Understanding while...else...

René Klačan rene.klacan at gmail.com
Wed Jan 23 12:03:09 CET 2013


they wouldnt be equivalent if #staff in version 1 did not cointain "break"
statement and this is common mistake

On Wed, Jan 23, 2013 at 1:39 AM, Oscar Benjamin
<oscar.j.benjamin at gmail.com>wrote:

> On 22 January 2013 23:41, Terry Reedy <tjreedy at udel.edu> wrote:
> > On 1/22/2013 3:09 PM, Ethan Furman wrote:
> >>
> >> On 01/22/2013 09:44 AM, Terry Reedy wrote:
> >>>
> [SNIP]
> >>> The else clause is executed if and when the condition is false.
> >>> Now use a real Python while statement to do the *same
> >>> thing*.
> >>>
> >>> while n > 0:
> >>>    n -= 1
> >>> else:
> >>>    n = None
> >>
> >>
> >> I understand how it works (although it did take a while for it to sink
> >> in); my gripe, and probably why it is misunderstood so often, is that
> >> nine times out of ten when I /want/ to use a while-else or for-else I
> >> only want the true/false check /once/, at the beginning of the loop.
> >
> >
> > I do not understand what you are saying. There already is only one
> > true/false check, at the beginning of the loop. If you only want the
> check
> > *performed* once, you would use if-else. But I presume you know this.
>
> I think he meant that he would use the else clause more often if it
> had the semantics so that the two blocks below were equivalent:
>
> # Version 1
> while condition:
>     # stuff
> else:
>     # other stuff
>
> # Version 2
> if condition:
>     while condition:
>         # stuff
> else:
>     # other stuff
>
> So he wants a convenient way to execute code only if the loop
> performed zero iterations. I think that often when people are confused
> about the else clause on while loops it is because they expect this
> behaviour (which would also be useful). The same confusion arises with
> for loops where people expect the else clause to execute if the
> iterable was empty so that these would be equivalent:
>
> # Version 1
> for x in iterable:
>     # stuff
> else:
>     # other stuff
>
> # Version 2
> iterated = False
> for x in iterable:
>     iterated = True
>     # stuff
> if not iterated:
>     # other stuff
>
>
> Oscar
> --
> http://mail.python.org/mailman/listinfo/python-list
>
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