From 41563507081637d4088d69e18a220394dadba9ea Mon Sep 17 00:00:00 2001
From: Alessio Quaglino <quaglino@google.com>
Date: Mon, 10 Feb 2025 04:32:32 -0800
Subject: [PATCH] Add to_file function to save the Spec to a file.

Fixes #2415.

PiperOrigin-RevId: 725161617
Change-Id: I6c374cc7f7cb3f1da1cc0da531dd7e5be5d62b6a
---
 python/mujoco/specs.cc      | 6 ++++++
 python/mujoco/specs_test.py | 8 +++-----
 2 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/python/mujoco/specs.cc b/python/mujoco/specs.cc
index 5031f05c..98faf44f 100644
--- a/python/mujoco/specs.cc
+++ b/python/mujoco/specs.cc
@@ -518,6 +518,12 @@ PYBIND11_MODULE(_specs, m) {
     }
     return std::string(buf.get());
   });
+  mjSpec.def("to_file", [](MjSpec& self, std::string& file) {
+    std::array<char, 1024> err;
+    if (mj_saveXML(self.ptr, file.c_str(), err.data(), err.size()) < 0) {
+      throw FatalError(std::string(err.data()));
+    }
+  });
   mjSpec.def(
       "add_default",
       [](MjSpec* spec, std::string& classname,
diff --git a/python/mujoco/specs_test.py b/python/mujoco/specs_test.py
index 2806d01a..d7eb9b0c 100644
--- a/python/mujoco/specs_test.py
+++ b/python/mujoco/specs_test.py
@@ -113,9 +113,7 @@ class SpecsTest(absltest.TestCase):
     self.assertEqual(model.nuser_site, 6)
     np.testing.assert_array_equal(model.site_user[0], [1, 2, 3, 4, 5, 6])
 
-    self.assertEqual(
-        spec.to_xml(),
-        textwrap.dedent("""\
+    xml = textwrap.dedent("""\
         <mujoco model="MuJoCo Model">
           <compiler angle="radian"/>
 
@@ -127,8 +125,8 @@ class SpecsTest(absltest.TestCase):
             </body>
           </worldbody>
         </mujoco>
-    """),
-    )
+    """)
+    self.assertEqual(spec.to_xml(), xml)
 
   def test_kwarg(self):
     # Create a spec.