[SPARK-8467] [MLLIB] [PYSPARK] Add LDAModel.describeTopics() in Python

Could jkbradley and davies review it?

- Create a wrapper class: `LDAModelWrapper` for `LDAModel`. Because we can't deal with the return value of`describeTopics` in Scala from pyspark directly. `Array[(Array[Int], Array[Double])]` is too complicated to convert it.
- Add `loadLDAModel` in `PythonMLlibAPI`. Since `LDAModel` in Scala is an abstract class and we need to call `load` of `DistributedLDAModel`.

[[SPARK-8467] Add LDAModel.describeTopics() in Python - ASF JIRA](https://issues.apache.org/jira/browse/SPARK-8467)

Author: Yu ISHIKAWA <yuu.ishikawa@gmail.com>

Closes #8643 from yu-iskw/SPARK-8467-2.

2 files changed, 57 insertions, 2 deletions

diff --git a/mllib/src/main/scala/org/apache/spark/mllib/api/python/LDAModelWrapper.scala b/mllib/src/main/scala/org/apache/spark/mllib/api/python/LDAModelWrapper.scala
new file mode 100644
index 0000000000..63282eee6e
--- /dev/null
+++ b/mllib/src/main/scala/org/apache/spark/mllib/api/python/LDAModelWrapper.scala
@@ -0,0 +1,46 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.spark.mllib.api.python
+
+import scala.collection.JavaConverters
+
+import org.apache.spark.SparkContext
+import org.apache.spark.mllib.clustering.LDAModel
+import org.apache.spark.mllib.linalg.Matrix
+
+/**
+ * Wrapper around LDAModel to provide helper methods in Python
+ */
+private[python] class LDAModelWrapper(model: LDAModel) {
+
+  def topicsMatrix(): Matrix = model.topicsMatrix
+
+  def vocabSize(): Int = model.vocabSize
+
+  def describeTopics(): Array[Byte] = describeTopics(this.model.vocabSize)
+
+  def describeTopics(maxTermsPerTopic: Int): Array[Byte] = {
+    val topics = model.describeTopics(maxTermsPerTopic).map { case (terms, termWeights) =>
+      val jTerms = JavaConverters.seqAsJavaListConverter(terms).asJava
+      val jTermWeights = JavaConverters.seqAsJavaListConverter(termWeights).asJava
+      Array[Any](jTerms, jTermWeights)
+    }
+    SerDe.dumps(JavaConverters.seqAsJavaListConverter(topics).asJava)
+  }
+
+  def save(sc: SparkContext, path: String): Unit = model.save(sc, path)
+}
diff --git a/mllib/src/main/scala/org/apache/spark/mllib/api/python/PythonMLLibAPI.scala b/mllib/src/main/scala/org/apache/spark/mllib/api/python/PythonMLLibAPI.scala
index 40c41806cd..54b03a9f90 100644
--- a/mllib/src/main/scala/org/apache/spark/mllib/api/python/PythonMLLibAPI.scala
+++ b/mllib/src/main/scala/org/apache/spark/mllib/api/python/PythonMLLibAPI.scala
@@ -517,7 +517,7 @@ private[python] class PythonMLLibAPI extends Serializable {
       topicConcentration: Double,
       seed: java.lang.Long,
       checkpointInterval: Int,
-      optimizer: String): LDAModel = {
+      optimizer: String): LDAModelWrapper = {
     val algo = new LDA()
       .setK(k)
       .setMaxIterations(maxIterations)
@@ -535,7 +535,16 @@ private[python] class PythonMLLibAPI extends Serializable {
         case _ => throw new IllegalArgumentException("input values contains invalid type value.")
       }
     }
-    algo.run(documents)
+    val model = algo.run(documents)
+    new LDAModelWrapper(model)
+  }
+
+  /**
+   * Load a LDA model
+   */
+  def loadLDAModel(jsc: JavaSparkContext, path: String): LDAModelWrapper = {
+    val model = DistributedLDAModel.load(jsc.sc, path)
+    new LDAModelWrapper(model)
   }