[Python-3000] __nonzero__ vs. __bool__

Brett Cannon brett at python.org
Tue Nov 21 21:32:05 CET 2006


On 11/21/06, Walter Dörwald <walter at livinglogic.de> wrote:
>
> tomer filiba wrote:
> > patch updated
>
> You're fast! ;)
>
> AFAICT now we have the following problem: If
> Objects/typeobject.c::slot_nb_bool() falls back to using __len__(), the
> return value of __len__() must be a bool.


Why can't the fallback usage just pass the return value from __len__ to
bool() (forget the C function name) and return that result?  It's just like
doing::

  def bool(obj):
      try:
          return obj.__bool__()
      except AttributeError:
          return bool(len(obj))

-Brett

Servus,
>    Walter
>
> > On 11/21/06, *Guido van Rossum* <guido at python.org
> > <mailto:guido at python.org>> wrote:
> >
> >     I think it would set a good example if __bool__ was *required* to
> >     return exactly True or False and no other object, not even int
> >     instances equal to 0 or 1. Surely that can't be much of a burden.
> Feel
> >     free to upload an improved patch.
> >
> >     On 11/21/06, Walter Dörwald <walter at livinglogic.de
> >     <mailto:walter at livinglogic.de>> wrote:
> >     > Guido van Rossum wrote:
> >     > > On 11/21/06, Ivan Krstić < krstic at solarsail.hcs.harvard.edu
> >     <mailto:krstic at solarsail.hcs.harvard.edu>> wrote:
> >     > >> Guido van Rossum wrote:
> >     > >>> Can anyone help out evaluating this patch? If it has to wait
> >     for me
> >     > >>> it's gonna be a looooooong wait...
> >     > >> Looks fine to me functionally, although it seems to
> >     gratuitously retab
> >     > >> some code that used to be aligned with tabstop 8 to a tabstop
> of 4.
> >     > >
> >     > > Thanks! I hope you meant "indentation level" instead of
> "tabstop".
> >     > > Tomer, can you fix the indentation to be whatever's prevailing
> >     in the
> >     > > file you're editing?
> >     >
> >     > Another question is whether __bool__() should be allowed to return
> an
> >     > int (or any other object), or if it *must* be a bool. (The patch
> >     > currently allows ints). What if __bool__() returns subclasses of
> int,
> >     > that overwrite __bool__() themself?
> >     >
> >     > Servus,
> >     >    Walter
> >     >
> >     >
> >
> >
> >     --
> >     --Guido van Rossum (home page: http://www.python.org/~guido/)
> >
> >
>
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