From cedb6ee009dfda5f437ae6cd11b62db9b15d1c18 Mon Sep 17 00:00:00 2001
From: Richard Shadrach <45562402+rhshadrach@users.noreply.github.com>
Date: Fri, 13 Nov 2020 22:31:46 -0500
Subject: [PATCH] Backport PR #37801 on branch 1.1.x: REGR: SeriesGroupBy where
 index has a tuple name fails

---
 doc/source/whatsnew/v1.1.5.rst                |  1 +
 pandas/core/series.py                         |  6 +++---
 pandas/tests/groupby/test_groupby.py          | 10 ++++++++++
 pandas/tests/series/indexing/test_indexing.py |  4 ++--
 4 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/doc/source/whatsnew/v1.1.5.rst b/doc/source/whatsnew/v1.1.5.rst
index 3b1f64e730830..2a598c489e809 100644
--- a/doc/source/whatsnew/v1.1.5.rst
+++ b/doc/source/whatsnew/v1.1.5.rst
@@ -15,6 +15,7 @@ including other versions of pandas.
 Fixed regressions
 ~~~~~~~~~~~~~~~~~
 - Regression in addition of a timedelta-like scalar to a :class:`DatetimeIndex` raising incorrectly (:issue:`37295`)
+- Fixed regression in :meth:`Series.groupby` raising when the :class:`Index` of the :class:`Series` had a tuple as its name (:issue:`37755`)
 -
 
 .. ---------------------------------------------------------------------------
diff --git a/pandas/core/series.py b/pandas/core/series.py
index 18a201674db65..277aa1d095350 100644
--- a/pandas/core/series.py
+++ b/pandas/core/series.py
@@ -889,7 +889,7 @@ def __getitem__(self, key):
 
                 return result
 
-            except KeyError:
+            except (KeyError, TypeError):
                 if isinstance(key, tuple) and isinstance(self.index, MultiIndex):
                     # We still have the corner case where a tuple is a key
                     # in the first level of our MultiIndex
@@ -953,7 +953,7 @@ def _get_values_tuple(self, key):
             return result
 
         if not isinstance(self.index, MultiIndex):
-            raise ValueError("key of type tuple not found and not a MultiIndex")
+            raise KeyError("key of type tuple not found and not a MultiIndex")
 
         # If key is contained, would have returned by now
         indexer, new_index = self.index.get_loc_level(key)
@@ -1009,7 +1009,7 @@ def __setitem__(self, key, value):
 
         except TypeError as err:
             if isinstance(key, tuple) and not isinstance(self.index, MultiIndex):
-                raise ValueError(
+                raise KeyError(
                     "key of type tuple not found and not a MultiIndex"
                 ) from err
 
diff --git a/pandas/tests/groupby/test_groupby.py b/pandas/tests/groupby/test_groupby.py
index bdb283ae445b1..35eab708412b8 100644
--- a/pandas/tests/groupby/test_groupby.py
+++ b/pandas/tests/groupby/test_groupby.py
@@ -2055,3 +2055,13 @@ def test_groups_repr_truncates(max_seq_items, expected):
 
         result = df.groupby(np.array(df.a)).groups.__repr__()
         assert result == expected
+
+
+def test_groupby_series_with_tuple_name():
+    # GH 37755
+    ser = Series([1, 2, 3, 4], index=[1, 1, 2, 2], name=("a", "a"))
+    ser.index.name = ("b", "b")
+    result = ser.groupby(level=0).last()
+    expected = Series([2, 4], index=[1, 2], name=("a", "a"))
+    expected.index.name = ("b", "b")
+    tm.assert_series_equal(result, expected)
diff --git a/pandas/tests/series/indexing/test_indexing.py b/pandas/tests/series/indexing/test_indexing.py
index fbdac2bb2d8e8..3b66939d9ddd2 100644
--- a/pandas/tests/series/indexing/test_indexing.py
+++ b/pandas/tests/series/indexing/test_indexing.py
@@ -387,9 +387,9 @@ def test_2d_to_1d_assignment_raises():
 def test_basic_getitem_setitem_corner(datetime_series):
     # invalid tuples, e.g. td.ts[:, None] vs. td.ts[:, 2]
     msg = "key of type tuple not found and not a MultiIndex"
-    with pytest.raises(ValueError, match=msg):
+    with pytest.raises(KeyError, match=msg):
         datetime_series[:, 2]
-    with pytest.raises(ValueError, match=msg):
+    with pytest.raises(KeyError, match=msg):
         datetime_series[:, 2] = 2
 
     # weird lists. [slice(0, 5)] will work but not two slices