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".

Answer 1

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__.

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()