[SPARK-5588] [SQL] support select/filter by SQL expression

```
df.selectExpr('a + 1', 'abs(age)')
df.filter('age > 3')
df[ df.age > 3 ]
df[ ['age', 'name'] ]
```

Author: Davies Liu <davies@databricks.com>

Closes #4359 from davies/select_expr and squashes the following commits:

d99856b [Davies Liu] support select/filter by SQL expression

1 files changed, 10 insertions, 1 deletions

diff --git a/core/src/main/scala/org/apache/spark/api/python/PythonUtils.scala b/core/src/main/scala/org/apache/spark/api/python/PythonUtils.scala
index b7cfc8bd9c..acbaba6791 100644
--- a/core/src/main/scala/org/apache/spark/api/python/PythonUtils.scala
+++ b/core/src/main/scala/org/apache/spark/api/python/PythonUtils.scala
@@ -17,8 +17,10 @@
 
 package org.apache.spark.api.python
 
-import java.io.{File, InputStream, IOException, OutputStream}
+import java.io.{File}
+import java.util.{List => JList}
 
+import scala.collection.JavaConversions._
 import scala.collection.mutable.ArrayBuffer
 
 import org.apache.spark.SparkContext
@@ -44,4 +46,11 @@ private[spark] object PythonUtils {
   def generateRDDWithNull(sc: JavaSparkContext): JavaRDD[String] = {
     sc.parallelize(List("a", null, "b"))
   }
+
+  /**
+   * Convert list of T into seq of T (for calling API with varargs)
+   */
+  def toSeq[T](cols: JList[T]): Seq[T] = {
+    cols.toList.toSeq
+  }
 }