Intervals are not reversible.

music_theory/scales.py

@@ -28,12 +28,22 @@ def interval_quality(note1, note2):
     norm_note2 = note2.upper()
     index1 = CHROMATIC_SCALE.index(norm_note1)
     index2 = CHROMATIC_SCALE.index(norm_note2)
-    return abs(index1 - index2) + 1
+
+    if index2 >= index1:
+        return index2 - index1 + 1
+    else:
+        index2 += len(CHROMATIC_SCALE)
+        return index2 - index1 + 1
 
 def interval_quantity(note1, note2):
     # TODO Work this in terms of minor and major intervals.
     plain_note1 = note1[0].upper()
     plain_note2 = note2[0].upper()
     index1 = NO_SHARPS.index(plain_note1)
     index2 = NO_SHARPS.index(plain_note2)
-    return abs(index1 - index2) + 1
+
+    if index2 >= index1:
+        return index2 - index1 + 1
+    else:
+        index2 += len(NO_SHARPS)
+        return index2 - index1 + 1

music_theory/tests/scales_tests.py

@@ -26,6 +26,9 @@ def test_interval_quantity(self):
         self.assertEqual(3, interval_quantity("A#", "c"))
         self.assertEqual(3, interval_quantity("a", "C#"))
 
+        self.assertNotEqual(interval_quantity("A", "C"), interval_quantity("C", "A"))
+        self.assertEqual(6, interval_quantity("C", "A"))
+
     def test_eval_flat(self):
         self.assertEqual("G#", eval_flat("Ab"))
         self.assertEqual("B", eval_flat("Cb"))
@@ -49,3 +52,6 @@ def test_interval_quality(self):
         self.assertEqual(5, interval_quality("a", "C#"))
         self.assertEqual(3, interval_quality("A#", "c"))
         self.assertEqual(5, interval_quality("a", "C#"))
+
+        self.assertNotEqual(interval_quality("A", "C"), interval_quality("C", "A"))
+        self.assertEqual(10, interval_quality("C", "A"))