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To: python-list@python.org
From: Terry Reedy <tjreedy@udel.edu>
Subject: Re: Try-except-finally paradox
Date: Fri, 31 Jan 2014 00:26:05 -0500
References: <9314ac52-a2be-4382-94ef-2c291f32be1a@googlegroups.com> <lcdf0c$bnl$2@ger.gmane.org>
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On 1/30/2014 7:05 AM, Dave Angel wrote:
>   Jessica Ross <deathweasel@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