[Bug Report] isaaclab.sh does not properly configure python version in install

[inkyant]

Describe the bug

I have set up an isaacsim/isaaclab install, and had it working for training a robot.
Now, I added a user and realized there is an issue with the install script.
In the IsaacLab install requirements, it clearly states that required python version is 3.11 for isaaclab 5.0 and 3.10 for isaaclab 4.5, yet when running the script ./isaaclab.sh --uv it creates a UV env with the python version of whatever the variable python_path is in the script. However I'm not completely clear on what the purpose of this variable is, since it often is unset, and thus UV just installs the same default version of python, which is for me 3.12. So what happens is that the default install script installs the wrong version of python!
Here is the line of code in ./isaaclab.sh
uv venv --clear --python "${python_path}" "${env_path}"
While env_path is set, python_path is not set.
To fix this, we can change:
uv venv --clear --python "${python_path}" "${env_path}"
to:
uv venv --clear --python "3.11" "${env_path}"
I would be willing to make a PR, if I can confirm that the python_path variable is unnecessary.

Steps to reproduce

I've created a new user.
Installing UV, and restarting the terminal

Go to my install folder for Isaaclab, which the user has permissions over:

Current version (no env) is python 3.12:

Creating the env:

Notice, it creates python version 3.12

This comes from the line of code in isaaclab.sh in the function setup_uv_env() which is uv venv --clear --python "${python_path}" "${env_path}"
We can see that the env variable python_path is unset by adding an echo statement:

We can see that when creating an env now, it echos a blank line (compare to before last screenshot of uv env setup):

So as python_path is unset, it installs simply 3.12, the existing python version.
To fix this, we can change:
uv venv --clear --python "${python_path}" "${env_path}"
to:
uv venv --clear --python "3.11" "${env_path}"
I would be willing to make a PR, if I can confirm that the python_path variable is unnecessary.

System Info

Describe the characteristic of your environment:

• Commit: fc2ff6c
• Isaac Sim Version: 5.0.0-rc.45+release.23960.184afb15.gl
• OS: Ubuntu 24.04
• GPU: RTX 5090
• CUDA: 12.8
• GPU Driver: 580.95.05

Checklist

• I have checked that there is no similar issue in the repo (required)
• I have checked that the issue is not in running Isaac Sim itself and is related to the repo

Acceptance Criteria

Add the criteria for which this task is considered done. If not known at issue creation time, you can add this once the issue is assigned.

• running ./isaaclab.sh --uv for any current python version installs the correct python version, 3.11. Or docs are updated to say that 3.12 is supported

[LemonFoxmere]

I was able to replicate this issue on the same environment under a new user. Adding on, after running ./isaaclab.sh -i after sourcing the 3.12 venv, I noticed that IsaacSim seemed to not be installed properly, and Pytorch completely failed to install. Check out the full installation log on this gist.

(env_test) stacy@gpu-rtx5090:/home/shared/IsaacLab$ python3
Python 3.12.3 (main, Aug 14 2025, 17:47:21) [GCC 13.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>
>>>
>>> import isaaclab
>>> import isaacsim
Unable to expose 'isaacsim.simulation_app' API: No module named 'carb._carb'
>>> import omni
>>> import torch
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/shared/IsaacLab/_isaac_sim/exts/omni.isaac.ml_archive/pip_prebundle/torch/__init__.py", line 994, in <module>
    raise ImportError(
ImportError: Failed to load PyTorch C extensions:
    It appears that PyTorch has loaded the `torch/_C` folder
    of the PyTorch repository rather than the C extensions which
    are expected in the `torch._C` namespace. This can occur when
    using the `install` workflow. e.g.
        $ python setup.py install && python -c "import torch"

    This error can generally be solved using the `develop` workflow
        $ python setup.py develop && python -c "import torch"  # This should succeed
    or by running Python from a different directory.
>>>

I was able to fix this by changing line 432. Instead of using the python_path variable, I changed it to use a static "3.11". E.g.,

Here is a gist for the full installation log now (note: Stacy was deleted and recreated as a new user)

(env_test_2) stacy@gpu-rtx5090:/home/shared/IsaacLab$ python3
Python 3.11.14 (main, Oct 14 2025, 21:26:53) [Clang 20.1.4 ] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import isaaclab
>>> import isaacsim
>>> import omni
>>> import torch
>>>

[inkyant]

I see there is actually PR #3560 addresses this, but the documentation should be updated as well. Not sure if I should code until the PR is accepted.