chore: Make Implementation.UNKNOWN._backend_version() safe

narwhals/_utils.py

@@ -632,9 +632,8 @@ def _import_native_namespace(module_name: str) -> ModuleType:
 def backend_version(implementation: Implementation, /) -> tuple[int, ...]:
     if not isinstance(implementation, Implementation):
         assert_never(implementation)
-    if implementation is Implementation.UNKNOWN:  # pragma: no cover
-        msg = "Cannot return backend version from UNKNOWN Implementation"
-        raise AssertionError(msg)
+    if implementation is Implementation.UNKNOWN:
+        return (0, 0, 0)
     into_version: ModuleType | str
     impl = implementation
     module_name = _IMPLEMENTATION_TO_MODULE_NAME.get(impl, impl.value)

tests/dependencies/imports_test.py

@@ -52,3 +52,20 @@ def test_to_native_namespace_unknown() -> None:
         AssertionError, match="Cannot return native namespace from UNKNOWN Implementation"
     ):
         impl.to_native_namespace()
+
+
+@pytest.mark.parametrize("impl", list(Implementation))
+def test_backend_version(impl: Implementation) -> None:
+    version: tuple[int, ...] | None
+    try:
+        version = impl._backend_version()
+    except (ModuleNotFoundError, ValueError) as err:
+        # NOTE: See https://github.com/narwhals-dev/narwhals/issues/2786
+        assert impl is not Implementation.UNKNOWN
+
+        with pytest.raises(type(err)):
+            impl.to_native_namespace()
+        version = None
+
+    if version is not None:
+        assert version >= (0, 0, 0)