Add + use a staticmethod to fetch the immediate callables

For callables that have requested to be called immediately
once the periodic worker starts (or is reset) a utility method
can be useful to avoid the duplicated logic to fetch these
callables (required in reset and in the worker constructor).

Change-Id: I130b60da94745f107e41ba713c85ce5960065876

taskflow/types/periodic.py

@@ -108,19 +108,27 @@ class PeriodicWorker(object):
         self._callables = tuple((cb, reflection.get_callable_name(cb))
                                 for cb in almost_callables)
         self._schedule = []
-        self._immediates = []
         now = _now()
         for i, (cb, cb_name) in enumerate(self._callables):
             spacing = cb._periodic_spacing
             next_run = now + spacing
             heapq.heappush(self._schedule, (next_run, i))
-        for (cb, cb_name) in reversed(self._callables):
-            if cb._periodic_run_immediately:
-                self._immediates.append((cb, cb_name))
+        self._immediates = self._fetch_immediates(self._callables)
 
     def __len__(self):
         return len(self._callables)
 
+    @staticmethod
+    def _fetch_immediates(callables):
+        immediates = []
+        # Reverse order is used since these are later popped off (and to
+        # ensure the popping order is first -> last we need to append them
+        # in the opposite ordering last -> first).
+        for (cb, cb_name) in reversed(callables):
+            if cb._periodic_run_immediately:
+                immediates.append((cb, cb_name))
+        return immediates
+
     @staticmethod
     def _safe_call(cb, cb_name, kind='periodic'):
         try:
@@ -173,7 +181,4 @@ class PeriodicWorker(object):
     def reset(self):
         """Resets the tombstone and re-queues up any immediate executions."""
         self._tombstone.clear()
-        self._immediates = []
-        for (cb, cb_name) in reversed(self._callables):
-            if cb._periodic_run_immediately:
-                self._immediates.append((cb, cb_name))
+        self._immediates = self._fetch_immediates(self._callables)