Why doesn’t this work as one may have naively expected?
class Foo(object): def __init__(self): self.bar = 3 def __bool__(self): return self.bar > 10 foo = Foo() if foo: print 'x' else: print 'y'
(The output is
For Python 2-3 compatibility, just add this to your example:
Foo.__nonzero__ = Foo.__bool__
or expand the original definition of Foo to include:
__nonzero__ = __bool__
You could of course define them in reverse too, where the method name is
__nonzero__ and you assign it to
__bool__, but I think the name
__nonzero__ is just a legacy of the original C-ishness of Python’s interpretation of objects as truthy or falsy based on their equivalence with zero. Just add the statement above and your code will work with Python 2.x, and will automatically work when you upgrade to Python 3.x (and eventually you an drop the assignment to
__bool__ method is used in Python 3. For Python 2, you want