Allow to specify default provides at task definition

It is now possible to specify what task provides when task class
(deriving from taskflow.task.Task) is defined, using 'default_provides'
class attribute or property. This specification may still be overridden
when task instance is created via 'provides' constructor parameter.

Change-Id: If300c3a35807422d06c0196e84891faebde83c02

taskflow/task.py

@@ -204,13 +204,19 @@ class Task(BaseTask):
     Adds following features to Task:
         - auto-generates name from type of self
         - adds all execute argument names to task requirements
+        - items provided by the task may be specified via
+          'default_provides' class attribute or property
     """
 
+    default_provides = None
+
     def __init__(self, name=None, provides=None, requires=None,
                  auto_extract=True, rebind=None):
         """Initialize task instance"""
         if name is None:
             name = reflection.get_callable_name(self)
+        if provides is None:
+            provides = self.default_provides
         super(Task, self).__init__(name,
                                    provides=provides)
         self.rebind = _build_arg_mapping(self.name, requires, rebind,

taskflow/tests/unit/test_task.py

@@ -31,6 +31,13 @@ class KwargsTask(task.Task):
         pass
 
 
+class DefaultProvidesTask(task.Task):
+    default_provides = 'def'
+
+    def execute(self):
+        return None
+
+
 class TaskTestCase(test.TestCase):
 
     def test_passed_name(self):
@@ -149,3 +156,13 @@ class TaskTestCase(test.TestCase):
     def test_rebind_list_bad_value(self):
         with self.assertRaisesRegexp(TypeError, '^Invalid rebind value:'):
             MyTask(rebind=object())
+
+    def test_default_provides(self):
+        task = DefaultProvidesTask()
+        self.assertEquals(task.provides, set(['def']))
+        self.assertEquals(task.save_as, {'def': None})
+
+    def test_default_provides_can_be_overridden(self):
+        task = DefaultProvidesTask(provides=('spam', 'eggs'))
+        self.assertEquals(task.provides, set(['spam', 'eggs']))
+        self.assertEquals(task.save_as, {'spam': 0, 'eggs': 1})