Make ascii_tree's optional df argument usable, and stop it mutating the caller's frame

src/ssvc/decision_tables/helpers.py

@@ -377,11 +377,13 @@ def ascii_tree(dt: DecisionTable, df: pd.DataFrame | None = None) -> str:
     Reads a Pandas data frame, builds a decision tree, and returns its ASCII representation.
     """
     # Check for the optional 'row' column and drop it if it exists.
-    if df == None:
+    if df is None:
         df = decision_table_to_longform_df(dt)
 
     if "row" in df.columns:
-        df.drop(columns="row", inplace=True)
+        # Not in place: df may belong to the caller, and dropping a column from
+        # under them is not something a read-only rendering helper should do.
+        df = df.drop(columns="row")
 
     # Separate feature columns from the outcome column.
     feature_cols = list(df.columns[:-1])

src/test/decision_tables/test_helpers.py

@@ -0,0 +1,74 @@
+#  Copyright (c) 2026 Carnegie Mellon University.
+#  NO WARRANTY. THIS CARNEGIE MELLON UNIVERSITY AND SOFTWARE
+#  ENGINEERING INSTITUTE MATERIAL IS FURNISHED ON AN "AS-IS" BASIS.
+#  CARNEGIE MELLON UNIVERSITY MAKES NO WARRANTIES OF ANY KIND,
+#  EITHER EXPRESSED OR IMPLIED, AS TO ANY MATTER INCLUDING, BUT
+#  NOT LIMITED TO, WARRANTY OF FITNESS FOR PURPOSE OR
+#  MERCHANTABILITY, EXCLUSIVITY, OR RESULTS OBTAINED FROM USE
+#  OF THE MATERIAL. CARNEGIE MELLON UNIVERSITY DOES NOT MAKE
+#  ANY WARRANTY OF ANY KIND WITH RESPECT TO FREEDOM FROM
+#  PATENT, TRADEMARK, OR COPYRIGHT INFRINGEMENT.
+#  Licensed under a MIT (SEI)-style license, please see LICENSE or contact
+#  permission@sei.cmu.edu for full terms.
+#  [DISTRIBUTION STATEMENT A] This material has been approved for
+#  public release and unlimited distribution. Please see Copyright notice
+#  for non-US Government use and distribution.
+#  This Software includes and/or makes use of Third-Party Software each
+#  subject to its own license.
+#  DM24-0278
+import unittest
+
+from ssvc.decision_tables.base import ascii_tree as base_ascii_tree
+from ssvc.decision_tables.example.to_play import LATEST as EXAMPLE_DT
+from ssvc.decision_tables.helpers import (
+    ascii_tree,
+    decision_table_to_longform_df,
+)
+
+
+class TestAsciiTree(unittest.TestCase):
+    def setUp(self) -> None:
+        self.dt = EXAMPLE_DT
+
+    def test_df_omitted(self) -> None:
+        # The path every caller in the repository takes.
+        self.assertGreater(len(ascii_tree(self.dt).splitlines()), 0)
+
+    def test_df_supplied(self) -> None:
+        # Passing the frame is what the signature invites, and `df == None`
+        # made it raise ValueError from DataFrame.__bool__ for every frame.
+        df = decision_table_to_longform_df(self.dt)
+        self.assertEqual(ascii_tree(self.dt, df), ascii_tree(self.dt))
+
+    def test_df_supplied_via_base(self) -> None:
+        # base re-exports the helper, so the same call has two public routes.
+        df = decision_table_to_longform_df(self.dt)
+        self.assertEqual(
+            base_ascii_tree(self.dt, df), base_ascii_tree(self.dt)
+        )
+
+    def test_caller_frame_is_not_modified(self) -> None:
+        # The "row" column was dropped in place. That only ever touched the
+        # frame this function built itself while `df` could not be supplied;
+        # once it can, an in-place drop takes a column off the caller's object.
+        df = decision_table_to_longform_df(self.dt)
+        df.insert(0, "row", range(len(df)))
+        columns_before = list(df.columns)
+
+        ascii_tree(self.dt, df)
+
+        self.assertEqual(list(df.columns), columns_before)
+
+    def test_row_column_is_excluded_from_the_tree(self) -> None:
+        # Dropping "row" must still happen, just not on the caller's frame.
+        df = decision_table_to_longform_df(self.dt)
+        without_row = ascii_tree(self.dt, df)
+
+        df_with_row = decision_table_to_longform_df(self.dt)
+        df_with_row.insert(0, "row", range(len(df_with_row)))
+
+        self.assertEqual(ascii_tree(self.dt, df_with_row), without_row)
+
+
+if __name__ == "__main__":
+    unittest.main()