add docs and fix initialization in ThreadedMapData (fix #293)

tensorpack/dataflow/prefetch.py

@@ -212,8 +212,9 @@ class ThreadedMapData(ProxyDataFlow):
     Note that the threads will only start in the process which calls
     `reset_state()`.
     """
-    class WorkerThread(StoppableThread):
+    class _WorkerThread(StoppableThread):
         def __init__(self, inq, outq, map_func):
+            super(ThreadedMapData._WorkerThread, self).__init__()
             self.inq = inq
             self.outq = outq
             self.func = map_func
@@ -228,7 +229,10 @@ class ThreadedMapData(ProxyDataFlow):
     def __init__(self, ds, nr_thread, map_func, buffer_size=200):
         """
         Args:
-            pass
+            ds (DataFlow): the dataflow to map
+            nr_thread (int): number of threads to use
+            map_func (callable): datapoint -> datapoint | None
+            buffer_size (int): number of datapoints in the buffer
         """
         super(ThreadedMapData, self).__init__(ds)
         self.infinite_ds = RepeatedData(ds, -1)
@@ -244,7 +248,7 @@ class ThreadedMapData(ProxyDataFlow):
             t.join()
         self._in_queue = queue.Queue()
         self._out_queue = queue.Queue()
-        self._threads = [ThreadedMapData.WorkerThread(
+        self._threads = [ThreadedMapData._WorkerThread(
             self._in_queue, self._out_queue, self.map_func)
             for _ in range(self.nr_thread)]
         for t in self._threads: