lazy_set.py

import itertools as it

def setness(f):
        setness.calls = dict()
        def decorator(arg):
                seen = setness.calls.get( arg, set() )
                for i in seen: yield i
                for i in f(arg):
                        if i not in seen:
                                seen.add(i)
                                yield i
                setness.calls[arg] = seen
        return decorator

def silence_generator_already_executing(generator):
        try:
                for element in generator:
                        yield element
        except ValueError, e:
                pass

class lazy_set():
        def __init__(self, hudge_input):
                self._input = hudge_input
        @setness
        def __iter__(self):
                for i in self._input: yield i
        def update(self, iterable):
                self._input = it.chain(self._input,
                        silence_generator_already_executing(iterable))
        def add(self, obj):
                self._input = it.chain([obj], self._input)

lazy_set_test.py

import unittest
import lazy_set

class LazySetTest(unittest.TestCase):
    def setUp(self):
        self.lazy_set = lazy_set.lazy_set(xrange(1, 4))
        self.starting_value = range(1, 4)

    def testCreation(self):
        self.assertEqual(
            sorted(self.lazy_set),
            self.starting_value
        )

    def testUpdate(self):
        self.lazy_set.update(2* x for x in range(4, 10))
        self.assertEqual(
            sorted(self.lazy_set),
            [1, 2, 3, 8, 10, 12, 14, 16, 18]
        )

    def testAdd(self):
        self.lazy_set.add(5)
        self.assertEqual(
            sorted(self.lazy_set),
            [1, 2, 3, 5]
        )

    def testUniqueness(self):
        self.lazy_set.update(xrange(5))
        self.lazy_set.update(xrange(3, 8))
        self.assertEqual(
            sorted(self.lazy_set),
            range(8)
        )

    def testSelfUpdateWIter(self):
        self.lazy_set.update(iter(self.lazy_set))
        self.assertEqual(
            sorted(self.lazy_set),
            self.starting_value
        )

    def testSelfUpdateWOIter(self):
        self.lazy_set.update(self.lazy_set)
        self.assertEqual(
            sorted(self.lazy_set),
            self.starting_value
        )

    def testSelfUpdateNotLats(self):
        self.lazy_set.update(iter(self.lazy_set))
        self.lazy_set.update(xrange(5, 8))
        self.assertEqual(
            sorted(self.lazy_set),
            range(1, 4) + range(5, 8)
        )

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