Refactor: slide

Start by testing things that are forbidden, and move to successful stuff

tests/unit/workflow/test_io.py

@@ -40,6 +40,13 @@ def test_assignment(self):
         with self.assertRaises(TypeError):
             self.input.foo = "not an input channel"
 
+        with self.assertRaises(TypeError):
+            # Right label, and a channel, but wrong type of channel
+            self.input.b = self.post_facto_output
+
+        with self.subTest("Successful channel assignment"):
+            self.output.b = self.post_facto_output
+
         with self.subTest("Can assign to a key that is not the label"):
             label_before_assignment = self.post_facto_output.label
             self.output.not_this_channels_name = self.post_facto_output
@@ -55,13 +62,6 @@ def test_assignment(self):
                     "collections"
             )
 
-        with self.assertRaises(TypeError):
-            # Right label, and a channel, but wrong type of channel
-            self.input.b = self.post_facto_output
-
-        with self.subTest("Successful channel assignment"):
-            self.output.b = self.post_facto_output
-
     def test_connection(self):
         self.input.x = self.input.y
         self.assertEqual(