Document parallel helper functions

Signed-off-by: Aanand Prasad <aanand.prasad@gmail.com>

compose/parallel.py

@@ -65,12 +65,19 @@ def _no_deps(x):
 
 
 class State(object):
+    """
+    Holds the state of a partially-complete parallel operation.
+
+    state.started:   objects being processed
+    state.finished:  objects which have been processed
+    state.failed:    objects which either failed or whose dependencies failed
+    """
     def __init__(self, objects):
         self.objects = objects
 
-        self.started = set()   # objects being processed
-        self.finished = set()  # objects which have been processed
-        self.failed = set()    # objects which either failed or whose dependencies failed
+        self.started = set()
+        self.finished = set()
+        self.failed = set()
 
     def is_done(self):
         return len(self.finished) + len(self.failed) >= len(self.objects)
@@ -80,6 +87,21 @@ class State(object):
 
 
 def parallel_execute_stream(objects, func, get_deps):
+    """
+    Runs func on objects in parallel while ensuring that func is
+    ran on object only after it is ran on all its dependencies.
+
+    Returns an iterator of tuples which look like:
+
+    # if func returned normally when run on object
+    (object, result, None)
+
+    # if func raised an exception when run on object
+    (object, None, exception)
+
+    # if func raised an exception when run on one of object's dependencies
+    (object, None, UpstreamError())
+    """
     if get_deps is None:
         get_deps = _no_deps
 
@@ -109,6 +131,10 @@ def parallel_execute_stream(objects, func, get_deps):
 
 
 def queue_producer(obj, func, results):
+    """
+    The entry point for a producer thread which runs func on a single object.
+    Places a tuple on the results queue once func has either returned or raised.
+    """
     try:
         result = func(obj)
         results.put((obj, result, None))
@@ -117,6 +143,13 @@ def queue_producer(obj, func, results):
 
 
 def feed_queue(objects, func, get_deps, results, state):
+    """
+    Starts producer threads for any objects which are ready to be processed
+    (i.e. they have no dependencies which haven't been successfully processed).
+
+    Shortcuts any objects whose dependencies have failed and places an
+    (object, None, UpstreamError()) tuple on the results queue.
+    """
     pending = state.pending()
     log.debug('Pending: {}'.format(pending))