Try-except-finally paradox
Terry Reedy
tjreedy at udel.edu
Fri Jan 31 00:26:05 EST 2014
On 1/30/2014 7:05 AM, Dave Angel wrote:
> Jessica Ross <deathweasel at gmail.com> Wrote in message:
>> 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?
>>
>
> The finally has to happen before any return inside the try or the
> except. And once you're in the finally clause you'll finish it
> before resuming the except clause. Since it has a return, that
> will happen before the other returns. The one in the except block
> will never get reached.
>
> It's the only reasonable behavior., to my mind.
Checking with the disassembled code, it appears that the except return
happens first and is then caught and the value over-written
2 0 SETUP_FINALLY 45 (to 48)
3 SETUP_EXCEPT 16 (to 22)
3 6 LOAD_GLOBAL 0 (Exception)
9 LOAD_CONST 1 ('Exception raised during try')
12 CALL_FUNCTION 1 (1 positional, 0 keyword pair)
15 RAISE_VARARGS 1
18 POP_BLOCK
19 JUMP_FORWARD 22 (to 44)
4 >> 22 POP_TOP
23 POP_TOP
24 POP_TOP
5 25 LOAD_GLOBAL 1 (print)
28 LOAD_CONST 2 ('Except after try')
31 CALL_FUNCTION 1 (1 positional, 0 keyword pair)
34 POP_TOP
6 35 LOAD_CONST 3 (True)
38 RETURN_VALUE
39 POP_EXCEPT
40 JUMP_FORWARD 1 (to 44)
43 END_FINALLY
>> 44 POP_BLOCK
45 LOAD_CONST 0 (None)
8 >> 48 LOAD_GLOBAL 1 (print)
51 LOAD_CONST 4 ('Finally')
54 CALL_FUNCTION 1 (1 positional, 0 keyword pair)
57 POP_TOP
9 58 LOAD_CONST 5 (False)
61 RETURN_VALUE
62 END_FINALLY
10 63 LOAD_CONST 0 (None)
66 RETURN_VALUE
--
Terry Jan Reedy
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