boolean context of a python object [duplicate]

Question

Can I use a python object in a desired boolean context? By default any object is True in boolean context.

>>> class Person():
...           pass
... 
>>> a=Person()
>>> bool(a)
True

Like bool(0) returns False and bool(1) returns True.Can i have any way to define an object to have it's boolean value either True or False. Correct me if i'm wrong anywhere,thanks.

Answer 1

class Something(object):
    def __nonzero__(self):
        return False  # Something is False always. 

print bool(Something())

Take note __nonzero__ has been renamed to __bool__ in Python 3.x

As pointed out in a comment, you would be better to define __len__ on container like collections. When __len__ returns 0, bool(x) will evaluate False. Otherwise any positive number above 1 will equate to True.

Answer 2

See the documentation on Truth Value Testing. It first checks if your instance has a __nonzero__ method, if not it uses a __len__ method. The instance is False if __nonzero__ is False or its length is 0.