TST: Add regression test for Series index.map failing (pandas-dev#20990)

Author: Mateusz Górski

pandas/tests/series/indexing/test_indexing.py

@@ -907,3 +907,18 @@ def test_head_tail(test_data):
     assert_series_equal(test_data.series.head(0), test_data.series[0:0])
     assert_series_equal(test_data.series.tail(), test_data.series[-5:])
     assert_series_equal(test_data.series.tail(0), test_data.series[0:0])
+
+
+def test_index_map():
+    # see GH20990
+    count = 6
+    index = pd.date_range("2018-01-01", periods=count, freq="M")
+
+    a = Series(np.arange(count), index=index)
+    a.index.name = "name"
+
+    b = a.copy()
+    try:
+        b.index = a.index.map(lambda x: (x.year, x.month))
+    except ValueError:
+        pytest.fail("Unexpected ValueError")