Added Choices().subset().

model_utils/choices.py

@@ -142,3 +142,17 @@ class Choices(object):
 
     def __deepcopy__(self, memo):
         return self.__class__(*copy.deepcopy(self._triples, memo))
+
+    def subset(self, *new_identifiers):
+        identifiers = set(self._identifier_map.keys())
+
+        if not identifiers.issuperset(new_identifiers):
+            raise ValueError(
+                'The following identifiers are not present: %s' %
+                identifiers.symmetric_difference(new_identifiers),
+            )
+
+        return self.__class__(*[
+            choice for choice in self._triples
+            if choice[1] in new_identifiers
+        ])

tests/test_choices.py

@@ -279,3 +279,30 @@ class IdentifierChoicesTests(ChoicesTests):
                 ('group b', [(3, 'three')]),
             ],
         )
+
+
+class SubsetChoicesTest(TestCase):
+
+    def setUp(self):
+        self.choices = Choices(
+            (0, 'a', 'A'),
+            (1, 'b', 'B'),
+        )
+
+    def test_nonexistent_identifiers_raise(self):
+        with self.assertRaises(ValueError):
+            self.choices.subset('a', 'c')
+
+    def test_solo_nonexistent_identifiers_raise(self):
+        with self.assertRaises(ValueError):
+            self.choices.subset('c')
+
+    def test_empty_subset_passes(self):
+        subset = self.choices.subset()
+
+        self.assertEqual(subset, Choices())
+
+    def test_subset_returns_correct_subset(self):
+        subset = self.choices.subset('a')
+
+        self.assertEqual(subset, Choices((0, 'a', 'A')))