[SPARK-11904][PYSPARK] reduceByKeyAndWindow does not require checkpointing when invFunc is None

when invFunc is None, `reduceByKeyAndWindow(func, None, winsize, slidesize)` is equivalent to

     reduceByKey(func).window(winsize, slidesize).reduceByKey(winsize, slidesize)

and no checkpoint is necessary. The corresponding Scala code does exactly that, but Python code always creates a windowed stream with obligatory checkpointing. The patch fixes this.

I do not know how to unit-test this.

Author: David Tolpin <david.tolpin@gmail.com>

Closes #9888 from dtolpin/master.

python/pyspark/streaming/dstream.py

@@ -542,31 +542,32 @@ class DStream(object):
 
         reduced = self.reduceByKey(func, numPartitions)
 
-        def reduceFunc(t, a, b):
-            b = b.reduceByKey(func, numPartitions)
-            r = a.union(b).reduceByKey(func, numPartitions) if a else b
-            if filterFunc:
-                r = r.filter(filterFunc)
-            return r
-
-        def invReduceFunc(t, a, b):
-            b = b.reduceByKey(func, numPartitions)
-            joined = a.leftOuterJoin(b, numPartitions)
-            return joined.mapValues(lambda kv: invFunc(kv[0], kv[1])
-                                    if kv[1] is not None else kv[0])
-
-        jreduceFunc = TransformFunction(self._sc, reduceFunc, reduced._jrdd_deserializer)
         if invFunc:
+            def reduceFunc(t, a, b):
+                b = b.reduceByKey(func, numPartitions)
+                r = a.union(b).reduceByKey(func, numPartitions) if a else b
+                if filterFunc:
+                    r = r.filter(filterFunc)
+                return r
+
+            def invReduceFunc(t, a, b):
+                b = b.reduceByKey(func, numPartitions)
+                joined = a.leftOuterJoin(b, numPartitions)
+                return joined.mapValues(lambda kv: invFunc(kv[0], kv[1])
+                                        if kv[1] is not None else kv[0])
+
+            jreduceFunc = TransformFunction(self._sc, reduceFunc, reduced._jrdd_deserializer)
             jinvReduceFunc = TransformFunction(self._sc, invReduceFunc, reduced._jrdd_deserializer)
+            if slideDuration is None:
+                slideDuration = self._slideDuration
+            dstream = self._sc._jvm.PythonReducedWindowedDStream(
+                reduced._jdstream.dstream(),
+                jreduceFunc, jinvReduceFunc,
+                self._ssc._jduration(windowDuration),
+                self._ssc._jduration(slideDuration))
+            return DStream(dstream.asJavaDStream(), self._ssc, self._sc.serializer)
         else:
-            jinvReduceFunc = None
-        if slideDuration is None:
-            slideDuration = self._slideDuration
-        dstream = self._sc._jvm.PythonReducedWindowedDStream(reduced._jdstream.dstream(),
-                                                             jreduceFunc, jinvReduceFunc,
-                                                             self._ssc._jduration(windowDuration),
-                                                             self._ssc._jduration(slideDuration))
-        return DStream(dstream.asJavaDStream(), self._ssc, self._sc.serializer)
+            return reduced.window(windowDuration, slideDuration).reduceByKey(func, numPartitions)
 
     def updateStateByKey(self, updateFunc, numPartitions=None, initialRDD=None):
         """