[SPARK-2130] End-user friendly String repr for StorageLevel in Python

JIRA issue https://issues.apache.org/jira/browse/SPARK-2130

This PR adds an end-user friendly String representation for StorageLevel
in Python, similar to ```StorageLevel.description``` in Scala.
```
>>> rdd = sc.parallelize([1,2])
>>> storage_level = rdd.getStorageLevel()
>>> storage_level
StorageLevel(False, False, False, False, 1)
>>> print(storage_level)
Serialized 1x Replicated
```

Author: Kan Zhang <kzhang@apache.org>

Closes #1096 from kanzhang/SPARK-2130 and squashes the following commits:

7c8b98b [Kan Zhang] [SPARK-2130] Prettier epydoc output
cc5bf45 [Kan Zhang] [SPARK-2130] End-user friendly String representation for StorageLevel in Python

1 files changed, 9 insertions, 0 deletions

diff --git a/python/pyspark/storagelevel.py b/python/pyspark/storagelevel.py
index 7b6660eab2..3a18ea54ea 100644
--- a/python/pyspark/storagelevel.py
+++ b/python/pyspark/storagelevel.py
@@ -36,6 +36,15 @@ class StorageLevel:
         return "StorageLevel(%s, %s, %s, %s, %s)" % (
             self.useDisk, self.useMemory, self.useOffHeap, self.deserialized, self.replication)
 
+    def __str__(self):
+        result = ""
+        result += "Disk " if self.useDisk else ""
+        result += "Memory " if self.useMemory else ""
+        result += "Tachyon " if self.useOffHeap else ""
+        result += "Deserialized " if self.deserialized else "Serialized "
+        result += "%sx Replicated" % self.replication
+        return result
+
 StorageLevel.DISK_ONLY = StorageLevel(True, False, False, False)
 StorageLevel.DISK_ONLY_2 = StorageLevel(True, False, False, False, 2)
 StorageLevel.MEMORY_ONLY = StorageLevel(False, True, False, True)