From d7e4038be844a10e612859b9aa15f4a84413e250 Mon Sep 17 00:00:00 2001
From: Yuval Tassa <tassa@google.com>
Date: Mon, 12 Jan 2026 03:20:08 -0800
Subject: [PATCH] Expose mj_copyData in Python bindings.

This allows calling `mujoco.mj_copyData(dest, model, src)` to copy data between existing `mjData` instances.

PiperOrigin-RevId: 855150139
Change-Id: If122cb7acf590846e25ee15f9770c3c087147169
---
 python/mujoco/bindings_test.py | 9 +++++++++
 python/mujoco/functions.cc     | 6 +++++-
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/python/mujoco/bindings_test.py b/python/mujoco/bindings_test.py
index b15dc261..42e758cf 100644
--- a/python/mujoco/bindings_test.py
+++ b/python/mujoco/bindings_test.py
@@ -495,6 +495,15 @@ class MuJoCoBindingsTest(parameterized.TestCase):
     mujoco.mj_forward(self.model, data_copy)
     self.assertEqual(data_copy.ncon, 4)
 
+    # Test copying into existing data.
+    data_copy.qpos[1] = 1.234
+    data_copy.geom_xpos[2] = 5.678
+    self.assertFalse(np.array_equal(self.data.qpos, data_copy.qpos))
+    self.assertFalse(np.array_equal(self.data.geom_xpos, data_copy.geom_xpos))
+    mujoco.mj_copyData(self.data, self.model, data_copy)
+    np.testing.assert_array_equal(self.data.qpos, data_copy.qpos)
+    np.testing.assert_array_equal(self.data.geom_xpos, data_copy.geom_xpos)
+
   def test_mjdata_can_read_warning_array(self):
     warnings = self.data.warning
     self.assertLen(warnings, mujoco.mjtWarning.mjNWARNING)
diff --git a/python/mujoco/functions.cc b/python/mujoco/functions.cc
index 5e5bb2c5..3ddb86fd 100644
--- a/python/mujoco/functions.cc
+++ b/python/mujoco/functions.cc
@@ -159,7 +159,11 @@ PYBIND11_MODULE(_functions, pymodule) {
   // Skipped: mj_deleteModel (have MjModel.__del__)
   Def<traits::mj_sizeModel>(pymodule);
   // Skipped: mj_makeData (have MjData.__init__)
-  // Skipped: mj_copyData (have MjData.__copy__, memory managed by MjData)
+  DEF_WITH_OMITTED_PY_ARGS(traits::mj_copyData)(
+      pymodule,
+      [](raw::MjData* dest, const raw::MjModel* m, const raw::MjData* src) {
+        InterceptMjErrors(::mj_copyData)(dest, m, src);
+      });
   Def<traits::mj_resetData>(pymodule);
   Def<traits::mj_resetDataDebug>(pymodule);
   Def<traits::mj_resetDataKeyframe>(pymodule);