fix: replace bare assert with runtime checks in engine, index, and export

[tombudd]

Summary

Replace 6 bare assert statements with proper raise ValueError/RuntimeError that persist in optimized Python builds. Found by UNA (autonomous security auditor, designed and built by Tom Budd — tom@tombudd.com).

1. Bounds validation in Camera.select() — engine.py

File: dm_control/mujoco/engine.py:975-987

Four assert statements validate that body_id, geom_id, flex_id, and skin_id are within the physics model's bounds. When Python runs with -O, these checks are stripped — allowing invalid IDs to silently pass through to MuJoCo, potentially causing out-of-bounds memory access in the underlying C library.

Fix: Replace with raise ValueError including the invalid ID and valid range in the error message.

2. Asset filename collision check — export_with_assets.py

File: dm_control/mjcf/export_with_assets.py:54

assert out_file_name not in assets guards against the output XML filename colliding with an existing asset. With -O, a collision would silently overwrite model data during MJCF export.

Fix: Replace with raise RuntimeError including the conflicting filename.

3. Mocap body name resolution — index.py

File: dm_control/mujoco/index.py:233

assert None not in mocap_body_names verifies all mocap body names were resolved from body_mocapid mappings. With -O, unresolved None values propagate into the name mapping, causing hard-to-diagnose errors downstream.

Fix: Replace with raise RuntimeError with a descriptive message.

Changes

File	Change	Lines
engine.py	4× assert → raise ValueError	+8 / -4
export_with_assets.py	1× assert → raise RuntimeError	+3 / -1
index.py	1× assert → raise RuntimeError	+4 / -1

Total: 3 files, +15 / -6 lines

About This Review

This security audit was performed by UNA (Unified Nexus Agent), an autonomous AI security auditor — a Governed Digital Organism (GDO) designed and built by Tom Budd (tom@tombudd.com | tombudd.com).

UNA audits open-source codebases for security vulnerabilities, code quality issues, and reliability concerns. All findings are human-verified before submission.

Note: Happy to sign the Google CLA at cla.developers.google.com if required.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.