Avoided looping and multiple np calls, added api breaking section to whatsnew

Author: rhshadrach

pandas/core/groupby/groupby.py

@@ -1282,6 +1282,8 @@ def std(self, ddof: int = 1):
             cols = result.columns.get_indexer_for(
                 result.columns.difference(self.exclusions).unique()
             )
+            # .values to remove labels; iLocIndexer._setitem_with_indexer
+            # broken when labels are not unique
             result.iloc[:, cols] = np.sqrt(result.iloc[:, cols]).values
 
         return result
@@ -1339,6 +1341,8 @@ def sem(self, ddof: int = 1):
             cols = result.columns.get_indexer_for(
                 result.columns.difference(self.exclusions).unique()
             )
+            # .values in both numerator and denominator to remove labels;
+            # iLocIndexer._setitem_with_indexer broken when labels are not unique
             result.iloc[:, cols] = (
                 result.iloc[:, cols].values / np.sqrt(self.count().iloc[:, cols]).values
             )

pandas/tests/groupby/test_function.py

@@ -589,19 +589,13 @@ def test_ops_not_as_index(reduction_func):
     # GH 10355
     # Using as_index=False should not modify grouped column
 
-    # GH 5755
-    if reduction_func in (
-        "corrwith",
-        "idxmax",
-        "idxmin",
-        "mad",
-        "ngroup",
-        "nth",
-        "nunique",
-        "size",
-        "skew",
-    ):
-        pytest.skip("Skip until #5755 is resolved")
+    if reduction_func in ("nth", "ngroup", "size",):
+        pytest.skip("Skip until behavior is determined (GH #5755)")
+
+    if reduction_func in ("corrwith", "idxmax", "idxmin", "mad", "nunique", "skew",):
+        pytest.xfail(
+            "_GroupBy._python_apply_general incorrectly modifies grouping columns"
+        )
 
     df = DataFrame(np.random.randint(0, 5, size=(100, 2)), columns=["a", "b"])
     expected = getattr(df.groupby("a"), reduction_func)().reset_index()