From 6321d5fcd185c1a3a20e25a212d8f0056ec4ea56 Mon Sep 17 00:00:00 2001
From: hill <hill@hilll.dev>
Date: Sun, 20 Sep 2026 23:26:01 +0200
Subject: [PATCH] Do not append to the caller's list in
 AbstractOctaveRepeatingScale

buildNetwork completed the octave with mode.append(iComplement), so the
list passed in came back longer:

    mode = ['M3', 'M3']
    scale.OctaveRepeatingScale('c4', mode)
    mode  # ['M3', 'M3', <music21.interval.Interval d4>]

Reusing that list for a second scale then built a different scale.
Build a new list instead.
---
 music21/scale/__init__.py        |  4 ++--
 music21/scale/test_scale_main.py | 11 ++++++++++-
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/music21/scale/__init__.py b/music21/scale/__init__.py
index ebb66478e..e3804a12f 100644
--- a/music21/scale/__init__.py
+++ b/music21/scale/__init__.py
@@ -6,7 +6,7 @@
 #               Michael Scott Asato Cuthbert
 #               Jose Cabal-Ugaz
 #
-# Copyright:    Copyright © 2009-2011 Michael Scott Asato Cuthbert
+# Copyright:    Copyright © 2009-2026 Michael Scott Asato Cuthbert
 # License:      BSD, see license.txt
 # ------------------------------------------------------------------------------
 '''
@@ -1020,7 +1020,7 @@ def buildNetwork(self, mode: t.Any = None) -> None:
         intervalSum = interval.add(mode)
         iComplement = intervalSum.complement
         if iComplement is not None:
-            mode.append(iComplement)
+            mode = [*mode, iComplement]  # a copy: mode is the caller's list
 
         self.tonicDegree = 1
         self._net = intervalNetwork.IntervalNetwork(mode,
diff --git a/music21/scale/test_scale_main.py b/music21/scale/test_scale_main.py
index ded1bd6d9..c479a928d 100644
--- a/music21/scale/test_scale_main.py
+++ b/music21/scale/test_scale_main.py
@@ -5,7 +5,7 @@
 # Authors:      Christopher Ariza
 #               Michael Scott Asato Cuthbert
 #
-# Copyright:    Copyright © 2010-2024 Michael Scott Asato Cuthbert
+# Copyright:    Copyright © 2010-2026 Michael Scott Asato Cuthbert
 # License:      BSD, see license.txt
 # ------------------------------------------------------------------------------
 from __future__ import annotations
@@ -233,6 +233,15 @@ def testBasic(self):
              <music21.pitch.Pitch F#5>],
  'notMatched': [<music21.pitch.Pitch C#5>, <music21.pitch.Pitch G#5>]}''', pformat(sc1.match(sc2)))
 
+    def testOctaveRepeatingScaleLeavesModeAlone(self):
+        mode = ['M3', 'M3']
+        sc = scale.OctaveRepeatingScale('c4', mode)
+        self.assertEqual(mode, ['M3', 'M3'])
+        self.assertEqual(self.pitchOut(sc.pitches), '[C4, E4, G#4, C5]')
+
+        again = scale.OctaveRepeatingScale('c4', mode)
+        self.assertEqual(self.pitchOut(again.pitches), '[C4, E4, G#4, C5]')
+
     def testCyclicalScales(self):
         sc = scale.CyclicalScale('c4', ['m2', 'm2'])