fix: report the real cause when cel-python or tree-sitter fails to import

CHANGELOG.md

@@ -568,6 +568,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- When `cel-python` or `tree-sitter-language-pack` fails to import, the
+  error now gives the real cause. Both guards said "not installed" for any
+  import error, which was wrong for an installed package with a missing
+  dependency or a broken release (#573).
 - Installs without the development dependencies work again (`uv tool install`
   from a checkout, and any install from a built wheel). Two things broke
   them: `tree-sitter-language-pack` 1.6.3, the newest release the old

packages/darnit-baseline/src/darnit_baseline/threat_model/parsing.py

@@ -29,15 +29,18 @@
     from tree_sitter_language_pack import get_language, get_parser
 
     TREE_SITTER_AVAILABLE = True
-except ImportError:  # pragma: no cover — dependency is required in pyproject.toml
+except ImportError as _import_error:  # pragma: no cover — dependency is required in pyproject.toml
     TREE_SITTER_AVAILABLE = False
     ts = None  # type: ignore[assignment]
+    # Keep the real cause. A release that installs without an importable module
+    # (tree-sitter-language-pack 1.6.3, #573) is not "not installed".
+    _IMPORT_FAILURE = f"tree-sitter-language-pack could not be imported: {_import_error}"
 
     def get_language(name: str) -> Any:  # type: ignore[misc]
-        raise ImportError("tree-sitter-language-pack is not installed")
+        raise ImportError(_IMPORT_FAILURE)
 
     def get_parser(name: str) -> Any:  # type: ignore[misc]
-        raise ImportError("tree-sitter-language-pack is not installed")
+        raise ImportError(_IMPORT_FAILURE)
 
 
 logger = logging.getLogger("darnit_baseline.threat_model.parsing")

packages/darnit/src/darnit/sieve/cel_evaluator.py

@@ -179,7 +179,7 @@ def _get_environment(self) -> Any:
             from celpy.c7nlib import C7N_Interpreted_Runner
         except ImportError as e:
             raise CELCompilationError(
-                "cel-python not installed. Install with: pip install cel-python"
+                f"cel-python could not be imported: {e}"
             ) from e
 
         # Create environment with C7N runner that supports custom functions
@@ -275,7 +275,7 @@ def compile(self, expression: str) -> CELProgram:
             import celpy
         except ImportError as e:
             raise CELCompilationError(
-                "cel-python not installed. Install with: pip install cel-python"
+                f"cel-python could not be imported: {e}"
             ) from e
 
         env = self._get_environment()
@@ -344,7 +344,7 @@ def _evaluate_with_timeout(
             import celpy
         except ImportError as e:
             raise CELEvaluationError(
-                "cel-python not installed. Install with: pip install cel-python"
+                f"cel-python could not be imported: {e}"
             ) from e
 
         result_container: list[Any] = []

tests/darnit/sieve/test_cel_import_failure.py

@@ -0,0 +1,27 @@
+"""An import failure inside cel-python reports its real cause, not "not installed"."""
+
+from __future__ import annotations
+
+import builtins
+
+import pytest
+
+from darnit.sieve.cel_evaluator import CELCompilationError, evaluate_cel
+
+
+def test_cel_import_failure_names_the_real_cause(monkeypatch) -> None:
+    real_import = builtins.__import__
+
+    def failing_import(name, *args, **kwargs):
+        if name == "celpy" or name.startswith("celpy."):
+            raise ImportError("No module named 'packaging.version'")
+        return real_import(name, *args, **kwargs)
+
+    monkeypatch.setattr(builtins, "__import__", failing_import)
+
+    with pytest.raises(CELCompilationError) as excinfo:
+        evaluate_cel("1 == 1", {})
+
+    message = str(excinfo.value)
+    assert "packaging.version" in message
+    assert "not installed" not in message