[Python-ideas] for/else syntax

[Gerald Britton]

Sory Carl, I did NOT get it dead wrong.  The else is executed if the
for loop falls through.  That's the whole idea.  Probably I should
have set i to -1 in the else clause of the toy example though:

for i, j in enumerate(something):
  # do something
  i += 1
else:
 i = -1

if i > 0:
  # we did something

If you don't think the else is executed when the for-loop falls
through, then you should re-read the documentation, here:

http://docs.python.org/reference/compound_stmts.html#the-for-statement

Where it says (just as I did, but in other words):

"When the items are exhausted (which is immediately when the sequence
is empty), the suite in the else clause, if present, is executed, and
the loop terminates."

On Fri, Oct 2, 2009 at 9:18 PM, Carl Johnson
<cmjohnson.mailinglist at gmail.com> wrote:
> Those who believe that the for/else construct is sufficiently clear,
> please read this email again:
>
> Gerald Britton:
>
>> I can't imagine why I'm still commenting on this thread, since there
>> is no chance that Python will remove the "else" from for/while or put
>> conditions on it, but here is an example of a use that has no break
>> statement:
>>
>> for i, j in enumerate(something):
>>  # suite
>>  i += 1
>> else:
>>  i = 0
>>
>> # i == number of things processed
>>
>> The thing here is, "i" won't be set by the "for" statement if
>> "something" is empty.  OTOH, if "something" is non-empty, i >= 1 at
>> the end of the for-loop.  Using the "else" clause in this way ensure
>> that "i" has the number of things processed at the end of the loop.
>>
>> To do this without the "else" I have to:
>>
>> i = 0
>> for i, j in enumerate(something):
>>  # suite
>>  i += 1
>> I can't imagine why I'm still commenting on this thread, since there
>> is no chance that Python will remove the "else" from for/while or put
>> conditions on it, but here is an example of a use that has no break
>> statement:
>>
>> for i, j in enumerate(something):
>>  # suite
>>  i += 1
>> else:
>>  i = 0
>>
>> # i == number of things processed
>>
>> Does it work?  Sure!  But it moves the setting of "i" outside the
>> for-construct, which I personally dislike.
>
> I don't mean to pick on Gerald, but he got the meaning of for/else
> dead wrong *in the middle of a debate about for/else*. This shows that
> even experienced Pythoneers who read Python-ideas can be confused
> about the meaning of for/else. Using it in production code means
> leaving a trap for the programmers who come after you and read your
> code to get confused and create a bug. Yes, when the others turn your
> working use of for/else into a bug, it's their fault for not reading
> the tutorial more closely so as not get confused by it, but ultimately
> no matter how much you blame the user, that won't stop the next guy
> who uses your code from misunderstanding it.
>
> The best solution to this inherent confusingness is to leave bare
> for/else alone for the foreseeable future, but to add a less confusing
> synonym for this feature. I propose "else not break:" although "elif
> not break:" is good too. Rewriting Gerald's example as
>
> for i, j in enumerate(something):
>  # suite
>  i += 1
> else if not break:
>  i = 0
>
> # i == number of things processed
>
> makes it 100% smack you in the face obvious that "Oh yeah, this is
> going to set i to 0 every time, since there's no break."
>
> In an unrelated but also good idea, it would be nice to be able to do
> what Gerald thought he was doing:
>
> for i, j in enumerate(something):
>  # suite
>  i += 1
> else if None: #or some other suitable phrase for "empty"
>  i = 0
>
> # i == number of things processed
>
> — Carl
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>



-- 
Gerald Britton