Add to_file function to save the Spec to a file.

Fixes #2415.

PiperOrigin-RevId: 725161617
Change-Id: I6c374cc7f7cb3f1da1cc0da531dd7e5be5d62b6a

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,

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(
+    xml = textwrap.dedent("""\
-        spec.to_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.