[Python-3000] __nonzero__ vs. __bool__

[Walter Dörwald]

Jack Diederich wrote:

> On Wed, Nov 22, 2006 at 09:36:09AM +0100, Georg Brandl wrote:
>> Terry Reedy schrieb:
>>> "Brett Cannon" <brett at python.org> wrote in message 
>>> news:bbaeab100611211232r53de3c33p915bee7b49dbdf9a at mail.gmail.com...
>>>
>>> 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))
>>> ------------
>>>
>>> If an object without __bool__ returned itself as its length, this would be 
>>> an infinite loop, at least in this Python version.  Do we worry about 
>>> something so crazy?
>> The length would have to be an integer, and this would have to be checked.
>>
> 
> It looks like the regular checks are happening on __len__ methods
> anyway so the explicit int check in slot_nb_bool is redundant.
> This is the first time I've looked at the new slots in py3k so
> feel free to correct. (using bool4.patch)
> 
> sprat:~/src/py3k-rw> ./python
> Python 3.0x (p3yk:52823M, Nov 22 2006, 11:57:34) 
> [GCC 4.0.3 (Ubuntu 4.0.3-1ubuntu5)] on linux2
> Type "help", "copyright", "credits" or "license" for more information.
>>>> class A(object):
>   def __len__(self):
>     return -1
> 
> a = A()
> print bool(a)
>   ... ... ... >>> >>> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> ValueError: __len__() should return >= 0

OK, I've updated the patch: bugs.python.org/1600346

Servus,
   Walter