MjSpec python bindings

[milutter]

My setup

MuJoCo==3.2.6, Python, Ubuntu 22.04

What's happening? What did you expect?

I started playing around with MjSpec and noticed some problems in the API. I commonly use the manual asset dict functionality to create a MjSpec or MjModel. For example mj_spec = MjSpec.from_file(filename, assets=assets) and mj_spec.compile(assets).

There seems to be some issue when doing mj_spec.compile(assets=assets), even though it looks like assets is a valid keyword. While doing mj_spec.compile(assets) seems to work just fine, providing the assets keyword the program dies and a lot of binary code is printed to the console.

When the assets are passed as positional argument, passing None via mj_spec.compile(None) does not work. Historically the assets argument has been dict[str, byte] | None but this does not seem to be true for MjSpec.

When previously working with mjcf, I could get the assets mjcf_root.get_assets(). MjSpec does not seem to provide this option. Would it be possible to add this feature?

Lastly, I am having problems with mujoco.MjSpec in a docker container. For example

import mujoco

xml = """
 <mujoco>
   <worldbody>
     <geom name="red_box" type="box" size=".2 .2 .2" rgba="1 0 0 1"/>
     <geom name="green_sphere" pos=".2 .2 .2" size=".1" rgba="0 1 0 1"/>
   </worldbody>
</mujoco>
"""

spec = mujoco.MjSpec.from_string(xml)
print(spec.geoms[0].name)

yields the error UnicodeDecodeError: 'utf-8' codec can't decode byte 0xc2 in position 1: invalid continuation byte. This error seems to be specific to my Docker setup as I do not observe the issues when running outside docker. Do you have any idea what could be causing this difference?

CC: @yuvaltassa @quagla

Confirmations

• I searched the latest documentation thoroughly before posting.
• I searched previous Issues and Discussions, I am certain this has not been raised before.

[quagla]

I have just submitted ba183ad which adds the attribute assets so you can do

specs.assets = asset_dict
model = spec.compile()

I have removed any keywords from compile so you also don't have the other issues and there is no ambiguity concerning the assets used for compilation vs those that belong to the spec. If you want to retrieve the assets associated to the spec, you can do spec.assets, so for example after attaching a spec you can do

parent.assets.update(child.assets)

In order to add the child assets to the parent assets. I am not sure about the Docker issue. Maybe you can check if you still have this issue now?

[yuvaltassa]

@quagla should we add a line about this here?
https://mujoco.readthedocs.io/en/latest/python.html#construction

[milutter]

Thanks for fixing the interface so fast. I can confirm this issue is resolved in 3.2.7.

+1 to @yuvaltassa point of adding it to the documentation. I think it is quite undocumented that one can pass the meshes explicitly which I prefer as this enables storing the meshes and models at different locations.

Don't worry about the encoding issue. I narrowed it down to how we build the mujoco python package in the docker.

[yuvaltassa]

@milutter do you want to update here and send a PR?

mujoco/doc/python.rst

Line 496 in a5ab7a9

Construction

[milutter]

Sure I can do that later this week