Why bool( object )?

Colin J. Williams cjw at ncf.ca
Tue Apr 28 14:22:01 CEST 2009


Lie Ryan 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.

I'm puzzled by the last sentence:

*** Python 2.6.2 (r262:71605, Apr 14 
2009, 22:40:02) [MSC v.1500 32 bit 
(Intel)] on win32. ***
>>> bool(0)
False
>>> bool(-1)
True
>>> bool(-100)
True
>>>

Colin W.



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