From 1b56b1e99250930df8d24c2eeae967eac2d2724b Mon Sep 17 00:00:00 2001
From: Aamir Abdul Azeez <165933255+aamir-azeez@users.noreply.github.com>
Date: Fri, 25 Sep 2026 23:05:57 -0400
Subject: [PATCH] Save periodic-message data prior to first update

---
 canopen/network.py   |  1 +
 test/test_network.py | 35 +++++++++++++++++++++++++++++++++++
 2 files changed, 36 insertions(+)

diff --git a/canopen/network.py b/canopen/network.py
index 0bd56fdf..3d87d14b 100644
--- a/canopen/network.py
+++ b/canopen/network.py
@@ -330,6 +330,7 @@ def __init__(
         self.msg = can.Message(is_extended_id=can_id > 0x7FF,
                                arbitration_id=can_id,
                                data=data, is_remote_frame=remote)
+        self.msg.data = bytearray(self.msg.data)
         self._start()
 
     def _start(self):
diff --git a/test/test_network.py b/test/test_network.py
index 8017f89c..6d940ee7 100644
--- a/test/test_network.py
+++ b/test/test_network.py
@@ -1,6 +1,7 @@
 import logging
 import time
 import unittest
+from unittest.mock import Mock
 
 import can
 
@@ -286,6 +287,40 @@ def wait_for_periodicity():
         if msg is not None:
             self.assertIsNone(bus.recv(PERIOD))
 
+    def test_network_send_periodic_mutable_data(self):
+        scheduled = []
+
+        def send_periodic(message, period):
+            task = Mock(spec=["stop"])
+            scheduled.append((bytes(message.data), task))
+            return task
+
+        self.network.bus = Mock(spec=["send_periodic"])
+        self.network.bus.send_periodic.side_effect = send_periodic
+        data = bytearray([1, 2])
+        task = self.network.send_periodic(0x123, data, 0.1)
+        data[0] = 3
+        task.update(data)
+        self.assertEqual([payload for payload, _ in scheduled], [b'\x01\x02', b'\x03\x02'])
+        scheduled[0][1].stop.assert_called_once_with()
+
+        task.update(data)
+        self.assertEqual(len(scheduled), 2)
+        scheduled[1][1].stop.assert_not_called()
+
+        data[0] = 4
+        task.update(data)
+        self.assertEqual(scheduled[2][0], b'\x04\x02')
+        scheduled[1][1].stop.assert_called_once_with()
+
+    def test_network_send_periodic_empty_data(self):
+        self.network.bus = Mock(spec=["send_periodic"])
+        for data, remote in ((None, False), (None, True), (b'\x01', True)):
+            with self.subTest(data=data, remote=remote):
+                task = self.network.send_periodic(0x123, data, 0.1, remote=remote)
+                self.assertEqual(task.msg.data, bytearray())
+                self.assertEqual(task.msg.is_remote_frame, remote)
+
     def test_network_connect_does_not_recreate_notifier(self):
         self.network.connect(interface="virtual")
         self.addCleanup(self.network.disconnect)