A __getattr__ example.

getattr-example.py

"""
An example of decorating a built-in type with another object.

Attributes and methods on the built-in type delegate to the other object.

"""

class WrappedInt(int):

    """
    An integer wrapper that decorates an integer with a set.

    """

    def __new__(cls, integer, elements):
        return int.__new__(cls, integer)

    def __init__(self, integer, elements):
        int.__init__(self, integer)
        self._set = set(elements)

    def __len__(self):
        return len(self._set)

    def __getattr__(self, attr):
        return getattr(self._set, attr)


a = WrappedInt(10, [1, 2, 3])

print(a + 20)  # prints 30
print(len(a))  # prints 3
print(a.union([4, 5, 6]))  # prints set([1, 2, 3, 4, 5, 6])