Failing test edited

Previous assertion replaced with a new check.
Added description to linking issue.

Co-Authored-By: Richard Shadrach <45562402+rhshadrach@users.noreply.github.com>

Author: dimithras

pandas/tests/groupby/test_grouping.py

@@ -408,8 +408,14 @@ def test_groupby_grouper_f_sanity_checked(self):
         # when the elements are Timestamp.
         # the result is Index[0:6], very confusing.
 
-        msg = r"Grouper result violates len\(labels\) == len\(data\)"
-        with pytest.raises(AssertionError, match=msg):
+        # GH36158
+        # issue references errors for groupby
+        # created with functions (lambda or named)
+        # after changes applied under that issue this test fails with a
+        # more sensible error than the previous assertion.
+
+        msg = r"'Timestamp' object is not subscriptable"
+        with pytest.raises(TypeError, match=msg):
             ts.groupby(lambda key: key[0:6])
 
     def test_grouping_error_on_multidim_input(self, df):