[SPARK-4398][PySpark] specialize sc.parallelize(xrange)

`sc.parallelize(range(1 << 20), 1).count()` may take 15 seconds to finish and the rdd object stores the entire list, making task size very large. This PR adds a specialized version for xrange.

JoshRosen davies

Author: Xiangrui Meng <meng@databricks.com>

Closes #3264 from mengxr/SPARK-4398 and squashes the following commits:

8953c41 [Xiangrui Meng] follow davies' suggestion
cbd58e3 [Xiangrui Meng] specialize sc.parallelize(xrange)

python/pyspark/context.py

@@ -289,12 +289,29 @@ class SparkContext(object):
 
     def parallelize(self, c, numSlices=None):
         """
-        Distribute a local Python collection to form an RDD.
+        Distribute a local Python collection to form an RDD. Using xrange
+        is recommended if the input represents a range for performance.
 
-        >>> sc.parallelize(range(5), 5).glom().collect()
-        [[0], [1], [2], [3], [4]]
+        >>> sc.parallelize([0, 2, 3, 4, 6], 5).glom().collect()
+        [[0], [2], [3], [4], [6]]
+        >>> sc.parallelize(xrange(0, 6, 2), 5).glom().collect()
+        [[], [0], [], [2], [4]]
         """
-        numSlices = numSlices or self.defaultParallelism
+        numSlices = int(numSlices) if numSlices is not None else self.defaultParallelism
+        if isinstance(c, xrange):
+            size = len(c)
+            if size == 0:
+                return self.parallelize([], numSlices)
+            step = c[1] - c[0] if size > 1 else 1
+            start0 = c[0]
+
+            def getStart(split):
+                return start0 + (split * size / numSlices) * step
+
+            def f(split, iterator):
+                return xrange(getStart(split), getStart(split + 1), step)
+
+            return self.parallelize([], numSlices).mapPartitionsWithIndex(f)
         # Calling the Java parallelize() method with an ArrayList is too slow,
         # because it sends O(n) Py4J commands.  As an alternative, serialized
         # objects are written to a file and loaded through textFile().