Question

If statements in Python allow you to do something like:

   if not x:
       print "X is false."

This works if you’re using an empty list, an empty dictionary, None, 0, etc, but what if you have your own custom class? Can you assign a false value for that class so that in the same style of conditional, it will return false?

Answer 1

Since Python 3, you need to implement the __bool__ method on your class. This should return True or False to determine the truth value:

class MyClass:
    def __init__(self, val):
        self.val = val
    def __bool__(self):
        return self.val != 0  #This is an example, you can use any condition

x = MyClass(0)
if not x:
    print('x is false')

If __bool__ has not been defined, the implementation will call __len__ and the instance will be considered True if it returned a nonzero value. If __len__ hasn’t been defined either, all instances will be considered True.

For further reading:

• https://docs.python.org/3/library/stdtypes.html#truth-value-testing

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In Python 2, the special method __nonzero__ was used instead of __bool__.