any(), all() and empty iterable

Eduardo O. Padoan eduardo.padoan at gmail.com
Sun Apr 12 19:59:10 CEST 2009


On Sun, Apr 12, 2009 at 8:53 AM, Tim Chase
<python.list at tim.thechases.com> wrote:
>>> From the docs:
>>
>> all(iterable)
>>                Return True if all elements of the iterable are true.
>> Equivalent
>>        to:
>>                def all(iterable):
>>            for element in iterable:
>>                if not element:
>>                    return False
>>            return True
>
> Then I'd say the comment is misleading.  An empty list has no item that is
> true (or false), yet it returns true.  The comment in the docs should read
> "Return False if any element of the iterable is not true" or "Return True if
> all elements of the iterable are true or if the iterable is empty."

I didn't knew about "Vacuous True" (my fault, it seems Discrete Math
101) until reading about on this thread (thanks everyone!), and
reading on wikipedia it answered this exact question.


> To get the behavior the original comment describes, would seem to require an
> implementation something like this:
>
>  def all(iterable):
>    iterable = iter(iterable)
>    try:
>      element = iterable.next()
>    except StopIteration:
>      raise UnderdefinedBecauseNoElementsToCompareToTrue
>    while element:
>      try:
>        element = iterable.next()
>      except StopIteration:
>        return True
>    return False
>
>
> Tweaking the documentation seems like an easier and more backwards
> compatible solution to me :)
>
> -tkc
>
>
>
> --
> http://mail.python.org/mailman/listinfo/python-list
>



-- 
    Eduardo de Oliveira Padoan
http://importskynet.blogspot.com
http://djangopeople.net/edcrypt/

"Distrust those in whom the desire to punish is strong."
   -- Goethe, Nietzsche, Dostoevsky



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