Fix bug when an AttributeError is raised, and add a unit test for tha…

…t too

lib/cylc/config.py

@@ -2103,13 +2103,14 @@ def load_graph(self):
                 else:
                     try:
                         if not callable(get_func(xtrig.func_name, self.fdir)):
-                            raise SuiteConfigError("ERROR, xtrigger function "
-                                                   "not callable: "
-                                                   "%s" % xtrig.func_name)
-                    except ImportError:
-                        raise SuiteConfigError("ERROR, xtrigger function "
-                                               "not found: "
-                                               "%s" % xtrig.func_name)
+                            raise SuiteConfigError(
+                                f"ERROR, "
+                                f"xtrigger function not callable: "
+                                f"{xtrig.func_name}")
+                    except (ImportError, AttributeError):
+                        raise SuiteConfigError(
+                            f"ERROR, "
+                            f"xtrigger function not found: {xtrig.func_name}")
                     self.xtrigger_mgr.add_trig(label, xtrig)
                     self.taskdefs[task_name].xtrig_labels.add(label)
 

lib/cylc/tests/test_config.py

@@ -76,6 +76,32 @@ def test_xfunction_import_error(self):
                     SuiteConfig(suite="caiman_suite", fpath=f.name)
                 assert "not found" in str(excinfo.value)
 
+    def test_xfunction_attribute_error(self):
+        """Test for error when a xtrigger function cannot be imported."""
+        with TemporaryDirectory() as temp_dir:
+            python_dir = Path(os.path.join(temp_dir, "lib", "python"))
+            python_dir.mkdir(parents=True)
+            capybara_file = python_dir / "capybara.py"
+            with capybara_file.open(mode="w") as f:
+                # NB: we are not returning a lambda, instead we have a scalar
+                f.write("""toucan = lambda: True""")
+                f.flush()
+            suite_rc = Path(temp_dir, "suite.rc")
+            with suite_rc.open(mode="w") as f:
+                f.write("""
+    [scheduling]
+        initial cycle point = 2018-01-01
+        [[xtriggers]]
+            oopsie = capybara()
+        [[dependencies]]
+            [[[R1]]]
+                graph = '@oopsie => qux'
+                """)
+                f.flush()
+                with pytest.raises(SuiteConfigError) as excinfo:
+                    SuiteConfig(suite="capybara_suite", fpath=f.name)
+                assert "not found" in str(excinfo.value)
+
     def test_xfunction_not_callable(self):
         """Test for error when a xtrigger function is not callable."""
         with TemporaryDirectory() as temp_dir: