[SPARK-6957] [SPARK-6958] [SQL] improve API compatibility to pandas

```
select(['cola', 'colb'])

groupby(['colA', 'colB'])
groupby([df.colA, df.colB])

df.sort('A', ascending=True)
df.sort(['A', 'B'], ascending=True)
df.sort(['A', 'B'], ascending=[1, 0])
```

cc rxin

Author: Davies Liu <davies@databricks.com>

Closes #5544 from davies/compatibility and squashes the following commits:

4944058 [Davies Liu] add docstrings
adb2816 [Davies Liu] Merge branch 'master' of github.com:apache/spark into compatibility
bcbbcab [Davies Liu] support ascending as list
8dabdf0 [Davies Liu] improve API compatibility to pandas

1 files changed, 3 insertions, 8 deletions

diff --git a/python/pyspark/sql/functions.py b/python/pyspark/sql/functions.py
index 1d65369528..bb47923f24 100644
--- a/python/pyspark/sql/functions.py
+++ b/python/pyspark/sql/functions.py
@@ -23,13 +23,11 @@ import sys
 if sys.version < "3":
     from itertools import imap as map
 
-from py4j.java_collections import ListConverter
-
 from pyspark import SparkContext
 from pyspark.rdd import _prepare_for_python_RDD
 from pyspark.serializers import PickleSerializer, AutoBatchedSerializer
 from pyspark.sql.types import StringType
-from pyspark.sql.dataframe import Column, _to_java_column
+from pyspark.sql.dataframe import Column, _to_java_column, _to_seq
 
 
 __all__ = ['countDistinct', 'approxCountDistinct', 'udf']
@@ -87,8 +85,7 @@ def countDistinct(col, *cols):
     [Row(c=2)]
     """
     sc = SparkContext._active_spark_context
-    jcols = ListConverter().convert([_to_java_column(c) for c in cols], sc._gateway._gateway_client)
-    jc = sc._jvm.functions.countDistinct(_to_java_column(col), sc._jvm.PythonUtils.toSeq(jcols))
+    jc = sc._jvm.functions.countDistinct(_to_java_column(col), _to_seq(sc, cols, _to_java_column))
     return Column(jc)
 
 
@@ -138,9 +135,7 @@ class UserDefinedFunction(object):
 
     def __call__(self, *cols):
         sc = SparkContext._active_spark_context
-        jcols = ListConverter().convert([_to_java_column(c) for c in cols],
-                                        sc._gateway._gateway_client)
-        jc = self._judf.apply(sc._jvm.PythonUtils.toSeq(jcols))
+        jc = self._judf.apply(_to_seq(sc, cols, _to_java_column))
         return Column(jc)