Refuse an odd keyword count in createPairs

src/ideaCollisionGenerator.py

@@ -36,6 +36,11 @@ def getKeywords(self):
         random.shuffle(self.keywords)
 
     def createPairs(self):
+        # an odd keyword would be left without a partner, so it is refused
+        # the same way the constructor refuses an odd numKeywords
+        if len(self.keywords) % 2 != 0:
+            raise ValueError("keywords must have an even count to be paired, got " + str(len(self.keywords)))
+
         # match keywords in pairs
         for i in range(0, len(self.keywords), 2):
             pair = []

tests/test_ideaCollisionGenerator.py

@@ -120,14 +120,21 @@ def testNoKeywordsProduceNoPairs(self):
         generator.createPairs()
         self.assertEqual(generator.pairs, [])
 
-    def testOddKeywordCountRaisesIndexError(self):
-        # current behaviour: keywords[i+1] is unguarded, so the final iteration
-        # indexes past the end (unreachable through the CLI, where the count is 10)
+    def testOddKeywordCountRaisesValueError(self):
+        # a trailing keyword with no partner is refused rather than indexed past
         generator = IdeaCollisionGenerator()
         generator.keywords = ["a", "b", "c"]
-        with self.assertRaises(IndexError):
+        with self.assertRaisesRegex(ValueError, "even count"):
             generator.createPairs()
 
+    def testOddKeywordCountCreatesNoPairs(self):
+        # the count is checked before any pair is made, so nothing half-built is left behind
+        generator = IdeaCollisionGenerator()
+        generator.keywords = ["a", "b", "c"]
+        with self.assertRaises(ValueError):
+            generator.createPairs()
+        self.assertEqual(generator.pairs, [])
+
 
 class TestPromptForIdeas(unittest.TestCase):
     def testCollectsOneIdeaPerPair(self):