question about True values

[Robert Kern]

George Sakkis wrote:
> Martin v. Löwis wrote:

>> Not-so-obviously, arbitrary user-defined values can also be
>> treated as false: If they implement __nonzero__, they are
>> false if False is returned from __nonzero__; otherwise,
>> if they implement __len__, they are false if 0 is returned
>> from __len__. Under these rules, array.array objects can
>> also be false, as can UserList and UserDict objects.
> 
> A notable exception are numarray arrays (probably true for numpy too, I
> haven't tested it though):
> 
>>>> from numarray import array
>>>> bool(array([1,2]))
> Traceback (most recent call last):
>   File "<stdin>", line 1, in ?
> RuntimeError: An array doesn't make sense as a truth value.  Use any(a)
> or all(a).

numpy also has this behavior. Numeric yielded to the temptation to guess any(a) 
in the face of this ambiguity. Much buggy code was written as a result.

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
  that is made terrible by our own mad attempt to interpret it as though it had
  an underlying truth."
   -- Umberto Eco