Support presets and arbitrary skipping keys in dump comparator (#19676)

test/registered/debug_utils/comparator/test_preset.py

@@ -0,0 +1,44 @@
+import pytest
+
+from sglang.srt.debug_utils.comparator.preset import PRESETS, expand_preset
+from sglang.test.ci.ci_register import register_cpu_ci
+
+register_cpu_ci(est_time=5, suite="default", nightly=True)
+
+
+class TestExpandPreset:
+    """Test preset expansion logic."""
+
+    def test_explicit_preset(self):
+        """--preset sglang_megatron expands into its argv."""
+        argv = [
+            "--baseline-path",
+            "/a",
+            "--preset",
+            "sglang_megatron",
+            "--diff-threshold",
+            "0.01",
+        ]
+        result = expand_preset(argv, presets=PRESETS)
+        assert "--preset" not in result
+        assert "--grouping-skip-keys" in result
+        assert "concat_steps" in result
+        assert "--baseline-path" in result
+        assert "--diff-threshold" in result
+
+    def test_default_preset_applied(self):
+        """No --preset and no --grouping-skip-keys triggers default preset."""
+        argv = ["--baseline-path", "/a"]
+        result = expand_preset(argv, presets=PRESETS)
+        assert "--grouping-skip-keys" in result
+
+    def test_explicit_skip_keys_prevents_default(self):
+        """Explicit --grouping-skip-keys prevents default preset injection."""
+        argv = ["--grouping-skip-keys", "rank", "--baseline-path", "/a"]
+        result = expand_preset(argv, presets=PRESETS)
+        assert result == argv
+
+    def test_unknown_preset_raises(self):
+        """Unknown preset name raises ValueError."""
+        with pytest.raises(ValueError, match="Unknown value for --preset"):
+            expand_preset(["--preset", "nonexistent"], presets=PRESETS)