Boolean value of objects in Python

Question

As we know, Python has boolean values for objects: If a class has a __len__ method, every instance of it for which __len__() happens to return 0 will be evaluated as a boolean False (for example, the empty list).

In fact, every iterable, empty custom object is evaluated as False if it appears in boolean expression.

Now suppose I have a class foo with attribute bar. How can I define its truth value, so that, say, it will be evaluated to True if bar % 2 == 0 and False otherwise?

For example:

myfoo = foo()
myfoo.bar = 3
def a(foo):
    if foo:
        print "spam"
    else:
        print "eggs"

so, a(myfoo) should print "eggs".

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_{To close questions that are specifically about debugging an error caused by using the 3.x-specific approach in 2.x, use How can I implement conversion to boolean for classes in 2.x? Why doesn't the "__bool__" magic method work?.}

Answer 1

Python 2.x

Use the __nonzero__ magic method. It's 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 !"

Python 3.x

Use __bool__ instead, in the same way.

Answer 2

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()