Add periodics.is_periodic to check if object is a periodic task

People implementing their own periodic tasks collection (as opposed
to using PeriodicWorker.create) need to know if something is
a periodic task without inspecting its private attributes.
This function returning True means something is safe to add
to a PeriodicWorker (will not raise ValueError).

Change-Id: Iee8da601b1bc994188c58ca459574a466270bf63

futurist/periodics.py

@@ -115,6 +115,16 @@ def _check_attrs(obj):
     return missing_attrs
 
 
+def is_periodic(obj):
+    """Check whether an object is a valid periodic callable.
+
+    :param obj: object to inspect
+    :type obj: anything
+    :return: True if obj is a periodic task, otherwise False
+    """
+    return callable(obj) and not _check_attrs(obj)
+
+
 def periodic(spacing, run_immediately=False, enabled=True):
     """Tags a method/function as wanting/able to execute periodically.
 

futurist/tests/test_periodics.py

@@ -234,6 +234,21 @@ class TestPeriodics(testscenarios.TestWithScenarios, base.TestCase):
         self.assertIsNotNone(w.add(add_me))
         self.assertEqual(1, len(w))
 
+    def test_is_periodic(self):
+
+        @periodics.periodic(0.5, enabled=False)
+        def no_add_me():
+            pass
+
+        @periodics.periodic(0.5)
+        def add_me():
+            pass
+
+        self.assertTrue(periodics.is_periodic(add_me))
+        self.assertTrue(periodics.is_periodic(no_add_me))
+        self.assertFalse(periodics.is_periodic(self.test_is_periodic))
+        self.assertFalse(periodics.is_periodic(42))
+
     def test_watcher(self):
 
         def cb():