Spark-3406 add a default storage level to python RDD persist API

Author: Holden Karau <holden@pigscanfly.ca>

Closes #2280 from holdenk/SPARK-3406-Python-RDD-persist-api-does-not-have-default-storage-level and squashes the following commits:

33eaade [Holden Karau] As Josh pointed out, sql also override persist. Make persist behave the same as in the underlying RDD as well
e658227 [Holden Karau] Fix the test I added
e95a6c5 [Holden Karau] The Python persist function did not have a default storageLevel unlike the Scala API. Noticed this issue because we got a bug report back from the book where we had documented it as if it was the same as the Scala API

python/pyspark/rdd.py

@@ -212,11 +212,16 @@ class RDD(object):
         self.persist(StorageLevel.MEMORY_ONLY_SER)
         return self
 
-    def persist(self, storageLevel):
+    def persist(self, storageLevel=StorageLevel.MEMORY_ONLY_SER):
         """
         Set this RDD's storage level to persist its values across operations
         after the first time it is computed. This can only be used to assign
         a new storage level if the RDD does not have a storage level set yet.
+        If no storage level is specified defaults to (C{MEMORY_ONLY_SER}).
+
+        >>> rdd = sc.parallelize(["b", "a", "c"])
+        >>> rdd.persist().is_cached
+        True
         """
         self.is_cached = True
         javaStorageLevel = self.ctx._getJavaStorageLevel(storageLevel)

python/pyspark/sql.py

@@ -29,6 +29,7 @@ from operator import itemgetter
 
 from pyspark.rdd import RDD, PipelinedRDD
 from pyspark.serializers import BatchedSerializer, PickleSerializer, CloudPickleSerializer
+from pyspark.storagelevel import StorageLevel
 
 from itertools import chain, ifilter, imap
 
@@ -1665,7 +1666,7 @@ class SchemaRDD(RDD):
         self._jschema_rdd.cache()
         return self
 
-    def persist(self, storageLevel):
+    def persist(self, storageLevel=StorageLevel.MEMORY_ONLY_SER):
         self.is_cached = True
         javaStorageLevel = self.ctx._getJavaStorageLevel(storageLevel)
         self._jschema_rdd.persist(javaStorageLevel)