update in multimap function in utils.py and added test for it

tests/test_utils.py

@@ -25,6 +25,11 @@ def test_count():
     assert count([True, False, True, True, False]) == 3
     assert count([5 > 1, len("abc") == 3, 3+1 == 5]) == 2
 
+def test_multimap():
+    assert multimap([(1, 2),(1, 3),(1, 4),(2, 3),(2, 4),(4, 5)]) == \
+        {1: [2, 3, 4], 2: [3, 4], 4: [5]}
+    assert multimap([("a", 2), ("a", 3), ("a", 4), ("b", 3), ("b", 4), ("c", 5)]) == \
+        {'a': [2, 3, 4], 'b': [3, 4], 'c': [5]}
 
 def test_product():
     assert product([1, 2, 3, 4]) == 24

utils.py

@@ -42,10 +42,10 @@ def count(seq): # TODO: replace with quantify
 
 def multimap(items):
     """Given (key, val) pairs, return {key: [val, ....], ...}."""
-    result = defaultdict(list)
+    result = collections.defaultdict(list)
     for (key, val) in items:
         result[key].append(val)
-    return result
+    return dict(result)
 
 def multimap_items(mmap):
     """Yield all (key, val) pairs stored in the multimap."""