[Python-ideas] Control Flow - Never Executed Loop Body

[Michael Selik]

> On Mar 23, 2016, at 1:13 PM, Henshaw, Andy <Andy.Henshaw at gtri.gatech.edu> wrote:
> 
>> -----Original Message-----
>> 
>> On Mon, Mar 21, 2016 at 7:01 PM, Stephen J. Turnbull
>> <stephen at xemacs.org> wrote:
>>>    item = _sentinel = object()
>>>    for item in iterable:
>>>        # do for each item
>>>    if item is _sentinel:
>>>        # do exactly when iterable raises StopIteration on the first
>>> pass
>> 
>> What if 'iterable' is locals().values()? Can you, with perfect reliability,
>> recognize that case? AIUI this is exactly why next() and
>> __next__() are defined to "return a value or raise", rather than "return a
>> value or return a magic no-more-values value", because there's always the
>> possibility that the no-more-values value is a legitimately-yielded value.
>> 
>> Maybe this situation isn't important enough or common enough to justify
>> dedicated syntax, but it's definitely a possibility.
>> 
>> ChrisA
> 
> is_empty = True
> for item in iterable:
>    is_empty = False
>    # do for each item
> if is_empty:
>    # do exactly when iterable raises StopIteration on the first
> 
> Maybe I'm missing something, but this seems to be an unnecessary addition.

It’s comparable in purpose to the ``else`` clause on a ``for`` or ``while``. It eliminates the need for a flag variable, like ``was_completed`` or ``is_empty``. It saves two lines of code, but more importantly avoids the accidental flip of True/False when setting the flag variable. Whether that warrants adding new syntax... I’m not convinced by the current proposals, but I can imagine finding a good keyword for the purpose. A keyword as good as ``else`` at least.