Change construction of "expected" for better readibility

Author: Mateusz Górski

pandas/tests/indexes/datetimes/test_datetime.py

@@ -466,9 +466,8 @@ def test_index_map(self, name):
         a = pd.Series(np.arange(count), index=index)
         a.index = a.index.map(lambda x: (x.year, x.month))
 
-        exp_index = pd.MultiIndex.from_arrays(
-            [[2018 for i in range(count)], [i for i in range(1, count + 1)]],
-            names=[name, name],
+        exp_index = pd.MultiIndex.from_product(
+            ((2018,), range(1, 7)), names=[name, name]
         )
         expected = pd.Series(np.arange(count), index=exp_index)