Guard set_merge_fixed_ignore_inertia for Isaac Sim version compatibility

[Isk5434]

Description

UrdfConverter._convert_asset unconditionally calls
import_config.set_merge_fixed_ignore_inertia(...). This method is not present on
every Isaac Sim URDF importer version, so importing a URDF raises

AttributeError: 'ImportConfig' object has no attribute 'set_merge_fixed_ignore_inertia'

on those builds, breaking asset conversion.

Fix

Call the setter only when it exists:

import_config.set_merge_fixed_joints(self.cfg.merge_fixed_joints)
if hasattr(import_config, "set_merge_fixed_ignore_inertia"):
    import_config.set_merge_fixed_ignore_inertia(self.cfg.merge_fixed_joints)

When the setter is absent, the importer behavior matches its default, so guarding the
call is safe and restores URDF import across Isaac Sim versions.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have run the pre-commit checks with ./isaaclab.sh --format
• I have signed off all commits (DCO)
• I have updated the changelog / docs as needed

[greptile-apps]

Greptile Summary

This PR guards the set_merge_fixed_ignore_inertia call in UrdfConverter._create_urdf_import_config with a hasattr check, preventing an AttributeError on Isaac Sim URDF importer versions that do not expose this method.

• The fix is a single-line guard that leaves the existing code path entirely unchanged on builds where the method is present, and silently skips the call on older builds.
• The argument passed (self.cfg.merge_fixed_joints) is identical to the original unconditional call, so no behavioral regression is introduced on supported versions.

Confidence Score: 4/5

Safe to merge; the change only adds a compatibility guard around a single setter call and does not alter behavior on Isaac Sim builds where the method exists.

On older Isaac Sim builds that lack set_merge_fixed_ignore_inertia, the importer's compiled-in default will be used instead of the user's configured value. The PR assumes that default matches the intended behavior, but this is not verified — users with merge_fixed_joints=True on older builds may silently get different fixed-joint inertia handling than on newer builds, with no log message to indicate why.

source/isaaclab/isaaclab/sim/converters/urdf_converter.py — consider adding a log warning in the fallback branch so users on older Isaac Sim builds are informed that fixed-joint inertia behavior may differ.

Important Files Changed

Filename	Overview
source/isaaclab/isaaclab/sim/converters/urdf_converter.py	Adds a hasattr guard around set_merge_fixed_ignore_inertia to prevent AttributeError on Isaac Sim builds that lack the method; one-line, targeted fix with no logic changes to the existing path.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[_create_urdf_import_config] --> B[set_merge_fixed_joints]
    B --> C{hasattr set_merge_fixed_ignore_inertia?}
    C -- Yes: newer Isaac Sim --> D[set_merge_fixed_ignore_inertia]
    C -- No: older Isaac Sim --> E[Skip — use importer default]
    D --> F[set_fix_base / other physics settings]
    E --> F

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_{Reviews (1): Last reviewed commit: "Guard set_merge_fixed_ignore_inertia for..." | Re-trigger Greptile}

[greptile-apps]

Silent version fallback could mask behavior differences

When set_merge_fixed_ignore_inertia is absent, the importer silently uses whatever its compiled-in default is for that setting. If a user sets merge_fixed_joints=True and is on an older Isaac Sim build, the behavior will differ from a newer build without any indication — potentially causing hard-to-diagnose articulation issues. A one-line carb.log_warn (or omni.log.warn) here would make the version difference visible in the output log, helping users on older builds understand why their fixed-joint inertia handling differs.

Note: If this suggestion doesn't match your team's coding style, reply to this and let me know. I'll remember it for next time!

[isaaclab-review-bot]

Review Summary

This PR adds a hasattr guard around import_config.set_merge_fixed_ignore_inertia(...) to prevent AttributeError on Isaac Sim versions where this method doesn't exist. The fix is minimal, targeted, and the fallback behavior (method absent → importer uses its default) is safe.

Overall: Looks good with minor suggestions.

---

Findings

1. 🟡 Consider logging when the method is unavailable

When set_merge_fixed_ignore_inertia is absent, the code silently skips it. This is fine for correctness, but a debug/warning log would help developers understand why behavior differs across Isaac Sim versions.

Suggestion:

if hasattr(import_config, "set_merge_fixed_ignore_inertia"):
    import_config.set_merge_fixed_ignore_inertia(self.cfg.merge_fixed_joints)
else:
    import omni.log
    omni.log.warn(
        "ImportConfig does not have 'set_merge_fixed_ignore_inertia'. "
        "Skipping — the importer will use its default behavior."
    )

2. 🟡 The guarded value reuses merge_fixed_joints — intentional but worth a comment

The call passes self.cfg.merge_fixed_joints (not a dedicated merge_fixed_ignore_inertia config field). Based on the class docstring explaining Isaac Sim 5.1's changed merge behavior, this coupling appears intentional — when merging fixed joints, you also want to ignore inertia to preserve legacy behavior.

However, a brief inline comment explaining why the same config flag drives both settings would help future maintainers:

# When merge_fixed_joints is enabled, also ignore inertia during merge
# to preserve pre-5.1 behavior (links with inertia still get merged).
if hasattr(import_config, "set_merge_fixed_ignore_inertia"):
    import_config.set_merge_fixed_ignore_inertia(self.cfg.merge_fixed_joints)

3. 🟢 Version-pinning context validates the approach

The converter already pins to isaacsim.asset.importer.urdf-2.4.31 on Isaac Sim ≥5.1 (lines 67–70). The hasattr guard adds defense-in-depth for environments where the pinned version still lacks the method (e.g., older Isaac Sim builds < 5.1 that don't have this API at all). This is sound.

4. 🔵 Minor: no test coverage for the guarded path

There's no unit test verifying that _get_urdf_import_config() succeeds when the ImportConfig object lacks set_merge_fixed_ignore_inertia. A simple mock-based test could guard against regressions:

def test_urdf_import_config_without_merge_fixed_ignore_inertia(monkeypatch):
    """Verify graceful handling when set_merge_fixed_ignore_inertia is unavailable."""
    # mock ImportConfig without the method
    ...

This is a nice-to-have given the simplicity of the fix, not a blocker.

---

Verdict

The fix is correct, minimal, and safe. The hasattr pattern is idiomatic Python for optional API support. The two minor suggestions (logging + comment) would improve observability and maintainability but are not blockers for merge.