Boolean value of objects in Python

In Python < 3.0 :

You have to use __nonzero__ to achieve what you want. It's a method that is called automatically by Python when evaluating an object in a boolean context. It must return a boolean that will be used as the value to evaluate.

E.G :

class Foo(object):

    def __init__(self, bar) :
        self.bar = bar

    def __nonzero__(self) :
        return self.bar % 2 == 0

if __name__ == "__main__":
     if (Foo(2)) : print "yess !"

In Python => 3.0 :

Same thing, except the method has been renamed to the much more obvious __bool__.

In Python 2, use __nonzero__:

Refer to the Python 2 docs for __nonzero__.

class foo(object):
    def __nonzero__( self) :
        return self.bar % 2 == 0

def a(foo):
    if foo:
        print "spam"
    else:
        print "eggs"

def main():
    myfoo = foo()
    myfoo.bar = 3
    a(myfoo)

if __name__ == "__main__":
    main()