Why bool( object )?

Steven D'Aprano steven at REMOVE.THIS.cybersource.com.au
Tue Apr 28 09:39:51 CEST 2009


On Mon, 27 Apr 2009 23:11:11 -0700, Aaron Brady wrote:

> What is the rationale for considering all instances true of a user-
> defined type?  Is it strictly a practical stipulation, or is there
> something conceptually true about objects?

Seven years ago, in an attempt to convince Guido *not* to include 
booleans in Python, Laura Creighton wrote a long, detailed post 
explaining her opposition.

At the heart of her argument is the observation that Python didn't need 
booleans, at least not the ints-in-fancy-hats booleans that we've ended 
up with, because Python already made a far more useful and fundamental 
distinction: between Something and Nothing.

http://groups.google.com/group/comp.lang.python/msg/2de5e1c8384c0360?hl=en

All objects are either Something or Nothing. The instances of some 
classes are always Something, just as the instances of some classes are 
always Nothing. By default, instances are Something, unless __nonzero__ 
returns False, or __len__ returns 0, then they are Nothing.

In a boolean (or truth) context, Something and Nothing behave like True 
and False in languages with real booleans:

if obj:
    print "I am Something"
else:
    print "I am Nothing"


To steal an idiom from Laura: Python has a float-shaped Nothing 0.0, a 
list-shaped Nothing [], a dict-shaped Nothing {}, an int-shaped Nothing 
0, a singleton Nothing None, and so forth. It also has many corresponding 
Somethings.

All bool() does is convert Something or Nothing into a canonical form, 
the subclassed ints True and False.

I'm not sure whether Guido ever used the terms Something vs Nothing when 
describing Python's truth-testing model, but it is clearly there, at the 
heart of Python. Python didn't even get a boolean type until version 2.3.




-- 
Steven



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