Only do differences on set objects.

taskflow/patterns/linear_flow.py

@@ -20,6 +20,16 @@ from taskflow import exceptions as exc
 from taskflow.patterns import ordered_flow
 
 
+def _convert_to_set(items):
+    if not items:
+        return set()
+    if isinstance(items, set):
+        return items
+    if isinstance(items, dict):
+        return items.keys()
+    return set(iter(items))
+
+
 class Flow(ordered_flow.Flow):
     """A linear chain of tasks that can be applied as one unit or
        rolled back as one unit. Each task in the chain may have requirements
@@ -39,14 +49,15 @@ class Flow(ordered_flow.Flow):
         return inputs
 
     def _validate_provides(self, task):
+        requires = _convert_to_set(task.requires())
         last_provides = set()
         last_provider = None
         if self._tasks:
             last_provider = self._tasks[-1]
-            last_provides = last_provider.provides()
+            last_provides = _convert_to_set(last_provider.provides())
         # Ensure that the last task provides all the needed input for this
         # task to run correctly.
-        req_diff = task.requires().difference(last_provides)
+        req_diff = requires.difference(last_provides)
         if req_diff:
             if last_provider is None:
                 msg = ("There is no previous task providing the outputs %s"