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".

_{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?.}

See also: __nonzero__()

Jonathan
– Jonathan

2015-08-18 23:27:37 +00:00
Commented Aug 18, 2015 at 23:27 — Jonathan
– Jonathan, Commented Aug 18, 2015 at 23:27

Karl Knechtel · Accepted Answer · 2023-01-15 06:27:50Z

59

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.

edited Jan 15, 2023 at 6:27

Karl Knechtel

61.4k14 gold badges134 silver badges193 bronze badges

answered Jul 6, 2009 at 14:05

Bite code

601k118 gold badges310 silver badges336 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Karl Knechtel Over a year ago

Would you object to further editing this, to prioritize the 3.x answer?

Flimm · Accepted Answer · 2021-08-02 10:02:52Z

38

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()

edited Aug 2, 2021 at 10:02

Flimm

154k49 gold badges282 silver badges295 bronze badges

answered Jul 6, 2009 at 14:04

sunqiang

6,5021 gold badge34 silver badges32 bronze badges

1 Comment

Nobody Over a year ago

Please refer to other answer for Python 3, this answer applies exclusively to Python 2.

Collectives™ on Stack Overflow

Boolean value of objects in Python

2 Answers 2

Python 2.x

Python 3.x

1 Comment

In Python 2, use `nonzero`:

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Python 2.x

Python 3.x

1 Comment

In Python 2, use __nonzero__:

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related

In Python 2, use `nonzero`: