Python#

MuJoCo comes with native Python bindings that are developed in C++ using pybind11. The Python API is consistent with the underlying C API. This leads to some non-Pythonic code structure (e.g. order of function arguments), but it has the benefit that the API documentation is applicable to both languages.

The Python bindings are distributed as the mujoco package on PyPI. These are low-level bindings that are meant to give as close to a direct access to the MuJoCo library as possible. However, in order to provide an API and semantics that developers would expect in a typical Python library, the bindings deliberately diverge from the raw MuJoCo API in a number of places, which are documented throughout this page.

Google DeepMind’s dm_control reinforcement learning library depends on the mujoco package and continues to be supported by Google DeepMind. For code that depends on dm_control versions prior to 1.0.0, consult the migration guide.

For mujoco-py users, we include migration notes below.

Tutorial notebook#

A MuJoCo tutorial using the Python bindings is available here: mjcolab

Installation#

The recommended way to install this package is via PyPI:

pip install mujoco

A copy of the MuJoCo library is provided as part of the package and does not need to be downloaded or installed separately.

Interactive viewer#

An interactive GUI viewer is provided as part of the Python package in the mujoco.viewer module. It is based on the same codebase as the simulate application that ships with the MuJoCo binary releases. Three distinct use cases are supported:

Standalone app#

  • python -m mujoco.viewer launches an empty visualization session, where a model can be loaded by drag-and-drop.

  • python -m mujoco.viewer --mjcf=/path/to/some/mjcf.xml launches a visualization session for the specified model file.

Managed viewer#

Called from a Python program/script, through the function viewer.launch. This function blocks user code to support precise timing of the physics loop. This mode should be used if user code is implemented as engine plugins or physics callbacks, and is called by MuJoCo during mj_step.

  • viewer.launch() launches an empty visualization session, where a model can be loaded by drag-and-drop.

  • viewer.launch(model) launches a visualization session for the given mjModel where the visualizer internally creates its own instance of mjData

  • viewer.launch(model, data) is the same as above, except that the visualizer operates directly on the given mjData instance – upon exit the data object will have been modified.

Passive viewer#

By calling viewer.launch_passive(model, data). This function does not block, allowing user code to continue execution. In this mode, the user’s script is responsible for timing and advancing the physics state, and mouse-drag perturbations will not work unless the user explicitly synchronizes incoming events.

Warning

On MacOS, launch_passive requires that the user script is executed via a special mjpython launcher. The mjpython command is installed as part of the mujoco package, and can be used as a drop-in replacement for the usual python command and supports an identical set of command line flags and arguments. For example, a script can be executed via mjpython my_script.py, and an IPython shell can be launched via mjpython -m IPython.

The launch_passive function returns a handle which can be used to interact with the viewer. It has the following attributes:

  • cam, opt, and pert properties: correspond to mjvCamera, mjvOption, and mjvPerturb structs, respectively.

  • lock(): provides a mutex lock for the viewer as a context manager. Since the viewer operates its own thread, user code must ensure that it is holding the viewer lock before modifying any physics or visualization state. These include the mjModel and mjData instance passed to launch_passive, and also the cam, opt, and pert properties of the viewer handle.

  • sync(): synchronizes state between mjModel, mjData, and GUI user inputs since the previous call to sync. In order to allow user scripts to make arbitrary modifications to mjModel and mjData without needing to hold the viewer lock, the passive viewer does not access or modify these structs outside of sync calls.

    User scripts must call sync in order for the viewer to reflect physics state changes. The sync function also transfers user inputs from the GUI back into mjOption (inside mjModel) and mjData, including enable/disable flags, control inputs, and mouse perturbations.

  • update_hfield(hfieldid): updates the height field data at the specified hfieldid for subsequent renderings.

  • update_mesh(meshid): updates the mesh data at the specified meshid for subsequent renderings.

  • update_texture(texid): updates the texture data at the specified texid for subsequent renderings.

  • close(): programmatically closes the viewer window. This method can be safely called without locking.

  • is_running(): returns True if the viewer window is running and False if it is closed. This method can be safely called without locking.

