[SPARK-4148][PySpark] fix seed distribution and add some tests for rdd.sample

The current way of seed distribution makes the random sequences from partition i and i+1 offset by 1.

~~~
In [14]: import random

In [15]: r1 = random.Random(10)

In [16]: r1.randint(0, 1)
Out[16]: 1

In [17]: r1.random()
Out[17]: 0.4288890546751146

In [18]: r1.random()
Out[18]: 0.5780913011344704

In [19]: r2 = random.Random(10)

In [20]: r2.randint(0, 1)
Out[20]: 1

In [21]: r2.randint(0, 1)
Out[21]: 0

In [22]: r2.random()
Out[22]: 0.5780913011344704
~~~

Note: The new tests are not for this bug fix.

Author: Xiangrui Meng <meng@databricks.com>

Closes #3010 from mengxr/SPARK-4148 and squashes the following commits:

869ae4b [Xiangrui Meng] move tests tests.py
c1bacd9 [Xiangrui Meng] fix seed distribution and add some tests for rdd.sample

1 files changed, 0 insertions, 3 deletions

diff --git a/python/pyspark/rdd.py b/python/pyspark/rdd.py
index 550c9dd805..4f025b9f11 100644
--- a/python/pyspark/rdd.py
+++ b/python/pyspark/rdd.py
@@ -316,9 +316,6 @@ class RDD(object):
         """
         Return a sampled subset of this RDD (relies on numpy and falls back
         on default random generator if numpy is unavailable).
-
-        >>> sc.parallelize(range(0, 100)).sample(False, 0.1, 2).collect() #doctest: +SKIP
-        [2, 3, 20, 21, 24, 41, 42, 66, 67, 89, 90, 98]
         """
         assert fraction >= 0.0, "Negative fraction value: %s" % fraction
         return self.mapPartitionsWithIndex(RDDSampler(withReplacement, fraction, seed).func, True)