"""Python 3.10+; demonstrate revision-scoped keys and pruning."""
import unittest


class FeedCache:
    def __init__(self):
        self.values = {}

    def get(self, feed, revision, stop, loader):
        key = (feed, revision, stop)
        if key not in self.values:
            self.values[key] = tuple(loader())
        return self.values[key]

    def prune(self, feed, keep_revision):
        self.values = {key: value for key, value in self.values.items()
                       if key[0] != feed or key[1] == keep_revision}


class RevisionTests(unittest.TestCase):
    def test_revision_and_feed_are_both_part_of_identity(self):
        cache = FeedCache()
        self.assertEqual(cache.get("bus", "r1", "S", lambda: ["10:00"]), ("10:00",))
        def must_not_reload():
            raise AssertionError("cache miss for an existing key")
        self.assertEqual(cache.get("bus", "r1", "S", must_not_reload), ("10:00",))
        self.assertEqual(cache.get("bus", "r2", "S", lambda: ["10:05"]), ("10:05",))
        self.assertEqual(cache.get("rail", "r1", "S", lambda: ["11:00"]), ("11:00",))
        cache.prune("bus", "r2")
        self.assertNotIn(("bus", "r1", "S"), cache.values)
        self.assertIn(("rail", "r1", "S"), cache.values)

    def test_failed_load_is_not_cached(self):
        cache = FeedCache()
        def broken():
            raise OSError("download failed")
        with self.assertRaises(OSError):
            cache.get("bus", "r1", "S", broken)
        self.assertEqual(cache.values, {})


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