  • user_scn: an mjvScene object that allows users to add change rendering flags and add custom visualization geoms to the rendered scene. This is separate from the mjvScene that the viewer uses internally to render the final scene, and is entirely under the user’s control. User scripts can call e.g. mjv_initGeom or mjv_makeConnector to add visualization geoms to user_scn, and upon the next call to sync(), the viewer will incorporate these geoms to future rendered images. Similarly, user scripts can make changes to user_scn.flags which would be picked up at the next call to sync(). The sync() call also copies changes to rendering flags made via the GUI back into user_scn to preserve consistency. For example:

    with mujoco.viewer.launch_passive(m, d, key_callback=key_callback) as viewer:

  # Enable wireframe rendering of the entire scene.
  viewer.user_scn.flags[mujoco.mjtRndFlag.mjRND_WIREFRAME] = 1
  viewer.sync()

  while viewer.is_running():
    ...
    # Step the physics.
    mujoco.mj_step(m, d)

    # Add a 3x3x3 grid of variously colored spheres to the middle of the scene.
    viewer.user_scn.ngeom = 0
    i = 0
    for x, y, z in itertools.product(*((range(-1, 2),) * 3)):
      mujoco.mjv_initGeom(
          viewer.user_scn.geoms[i],
          type=mujoco.mjtGeom.mjGEOM_SPHERE,
          size=[0.02, 0, 0],
          pos=0.1*np.array([x, y, z]),
          mat=np.eye(3).flatten(),
          rgba=0.5*np.array([x + 1, y + 1, z + 1, 2])
      )
      i += 1
    viewer.user_scn.ngeom = i
    viewer.sync()
    ...

The viewer handle can also be used as a context manager which calls close() automatically upon exit. A minimal example of a user script that uses launch_passive might look like the following. (Note that example is a simple illustrative example that does not necessarily keep the physics ticking at the correct wallclock rate.)

import time

import mujoco
import mujoco.viewer

m = mujoco.MjModel.from_xml_path('/path/to/mjcf.xml')
d = mujoco.MjData(m)

with mujoco.viewer.launch_passive(m, d) as viewer:
  # Close the viewer automatically after 30 wall-seconds.
  start = time.time()
  while viewer.is_running() and time.time() - start < 30:
    step_start = time.time()

    # mj_step can be replaced with code that also evaluates
    # a policy and applies a control signal before stepping the physics.
    mujoco.mj_step(m, d)

    # Example modification of a viewer option: toggle contact points every two seconds.
    with viewer.lock():
      viewer.opt.flags[mujoco.mjtVisFlag.mjVIS_CONTACTPOINT] = int(d.time % 2)

    # Pick up changes to the physics state, apply perturbations, update options from GUI.
    viewer.sync()

    # Rudimentary time keeping, will drift relative to wall clock.
    time_until_next_step = m.opt.timestep - (time.time() - step_start)
    if time_until_next_step > 0:
      time.sleep(time_until_next_step)

Optionally, viewer.launch_passive accepts the following keyword arguments.

  • key_callback: A callable which gets called each time a keyboard event occurs in the viewer window. This allows user scripts to react to various key presses, e.g., pause or resume the run loop when the spacebar is pressed.

    paused = False

def key_callback(keycode):
  if chr(keycode) == ' ':
    nonlocal paused
    paused = not paused

...

with mujoco.viewer.launch_passive(m, d, key_callback=key_callback) as viewer:
  while viewer.is_running():
    ...
    if not paused:
      mujoco.mj_step(m, d)
      viewer.sync()
    ...

  • show_left_ui and show_right_ui: Boolean arguments indicating whether UI panels should be visible or hidden when the viewer is launched. Note that regardless of the values specified, the user can still toggle the visibility of these panels after launch by pressing Tab or Shift+Tab.

Basic usage#

Once installed, the package can be imported via import mujoco. Structs, functions, constants, and enums are available directly from the top-level mujoco module.

Structs#

The bindings include Python classes that expose MuJoCo data structures. For maximum performance, these classes provide access to the raw memory used by MuJoCo without copying or buffering. This means that some MuJoCo functions (e.g., mj_step) change the content of fields in place. The user is therefore advised to create copies where required. For example, when logging the position of a body, one could write positions.append(data.body('my_body').xpos.copy()). Without the .copy(), the list would contain identical elements, all pointing to the most recent value.

In order to conform to PEP 8 naming guidelines, struct names begin with a capital letter, for example mjData becomes mujoco.MjData in Python.

All structs other than mjModel have constructors in Python. For structs that have an mj_defaultFoo-style initialization function, the Python constructor calls the default initializer automatically, so for example mujoco.MjOption() creates a new mjOption instance that is pre-initialized with mj_defaultOption. Otherwise, the Python constructor zero-initializes the underlying C struct.

Structs with a mj_makeFoo-style initialization function have corresponding constructor overloads in Python, for example mujoco.MjvScene(model, maxgeom=10) in Python creates a new mjvScene instance that is initialized with mjv_makeScene(model, [the new mjvScene instance], 10) in C. When this form of initialization is used, the corresponding deallocation function mj_freeFoo/mj_deleteFoo is automatically called when the Python object is deleted. The user does not need to manually free resources.

The mujoco.MjModel class does not a have Python constructor. Instead, we provide three static factory functions that create a new mjModel instance: mujoco.MjModel.from_xml_string, mujoco.MjModel.from_xml_path, and mujoco.MjModel.from_binary_path. The first function accepts a model XML as a string, while the latter two functions accept the path to either an XML or MJB model file. All three functions optionally accept a Python dictionary which is converted into a MuJoCo Virtual file system for use during model compilation.

Functions#

MuJoCo functions are exposed as Python functions of the same name. Unlike with structs, we do not attempt to make the function names PEP 8-compliant, as MuJoCo uses both underscores and CamelCases. In most cases, function arguments appear exactly as they do in C, and keyword arguments are supported with the same names as declared in mujoco.h. Python bindings to C functions that accept array input arguments expect NumPy arrays or iterable objects that are convertible to NumPy arrays (e.g. lists). Output arguments (i.e. array arguments that MuJoCo expect to write values back to the caller) must always be writeable NumPy arrays.

In the C API, functions that take dynamically-sized arrays as inputs expect a pointer argument to the array along with an integer argument that specifies the array’s size. In Python, the size arguments are omitted since we can automatically (and indeed, more safely) deduce it from the NumPy array. When calling these functions, pass all arguments other than array sizes in the same order as they appear in mujoco.h, or use keyword arguments. For example, mj_jac should be called as mujoco.mj_jac(m, d, jacp, jacr, point, body) in Python.

The bindings releases the Python Global Interpreter Lock (GIL) before calling the underlying MuJoCo function. This allows for some thread-based parallelism, however users should bear in mind that the GIL is only released for the duration of the MuJoCo C function itself, and not during the execution of any other Python code.

Note

One place where the bindings do offer added functionality is the top-level mj_step function. Since it is often called in a loop, we have added an additional nstep argument, indicating how many times the underlying mj_step should be called. If not specified, nstep takes the default value of 1. The following two code snippets perform the same computation, but the first one does so without acquiring the GIL in between subsequent physics steps:

mj_step(model, data, nstep=20)

for _ in range(20):
  mj_step(model, data)

Enums and constants#

MuJoCo enums are available as mujoco.mjtEnumType.ENUM_VALUE, for example mujoco.mjtObj.mjOBJ_SITE. MuJoCo constants are available with the same name directly under the mujoco module, for example mujoco.mjVISSTRING.

Minimal example#

import mujoco

XML=r"""
<mujoco>
  <asset>
    <mesh file="gizmo.stl"/>
  </asset>
  <worldbody>
    <body>
      <freejoint/>
      <geom type="mesh" name="gizmo" mesh="gizmo"/>
    </body>
  </worldbody>
</mujoco>
"""

ASSETS=dict()
with open('/path/to/gizmo.stl', 'rb') as f:
  ASSETS['gizmo.stl'] = f.read()

model = mujoco.MjModel.from_xml_string(XML, ASSETS)
data = mujoco.MjData(model)
while data.time < 1:
  mujoco.mj_step(model, data)
  print(data.geom_xpos)

Named access#

Most well-designed MuJoCo models assign names to objects (joints, geoms, bodies, etc.) of interest. When the model is compiled down to an mjModel instance, these names become associated with numeric IDs that are used to index into the various array members. For convenience and code readability, the Python bindings provide “named access” API on MjModel and MjData. Each name_fooadr field in the mjModel struct defines a name category foo.

For each name category foo, mujoco.MjModel and mujoco.MjData objects provide a method foo that takes a single string argument, and returns an accessor object for all arrays corresponding to the entity foo of the given name. The accessor object contains attributes whose names correspond to the fields of either mujoco.MjModel or mujoco.MjData but with the part before the underscore removed. In addition, accessor objects also provide id and name properties, which can be used as replacements for mj_name2id and mj_id2name respectively. For example:

  • m.geom('gizmo') returns an accessor for arrays in the MjModel object m associated with the geom named “gizmo”.

  • m.geom('gizmo').rgba is a NumPy array view of length 4 that specifies the RGBA color for the geom. Specifically, it corresponds to the portion of m.geom_rgba[4*i:4*i+4] where i = mujoco.mj_name2id(m, mujoco.mjtObj.mjOBJ_GEOM, 'gizmo').

  • m.geom('gizmo').id is the same number as returned by mujoco.mj_name2id(m, mujoco.mjtObj.mjOBJ_GEOM, 'gizmo').

  • m.geom(i).name is 'gizmo', where i = mujoco.mj_name2id(m, mujoco.mjtObj.mjOBJ_GEOM, 'gizmo').

Additionally, the Python API define a number of aliases for some name categories corresponding to the XML element name in the MJCF schema that defines an entity of that category. For example, m.joint('foo') is the same as m.jnt('foo'). A complete list of these aliases are provided below.

The accessor for joints is somewhat different that of the other categories. Some mjModel and mjData fields (those of size size nq or nv) are associated with degrees of freedom (DoFs) rather than joints. This is because different types of joints have different numbers of DoFs. We nevertheless associate these fields to their corresponding joints, for example through d.joint('foo').qpos and d.joint('foo').qvel, however the size of these arrays would differ between accessors depending on the joint’s type.

Named access is guaranteed to be O(1) in the number of entities in the model. In other words, the time it takes to access an entity by name does not grow with the number of names or entities in the model.

For completeness, we provide here a complete list of all name categories in MuJoCo, along with their corresponding aliases defined in the Python API.

  • body

  • jnt or joint

  • geom

  • site

  • cam or camera

  • light

  • mesh

  • skin

  • hfield

  • tex or texture

  • mat or material

  • pair

  • exclude

  • eq or equality

  • tendon or ten

  • actuator

  • sensor

  • numeric

  • text

  • tuple

  • key or keyframe

Rendering#

MuJoCo itself expects users to set up a working OpenGL context before calling any of its mjr_ rendering routine. The Python bindings provide a basic class mujoco.GLContext that helps users set up such a context for offscreen rendering. To create a context, call ctx = mujoco.GLContext(max_width, max_height). Once the context is created, it must be made current before MuJoCo rendering functions can be called, which you can do so via ctx.make_current(). Note that a context can only be made current on one thread at any given time, and all subsequent rendering calls must be made on the same thread.

The context is freed automatically when the ctx object is deleted, but in some multi-threaded scenario it may be necessary to explicitly free the underlying OpenGL context. To do so, call ctx.free(), after which point it is the user’s responsibility to ensure that no further rendering calls are made on the context.

Once the context is created, users can follow MuJoCo’s standard rendering, for example as documented in the Visualization section.

Error handling#

MuJoCo reports irrecoverable errors via the mju_error mechanism, which immediately terminates the entire process. Users are permitted to install a custom error handler via the mju_user_error callback, but it too is expected to terminate the process, otherwise the behavior of MuJoCo after the callback returns is undefined. In actuality, it is sufficient to ensure that error callbacks do not return to MuJoCo, but it is permitted to use longjmp to skip MuJoCo’s call stack back to the external callsite.

The Python bindings utilizes longjmp to allow it to convert irrecoverable MuJoCo errors into Python exceptions of type mujoco.FatalError that can be caught and processed in the usual Pythonic way. Furthermore, it installs its error callback in a thread-local manner using a currently private API, thus allowing for concurrent calls into MuJoCo from multiple threads.

Callbacks#

MuJoCo allows users to install custom callback functions to modify certain parts of its computation pipeline. For example, mjcb_sensor can be used to implement custom sensors, and mjcb_control can be used to implement custom actuators. Callbacks are exposed through the function pointers prefixed mjcb_ in mujoco.h.

For each callback mjcb_foo, users can set it to a Python callable via mujoco.set_mjcb_foo(some_callable). To reset it, call mujoco.set_mjcb_foo(None). To retrieve the currently installed callback, call mujoco.get_mjcb_foo(). (The getter should not be used if the callback is not installed via the Python bindings.) The bindings automatically acquire the GIL each time the callback is entered, and release it before reentering MuJoCo. This is likely to incur a severe performance impact as callbacks are triggered several times throughout MuJoCo’s computation pipeline and is unlikely to be suitable for “production” use case. However, it is expected that this feature will be useful for prototyping complex models.

Alternatively, if a callback is implemented in a native dynamic library, users can use ctypes to obtain a Python handle to the C function pointer and pass it to mujoco.set_mjcb_foo. The bindings will then retrieve the underlying function pointer and assign it directly to the raw callback pointer, and the GIL will not be acquired each time the callback is entered.

Model editing#

The Model Editing framework which allows for procedural model manipulation is exposed via Python. In many ways this API is conceptually similar to dm_control’s PyMJCF module, where MjSpec plays the role of mjcf_model. The largest difference between these two APIs is speed. Native model manipulation via MjSpec is around ~100x faster than PyMJCF.

Below is a simple example of how to use the model editing API. For more examples, please refer to specs_test.py.

import mujoco
spec = mujoco.MjSpec()
body = spec.worldbody.add_body()
body.pos = [1, 2, 3]
body.quat = [0, 1, 0, 0]
geom = body.add_geom()
geom.name = 'my_geom'
geom.type = mujoco.mjtGeom.mjGEOM_SPHERE
geom.size[0] = 1
geom.rgba = [1, 0, 0, 1]
...
model = spec.compile()

Missing features

We are aware of multiple missing features in the Python API, including:

  • Convenient constructors like:
    geom = body.add_geom(name='my_geom', size=[1, 1, 1], rgba=[1, 0, 0, 1])

  • Better tree traversal utilities like children = body.children() etc.

  • PyMJCF’s notion of “binding”, allowing access to mjModel and mjData values via the associated mjs elements.

There are certainly other missing features that we are not aware of. Please contact us on GitHub with feature requests or bug reports and we will prioritize accordingly.

Building from source#

Note

Building from source is only necessary if you are modifying the Python bindings (or are trying to run on exceptionally old Linux systems). If that’s not the case, then we recommend installing the prebuilt binaries from PyPI.

  1. Make sure you have CMake and a C++17 compiler installed.

  2. Download the latest binary release from GitHub. On macOS, the download corresponds to a DMG file which you can mount by double-clicking or running hdiutil attach <dmg_file>.

  3. Clone the entire mujoco repository from GitHub and cd into the python directory:

    git clone https://github.com/google-deepmind/mujoco.git
cd mujoco/python

  4. Create a virtual environment:

    python3 -m venv /tmp/mujoco
source /tmp/mujoco/bin/activate

  5. Generate a source distribution tarball with the make_sdist.sh script.

    bash make_sdist.sh

    The make_sdist.sh script generates additional C++ header files that are needed to build the bindings, and also pulls in required files from elsewhere in the repository outside the python directory into the sdist. Upon completion, the script will create a dist directory with a mujoco-x.y.z.tar.gz file (where x.y.z is the version number).

  6. Use the generated source distribution to build and install the bindings. You’ll need to specify the path to the MuJoCo library you downloaded earlier in the MUJOCO_PATH environment variable, and the path to the MuJoCo plugin directory in the MUJOCO_PLUGIN_PATH environment variable.

    Note

    For macOS, the files need to be extracted from the DMG. Once you mounted it as in step 2, the mujoco.framework directory can be found in /Volumes/MuJoCo, and the plugins directory can be found in /Volumes/MuJoCo/MuJoCo.app/Contents/MacOS/mujoco_plugin. Those two directories can be copied out somewhere convenient, or you can use MUJOCO_PATH=/Volumes/MuJoCo MUJOCO_PLUGIN_PATH=/Volumes/MuJoCo/MuJoCo.app/Contents/MacOS/mujoco_plugin.

    cd dist
MUJOCO_PATH=/PATH/TO/MUJOCO \
MUJOCO_PLUGIN_PATH=/PATH/TO/MUJOCO_PLUGIN \
pip install mujoco-x.y.z.tar.gz

The Python bindings should now be installed! To check that they’ve been successfully installed, cd outside of the mujoco directory and run python -c "import mujoco".

Tip

As a reference, a working build configuration can be found in MuJoCo’s continuous integration setup on GitHub.

