In Python, what operator to override for "if object:"?

Question

I find it very handy to check if an object is "empty" with the following construct:

l=[]
if l:
     do_stuff()

For a standard python list, the if will be executed only if the list is not empty.

My question is, how can I implement the same idea for my own objects?

Answer 1

Define a method __bool__ (Python 3.x) or __nonzero__ (2.x). Or define both for portability, with one returning the result of the other.

Answer 2

Implement __nonzero__ for Python 2 and __bool__ for Python 3:

class AlwaysTrueObject:
    def __bool__(self):
        return True
    __nonzero__ = __bool__