[SPARK-12569][PYSPARK][ML] DecisionTreeRegressor: provide variance of prediction: Python API

## What changes were proposed in this pull request?

A new column VarianceCol has been added to DecisionTreeRegressor in ML scala code.

This patch adds the corresponding Python API, HasVarianceCol, to class DecisionTreeRegressor.

## How was this patch tested?
./dev/lint-python
PEP8 checks passed.
rm -rf _build/*
pydoc checks passed.

./python/run-tests --python-executables=python2.7 --modules=pyspark-ml
Running PySpark tests. Output is in /Users/mwang/spark_ws_0904/python/unit-tests.log
Will test against the following Python executables: ['python2.7']
Will test the following Python modules: ['pyspark-ml']
Finished test(python2.7): pyspark.ml.evaluation (12s)
Finished test(python2.7): pyspark.ml.clustering (18s)
Finished test(python2.7): pyspark.ml.classification (30s)
Finished test(python2.7): pyspark.ml.recommendation (28s)
Finished test(python2.7): pyspark.ml.feature (43s)
Finished test(python2.7): pyspark.ml.regression (31s)
Finished test(python2.7): pyspark.ml.tuning (19s)
Finished test(python2.7): pyspark.ml.tests (34s)

(If this patch involves UI changes, please attach a screenshot; otherwise, remove this)

Author: wm624@hotmail.com <wm624@hotmail.com>

Closes #12116 from wangmiao1981/fix_api.

2 files changed, 27 insertions, 1 deletions

diff --git a/python/pyspark/ml/param/_shared_params_code_gen.py b/python/pyspark/ml/param/_shared_params_code_gen.py
index 715fa9e9f8..a7615c43be 100644
--- a/python/pyspark/ml/param/_shared_params_code_gen.py
+++ b/python/pyspark/ml/param/_shared_params_code_gen.py
@@ -146,7 +146,9 @@ if __name__ == "__main__":
         ("weightCol", "weight column name. If this is not set or empty, we treat " +
          "all instance weights as 1.0.", None, "TypeConverters.toString"),
         ("solver", "the solver algorithm for optimization. If this is not set or empty, " +
-         "default value is 'auto'.", "'auto'", "TypeConverters.toString")]
+         "default value is 'auto'.", "'auto'", "TypeConverters.toString"),
+        ("varianceCol", "column name for the biased sample variance of prediction.",
+         None, "TypeConverters.toString")]
 
     code = []
     for name, doc, defaultValueStr, typeConverter in shared:
diff --git a/python/pyspark/ml/param/shared.py b/python/pyspark/ml/param/shared.py
index d79d55e463..c9e975525c 100644
--- a/python/pyspark/ml/param/shared.py
+++ b/python/pyspark/ml/param/shared.py
@@ -559,6 +559,30 @@ class HasSolver(Params):
         return self.getOrDefault(self.solver)
 
 
+class HasVarianceCol(Params):
+    """
+    Mixin for param varianceCol: column name for the biased sample variance of prediction.
+    """
+
+    varianceCol = Param(Params._dummy(), "varianceCol", "column name for the biased sample variance of prediction.", typeConverter=TypeConverters.toString)
+
+    def __init__(self):
+        super(HasVarianceCol, self).__init__()
+
+    def setVarianceCol(self, value):
+        """
+        Sets the value of :py:attr:`varianceCol`.
+        """
+        self._set(varianceCol=value)
+        return self
+
+    def getVarianceCol(self):
+        """
+        Gets the value of varianceCol or its default value.
+        """
+        return self.getOrDefault(self.varianceCol)
+
+
 class DecisionTreeParams(Params):
     """
     Mixin for Decision Tree parameters.