How to chain attribute lookups that might return None in Python?

Question

My problem is a general one, how to chain a series of attribute lookups when one of the intermediate ones might return None, but since I ran into this problem t

Answer 1

One solution would be to wrap the outer object inside a Proxy that handles None values for you. See below for a beginning implementation.

import unittest

class SafeProxy(object):

    def __init__(self, instance):
        self.__dict__["instance"] = instance

    def __eq__(self, other):
        return self.instance==other

    def __call__(self, *args, **kwargs):
        return self.instance(*args, **kwargs)

    # TODO: Implement other special members

    def __getattr__(self, name):
        if hasattr(self.__dict__["instance"], name):
            return SafeProxy(getattr(self.instance, name))

        if name=="val":
            return lambda: self.instance

        return SafeProxy(None)

    def __setattr__(self, name, value):
        setattr(self.instance, name, value)


# Simple stub for creating objects for testing
class Dynamic(object):
    def __init__(self, **kwargs):
        for name, value in kwargs.iteritems():
            self.__setattr__(name, value)

    def __setattr__(self, name, value):
        self.__dict__[name] = value


class Test(unittest.TestCase):

    def test_nestedObject(self):
        inner = Dynamic(value="value")
        middle = Dynamic(child=inner)
        outer = Dynamic(child=middle)
        wrapper = SafeProxy(outer)
        self.assertEqual("value", wrapper.child.child.value)
        self.assertEqual(None, wrapper.child.child.child.value)

    def test_NoneObject(self):
        self.assertEqual(None, SafeProxy(None))

    def test_stringOperations(self):
        s = SafeProxy("string")
        self.assertEqual("String", s.title())
        self.assertEqual(type(""), type(s.val()))
        self.assertEqual()

if __name__=="__main__":
    unittest.main()

NOTE: I am personally not sure wether I would use this in an actual project, but it makes an interesting experiment and I put it here to get people thoughts on this.