def test_simple():
    class A(object):
        def __init__(self):
            self.a = 12
            self.b = 13
            self.counter = 0
        def compute(self):
            try:
                return self.__dict__['c']
            except KeyError:
                self.counter += 1
                self.__dict__['c'] = result = self.a * self.b
                return result
        def __getattr__(self, attr):
            if attr == "c":
                return self.compute()
            else:
                raise AttributeError

    a = A()
    assert a.counter == 0
    assert a.c == 12 * 13
    assert a.counter == 1
    assert a.c == 12 * 13
    assert a.counter == 1

    class B(object):
        def __init__(self):
            self.a = 12
            self.b = 13
            self.counter = 0

        def compute(self):
            self.counter += 1
            return self.a * self.b
        c = LazyAttribute(compute, "c")
    b = B()
    assert b.counter == 0
    print b.__dict__
    assert b.c == 12 * 13
    assert b.counter == 1
    print b.__dict__
    assert b.c == 12 * 13
    assert b.counter == 1
    b2 = B()
    b2.a = 100
    assert b2.c == 1300

class LazyAttribute(object):
    def __init__(self, computer, attr):
        self.computer = computer
        self.attr = attr

    def __get__(self, inst, cls):
        try:
            return inst.__dict__[self.attr]
        except KeyError:
            inst.__dict__[self.attr] = result = self.computer(inst)
            return result