Set a no-op functor when none is provided

Instead of having later checks to check for
none when calling the thread bundle callback
just initially set it to a no-op functor that
does nothing at binding time instead; this avoids
the need to do repeated checks at start/stop
time.

Change-Id: I5ab4f801bf4767c802ea607fdf864d4852e6c84d

taskflow/utils/threading_utils.py

@@ -82,16 +82,19 @@ _ThreadBuilder = collections.namedtuple('_ThreadBuilder',
                                         ['thread_factory',
                                          'before_start', 'after_start',
                                          'before_join', 'after_join'])
-_ThreadBuilder.callables = tuple([
-    # Attribute name -> none allowed as a valid value...
-    ('thread_factory', False),
-    ('before_start', True),
-    ('after_start', True),
-    ('before_join', True),
-    ('after_join', True),
+_ThreadBuilder.fields = tuple([
+    'thread_factory',
+    'before_start',
+    'after_start',
+    'before_join',
+    'after_join',
 ])
 
 
+def no_op(*args, **kwargs):
+    """Function that does nothing."""
+
+
 class ThreadBundle(object):
     """A group/bundle of threads that start/stop together."""
 
@@ -110,13 +113,19 @@ class ThreadBundle(object):
                         in dead-lock since the lock on this object is not
                         meant to be (and is not) reentrant...
         """
+        if before_start is None:
+            before_start = no_op
+        if after_start is None:
+            after_start = no_op
+        if before_join is None:
+            before_join = no_op
+        if after_join is None:
+            after_join = no_op
         builder = _ThreadBuilder(thread_factory,
                                  before_start, after_start,
                                  before_join, after_join)
-        for attr_name, none_allowed in builder.callables:
+        for attr_name in builder.fields:
             cb = getattr(builder, attr_name)
-            if cb is None and none_allowed:
-                continue
             if not six.callable(cb):
                 raise ValueError("Provided callback for argument"
                                  " '%s' must be callable" % attr_name)
@@ -130,11 +139,6 @@ class ThreadBundle(object):
                 False,
             ])
 
-    @staticmethod
-    def _trigger_callback(callback, thread):
-        if callback is not None:
-            callback(thread)
-
     def start(self):
         """Creates & starts all associated threads (that are not running)."""
         count = 0
@@ -145,11 +149,11 @@ class ThreadBundle(object):
                     continue
                 if not thread:
                     self._threads[i][1] = thread = builder.thread_factory()
-                self._trigger_callback(builder.before_start, thread)
+                builder.before_start(thread)
                 thread.start()
                 count += 1
                 try:
-                    self._trigger_callback(builder.after_start, thread)
+                    builder.after_start(thread)
                 finally:
                     # Just incase the 'after_start' callback blows up make sure
                     # we always set this...
@@ -164,11 +168,11 @@ class ThreadBundle(object):
             for i, (builder, thread, started) in it:
                 if not thread or not started:
                     continue
-                self._trigger_callback(builder.before_join, thread)
+                builder.before_join(thread)
                 thread.join()
                 count += 1
                 try:
-                    self._trigger_callback(builder.after_join, thread)
+                    builder.after_join(thread)
                 finally:
                     # Just incase the 'after_join' callback blows up make sure
                     # we always set/reset these...