Merge pull request numba#6165 from stuartarchibald/fix/pr5425

numba#5425 continued

numba/core/withcontexts.py

@@ -225,8 +225,13 @@ def strip_var_ver(x):
         # Verify that all outputs are annotated
         not_annotated = set(stripped_outs) - set(typeanns)
         if not_annotated:
-            msg = 'missing type annotation on outgoing variables: {}'
-            raise errors.TypingError(msg.format(not_annotated))
+            msg = (
+                'Missing type annotation on outgoing variable(s): {0}\n\n'
+                'Example code: with objmode({1}=\'<'
+                'add_type_as_string_here>\')\n'
+            )
+            stable_ann = sorted(not_annotated)
+            raise errors.TypingError(msg.format(stable_ann, stable_ann[0]))
 
         # Get output types
         outtup = types.Tuple([typeanns[v] for v in stripped_outs])

numba/tests/test_withlifting.py

@@ -522,15 +522,21 @@ def test_case07_mystery_key_error(self):
         def foo(x):
             with objmode_context():
                 t = {'a': x}
-            return x, t
+                u = 3
+            return x, t, u
         x = np.array([1, 2, 3])
         cfoo = njit(foo)
+
         with self.assertRaises(errors.TypingError) as raises:
             cfoo(x)
-        self.assertIn(
-            "missing type annotation on outgoing variables",
-            str(raises.exception),
-            )
+
+        exstr = str(raises.exception)
+        self.assertIn("Missing type annotation on outgoing variable(s): "
+                      "['t', 'u']",
+                      exstr)
+        self.assertIn("Example code: with objmode"
+                      "(t='<add_type_as_string_here>')",
+                      exstr)
 
     def test_case08_raise_from_external(self):
         # this segfaults, expect its because the dict needs to raise as '2' is