From dea162b463a3dea0f2d2ade3bceebbcdd65ec8b5 Mon Sep 17 00:00:00 2001
From: Richard Shadrach <rhshadrach@gmail.com>
Date: Mon, 9 Nov 2020 17:23:43 -0500
Subject: [PATCH 1/2] ENH: Raise when subsetting columns on groupby with axis=1

---
 pandas/core/groupby/generic.py       | 3 +++
 pandas/tests/groupby/test_groupby.py | 9 ++++++++-
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/pandas/core/groupby/generic.py b/pandas/core/groupby/generic.py
index d0b58e8abc4ee..6f86819303537 100644
--- a/pandas/core/groupby/generic.py
+++ b/pandas/core/groupby/generic.py
@@ -1530,6 +1530,9 @@ def filter(self, func, dropna=True, *args, **kwargs):
         return self._apply_filter(indices, dropna)
 
     def __getitem__(self, key):
+        if self.axis == 1:
+            # GH 37725
+            raise ValueError("Cannot subset columns when using axis=1")
         # per GH 23566
         if isinstance(key, tuple) and len(key) > 1:
             # if len == 1, then it becomes a SeriesGroupBy and this is actually
diff --git a/pandas/tests/groupby/test_groupby.py b/pandas/tests/groupby/test_groupby.py
index a0c228200e73a..3a4abd58f0d39 100644
--- a/pandas/tests/groupby/test_groupby.py
+++ b/pandas/tests/groupby/test_groupby.py
@@ -2081,7 +2081,6 @@ def test_group_on_two_row_multiindex_returns_one_tuple_key():
 @pytest.mark.parametrize(
     "klass, attr, value",
     [
-        (DataFrame, "axis", 1),
         (DataFrame, "level", "a"),
         (DataFrame, "as_index", False),
         (DataFrame, "sort", False),
@@ -2120,6 +2119,14 @@ def test_subsetting_columns_keeps_attrs(klass, attr, value):
     assert getattr(result, attr) == getattr(expected, attr)
 
 
+def test_subsetting_columns_axis_1():
+    # GH 37725
+    g = DataFrame({"A": [1], "B": [2], "C": [3]}).groupby([0, 0, 1], axis=1)
+    match = "Cannot subset columns when using axis=1"
+    with pytest.raises(ValueError, match=match):
+        g[["A", "B"]].sum()
+
+
 @pytest.mark.parametrize("func", ["sum", "any", "shift"])
 def test_groupby_column_index_name_lost(func):
     # GH: 29764 groupby loses index sometimes

From 0d2630185b937cce362e8c9e34a1fc4bb3d03ad5 Mon Sep 17 00:00:00 2001
From: Richard Shadrach <rhshadrach@gmail.com>
Date: Mon, 9 Nov 2020 17:28:45 -0500
Subject: [PATCH 2/2] whatsnew

---
 doc/source/whatsnew/v1.2.0.rst | 1 +
 1 file changed, 1 insertion(+)

diff --git a/doc/source/whatsnew/v1.2.0.rst b/doc/source/whatsnew/v1.2.0.rst
index d4d28dde52d58..a5d60ca90d92f 100644
--- a/doc/source/whatsnew/v1.2.0.rst
+++ b/doc/source/whatsnew/v1.2.0.rst
@@ -232,6 +232,7 @@ Other enhancements
 - :meth:`testing.assert_index_equal` now has a ``check_order`` parameter that allows indexes to be checked in an order-insensitive manner (:issue:`37478`)
 - :func:`read_csv` supports memory-mapping for compressed files (:issue:`37621`)
 - Improve error reporting for :meth:`DataFrame.merge()` when invalid merge column definitions were given (:issue:`16228`)
+- Improved error reporting for subsetting columns of a :class:`DataFrameGroupBy` with ``axis=1`` (:issue:`37725`)
 
 .. _whatsnew_120.api_breaking.python: