[Chicago] falsy objects?

Jonathan Hayward christos.jonathan.hayward at gmail.com
Fri Mar 19 15:54:51 CET 2010

There was something I thought I'd read but couldn't track down in the

User-defined classes normally evaluate to true, i.e. if you define:

class foo:

bar = foo()

if bar:
    print "True"
    print "False"

then the output will be "True", and adding real functionality to foo does
not change this.

I thought there was supposed to be a method you could define that would
override this behavior, named something like is_true() or __is_true__(), so
that an object could be set to evaluate to false. However, looking through
the documentation did not confirm anything like:

class foo:
    def is_true(self):
        return False

bar = foo()

if bar:
    print "True"
    print "False"

which would print "False".

Is there such a method that can be defined, or is it non-negotiable that a
user-defined class (which does not extend a class that can be falsy) will
evaluate to true?

→ Jonathan Hayward, a Senior Web Developer who cares deeply about usability
→ www.linkedin.com/in/jonathanhayward • jonathan.hayward at pobox.com
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