
Thu Jan 30 13:30:21 EST 2014
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~
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