Make the graph '_unsatisfied_requires' be a staticmethod

To make it possible for users to subclass this flow and
replace this method (if they so choose to) make it a static
method that is directed to the current module level function
and call into the staticmethod when used (users can override
staticmethods and change them in subclasses).

Change-Id: I747c45636596ca5a8ad81dabcbba12ab55ce77d7

taskflow/patterns/graph_flow.py

@@ -22,7 +22,6 @@ from taskflow.types import graph as gr
 
 
 def _unsatisfied_requires(node, graph, *additional_provided):
-    """Extracts the unsatisified symbol requirements of a single node."""
     requires = set(node.requires)
     if not requires:
         return requires
@@ -64,6 +63,9 @@ class Flow(flow.Flow):
         self._graph = gr.DiGraph()
         self._graph.freeze()
 
+    #: Extracts the unsatisified symbol requirements of a single node.
+    _unsatisfied_requires = staticmethod(_unsatisfied_requires)
+
     def link(self, u, v):
         """Link existing node u as a runtime dependency of existing node v."""
         if not self._graph.has_node(u):
@@ -153,7 +155,7 @@ class Flow(flow.Flow):
                 provided[value].append(self._retry)
 
         for item in self._graph.nodes_iter():
-            for value in _unsatisfied_requires(item, self._graph,
+            for value in self._unsatisfied_requires(item, self._graph,
                                                     retry_provides):
                 required[value].append(item)
             for value in item.provides:
@@ -168,7 +170,7 @@ class Flow(flow.Flow):
 
             # Try to find a valid provider.
             if resolve_requires:
-                for value in _unsatisfied_requires(item, tmp_graph,
+                for value in self._unsatisfied_requires(item, tmp_graph,
                                                         retry_provides):
                     if value in provided:
                         providers = provided[value]
@@ -232,7 +234,8 @@ class Flow(flow.Flow):
             retry_provides.update(self._retry.provides)
         g = self._get_subgraph()
         for item in g.nodes_iter():
-            requires.update(_unsatisfied_requires(item, g, retry_provides))
+            requires.update(self._unsatisfied_requires(item, g,
+                                                       retry_provides))
         return frozenset(requires)