[SPARK-16086] [SQL] fix Python UDF without arguments (for 1.6)

## What changes were proposed in this pull request?

Fix the bug for Python UDF that does not have any arguments.

## How was this patch tested?

Added regression tests.

Author: Davies Liu <davies.liu@gmail.com>

Closes apache#13793 from davies/fix_no_arguments.

(cherry picked from commit abe36c5)

python/pyspark/sql/tests.py

@@ -305,6 +305,11 @@ def test_udf2(self):
         [res] = self.sqlCtx.sql("SELECT strlen(a) FROM test WHERE strlen(a) > 1").collect()
         self.assertEqual(4, res[0])
 
+    def test_udf_without_arguments(self):
+        self.sqlCtx.registerFunction("foo", lambda: "bar")
+        [row] = self.sqlCtx.sql("SELECT foo()").collect()
+        self.assertEqual(row[0], "bar")
+
     def test_udf_with_array_type(self):
         d = [Row(l=list(range(3)), d={"key": list(range(5))})]
         rdd = self.sc.parallelize(d)

python/pyspark/sql/types.py

@@ -1195,19 +1195,16 @@ def __new__(self, *args, **kwargs):
         if args and kwargs:
             raise ValueError("Can not use both args "
                              "and kwargs to create Row")
-        if args:
-            # create row class or objects
-            return tuple.__new__(self, args)
-
-        elif kwargs:
+        if kwargs:
             # create row objects
             names = sorted(kwargs.keys())
             row = tuple.__new__(self, [kwargs[n] for n in names])
             row.__fields__ = names
             return row
 
         else:
-            raise ValueError("No args or kwargs")
+            # create row class or objects
+            return tuple.__new__(self, args)
 
     def asDict(self, recursive=False):
         """