Put provides and requires code to basic Flow

Code that calculates provides and requires for flow is almost
identical for all patterns, so this change makes it completely
identical and puts it to the base class. Other patterns are
still allowed to override these properties for sake of customization
or optimization.

Change-Id: I6e875e863047b5287ec727fc9a491f252f144ecf

taskflow/flow.py

@@ -43,16 +43,10 @@ class Flow(object):
     def __init__(self, name, retry=None):
         self._name = six.text_type(name)
         self._retry = retry
-        # If retry doesn't have a name,
+        # NOTE(akarpinska): if retry doesn't have a name,
         # the name of its owner will be assigned
-        if self._retry:
-            self._retry_provides = self.retry.provides
-            self._retry_requires = self.retry.requires
-            if not self._retry.name:
+        if self._retry and self._retry.name is None:
                 self._retry.set_name(self.name + "_retry")
-        else:
-            self._retry_provides = set()
-            self._retry_requires = set()
 
     @property
     def name(self):
@@ -66,6 +60,10 @@ class Flow(object):
         """
         return self._retry
 
+    @abc.abstractmethod
+    def add(self, *items):
+        """Adds a given item/items to this flow."""
+
     @abc.abstractmethod
     def __len__(self):
         """Returns how many items are in this flow."""
@@ -90,14 +88,29 @@ class Flow(object):
         lines.append("%s" % (len(self)))
         return "; ".join(lines)
 
-    @abc.abstractmethod
-    def add(self, *items):
-        """Adds a given item/items to this flow."""
-
-    @abc.abstractproperty
-    def requires(self):
-        """Browse argument requirement names this flow requires to run."""
-
-    @abc.abstractproperty
+    @property
     def provides(self):
-        """Browse argument names provided by the flow."""
+        """Set of result names provided by the flow.
+
+        Includes names of all the outputs provided by atoms of this flow.
+        """
+        provides = set()
+        if self._retry:
+            provides.update(self._retry.provides)
+        for subflow in self:
+            provides.update(subflow.provides)
+        return provides
+
+    @property
+    def requires(self):
+        """Set of argument names required by the flow.
+
+        Includes names of all the inputs required by atoms of this
+        flow, but not provided within the flow itself.
+        """
+        requires = set()
+        if self._retry:
+            requires.update(self._retry.requires)
+        for subflow in self:
+            requires.update(subflow.requires)
+        return requires - self.provides