Boolean value of objects in Python

boolean.py

>>> class Foo:
...   def __init__(self, value):
...     self.value = value
...   def __nonzero__(self):
...     return self.value
... 
>>> a = Foo(1)
>>> b = Foo(0)
>>> assert a
>>> assert b
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError