[SPARK-29442][SQL] Set default mode should override the existing mode

python/pyspark/sql/readwriter.py

@@ -593,7 +593,7 @@ def mode(self, saveMode):
 
         * `append`: Append contents of this :class:`DataFrame` to existing data.
         * `overwrite`: Overwrite existing data.
-        * `error` or `errorifexists`: Throw an exception if data already exists.
+        * `error`, `errorifexists`, `default`: Throw an exception if data already exists.
         * `ignore`: Silently ignore this operation if data already exists.
 
         >>> df.write.mode('append').parquet(os.path.join(tempfile.mkdtemp(), 'data'))

sql/core/src/main/scala/org/apache/spark/sql/DataFrameWriter.scala

@@ -77,7 +77,7 @@ final class DataFrameWriter[T] private[sql](ds: Dataset[T]) {
    * <li>`overwrite`: overwrite the existing data.</li>
    * <li>`append`: append the data.</li>
    * <li>`ignore`: ignore the operation (i.e. no-op).</li>
-   * <li>`error` or `errorifexists`: default option, throw an exception at runtime.</li>
+   * <li>`error`, `errorifexists`, or `default`: default option, throw an exception at runtime.</li>
    * </ul>
    *
    * @since 1.4.0
@@ -87,10 +87,9 @@ final class DataFrameWriter[T] private[sql](ds: Dataset[T]) {
       case "overwrite" => mode(SaveMode.Overwrite)
       case "append" => mode(SaveMode.Append)
       case "ignore" => mode(SaveMode.Ignore)
-      case "error" | "errorifexists" => mode(SaveMode.ErrorIfExists)
-      case "default" => this
-      case _ => throw new IllegalArgumentException(s"Unknown save mode: $saveMode. " +
-        "Accepted save modes are 'overwrite', 'append', 'ignore', 'error', 'errorifexists'.")
+      case "error" | "errorifexists" | "default" => mode(SaveMode.ErrorIfExists)
+      case _ => throw new IllegalArgumentException(s"Unknown save mode: $saveMode. Accepted " +
+        "save modes are 'overwrite', 'append', 'ignore', 'error', 'errorifexists', 'default'.")
     }
   }
 

sql/core/src/test/scala/org/apache/spark/sql/DataFrameSuite.scala

@@ -2203,4 +2203,12 @@ class DataFrameSuite extends QueryTest with SharedSparkSession {
           |*(1) Range (0, 10, step=1, splits=2)""".stripMargin))
     }
   }
+
+  test("SPARK-29442 Set `default` mode should override the existing mode") {
+    val df = Seq(Tuple1(1)).toDF()
+    val writer = df.write.mode("overwrite").mode("default")
+    val modeField = classOf[DataFrameWriter[Tuple1[Int]]].getDeclaredField("mode")
+    modeField.setAccessible(true)
+    assert(SaveMode.ErrorIfExists === modeField.get(writer).asInstanceOf[SaveMode])
+  }
 }