Why bool( object )?
lie.1296 at gmail.com
Tue Apr 28 08:35:44 CEST 2009
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):
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.
> 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
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