Modules#

The mujoco package contains two sub-modules: mujoco.rollout and mujoco.minimize

rollout#

mujoco.rollout shows how to add additional C/C++ functionality, exposed as a Python module via pybind11. It is implemented in rollout.cc and wrapped in rollout.py. The module performs a common functionality where tight loops implemented outside of Python are beneficial: rolling out a trajectory (i.e., calling mj_step in a loop), given an intial state and sequence of controls, and returning subsequent states and sensor values. The basic usage form is

state, sensordata = rollout.rollout(model, data, initial_state, control)

initial_state is an nroll x nstate array, with nroll initial states of size nstate, where nstate = mj_stateSize(model, mjtState.mjSTATE_FULLPHYSICS) is the size of the full physics state. control is a nroll x nstep x ncontrol array of controls. Controls are by default the mjModel.nu standard actuators, but any combination of user input arrays can be specified by passing an optional control_spec bitflag.

If a rollout diverges, the current state and sensor values are used to fill the remainder of the trajectory. Therefore, non-increasing time values can be used to detect diverged rollouts.

The rollout function is designed to be completely stateless, so all inputs of the stepping pipeline are set and any values already present in the given MjData instance will have no effect on the output.

Since the Global Interpreter Lock can be released, this function can be efficiently threaded using Python threads. See the test_threading function in rollout_test.py for an example of threaded operation (and more generally for usage examples).

minimize#

This module contains optimization-related utilities.

The minimize.least_squares() function implements a nonlinear Least Squares optimizer solving sequential Quadratic Programs with mju_boxQP. It is documented in the associated notebook: lscolab

Utilities#

The python/mujoco directory also contains utility scripts.

msh2obj.py#

The msh2obj.py script converts the legacy .msh format for surface meshes (different from the possibly-volumetric gmsh format also using .msh), to OBJ files. The legacy format is deprecated and will be removed in a future release. Please convert all legacy files to OBJ.

mujoco-py migration#

In mujoco-py, the main entry point is the MjSim class. Users construct a stateful MjSim instance from an MJCF model (similar to dm_control.Physics), and this instance holds references to an mjModel instance and its associated mjData. In contrast, the MuJoCo Python bindings (mujoco) take a more low-level approach, as explained above: following the design principle of the C library, the mujoco module itself is stateless, and merely wraps the underlying native structs and functions.

While a complete survey of mujoco-py is beyond the scope of this document, we offer below implementation notes for a non-exhaustive list of specific mujoco-py features:

mujoco_py.load_model_from_xml(bstring)

This factory function constructs a stateful MjSim instance. When using mujoco, the user should call the factory function mujoco.MjModel.from_xml_* as described above. The user is then responsible for holding the resulting MjModel struct instance and explicitly generating the corresponding MjData by calling mujoco.MjData(model).

sim.reset(), sim.forward(), sim.step()

Here as above, mujoco users needs to call the underlying library functions, passing instances of MjModel and MjData: mujoco.mj_resetData(model, data), mujoco.mj_forward(model, data), and mujoco.mj_step(model, data).

sim.get_state(), sim.set_state(state), sim.get_flattened_state(), sim.set_state_from_flattened(state)

The MuJoCo library’s computation is deterministic given a specific input, as explained in the Programming section. mujoco-py implements methods for getting and setting some of the relevant fields (and similarly dm_control.Physics offers methods that correspond to the flattened case). mujoco do not offer such abstraction, and the user is expected to get/set the values of the relevant fields explicitly.

sim.model.get_joint_qvel_addr(joint_name)

This is a convenience method in mujoco-py that returns a list of contiguous indices corresponding to this joint. The list starts from model.jnt_qposadr[joint_index], and its length depends on the joint type. mujoco doesn’t offer this functionality, but this list can be easily constructed using model.jnt_qposadr[joint_index] and xrange.

sim.model.*_name2id(name)

mujoco-py creates dicts in MjSim that allow for efficient lookup of indices for objects of different types: site_name2id, body_name2id etc. These functions replace the function mujoco.mj_name2id(model, type_enum, name). mujoco offers a different approach for using entity names – named access, as well as access to the native mj_name2id.

sim.save(fstream, format_name)

This is the one context in which the MuJoCo library (and therefore also mujoco) is stateful: it holds a copy in memory of the last XML that was compiled, which is used in mujoco.mj_saveLastXML(fname). Note that mujoco-py’s implementation has a convenient extra feature, whereby the pose (as determined by sim.data’s state) is transformed to a keyframe that’s added to the model before saving. This extra feature is not currently available in mujoco.