Why bool( object )?

[Aaron Brady]

On Apr 28, 1:35 am, Lie Ryan <lie.1... at gmail.com> wrote:
> Aaron Brady wrote:
> > What is the rationale for considering all instances true of a user-
> > defined type?  
>
> User-defined objects (or type) can override .__len__() [usually
> container types] or .__nonzero__() to make bool() returns False.
>
> > Is it strictly a practical stipulation, or is there
> > something conceptually true about objects?
>
> Objects are true unless they define themself as false. The practical
> implication is we can do this:
>
> def foo(args = None):
>      if args:
>          ...
>
> In python all objects are true except: None, False, 0/0L/0.0/0j, empty
> sequence or container, and on objects that defines .__len__() or
> .__nonzero__() that returns 0 or False.
>
>
>
> > '''
> > object.__bool__(self)
> > If a class defines neither __len__() nor __bool__(), all its instances
> > are considered true.
> > '''
>
> > This makes it so all objects except False, None, 0, and empty
> > containers are true by default.  I am not convinced that 'if <a
> > generic object>' should have any meaning; it should probably throw an
> > exception.  Is it part of Python's look and feel or its mentality?  Is
> > it part of the Zen?  Certainly other ideal types can't be cast from
> > generic objects, so why booleans?  Is it an ineffable component of the
> > author's vision for the language?  I think that giving arbitrary
> > syntactic constructs meaning is just space-filler.  It's worse than
> > syntactic sugar, it's semantic sugar.  Why not assign meanings willy-
> > nilly to other random juxtapositions of tokens?
>
> It's part of the design decision. In almost all cases (in any language),
> a so-called "Design Decision" is rather random and prone to subjective
> judgment, just as the decision to make bool(int) returns False only on
> 0, -1, or for all negative values; whether to make bool(100) and
> exception or True; or round() rounds down or up or even-odd; or the use
> of brackets vs. indentation; or whether to treat empty list as True or
> False.

Whether it's a good feature and whether it's a popular feature are two
different things.  (It's not always clear that the former is even real
anyway.)

I'm actually not sure that my question was /entirely/ scientifically
motivated.  Perhaps I'm expressing admiration (or maybe envy).  Maybe
I want to design a popular language too.

If I was really being scientific, I would have been wondering, what is
the balance of pros and cons of the decision?  pros - cons = ?  Is
that an objective expression?  Are there only measurable quantities in
both operands?  If it's a close call, I might not agree, or might not
have followed the same experimental method.

Trivially the answer is, 'the decision maker found sum( pros ) > sum
( cons )'.  Many arts and disciplines do guesswork to a degree in
evaluating their 'sum( pros )', so perhaps I should expect a degree of
variation in what the experts all say that quantity is.

On a more idealist's level, perhaps I am asking the outside world
what /my/ evaluation of 'sum( pros )' is.  What's yours?