[SPARK-14071][PYSPARK][ML] Change MLWritable.write to be a property

Add property to MLWritable.write method, so we can use .write instead of .write()

Add a new test to ml/test.py to check whether the write is a property.
./python/run-tests --python-executables=python2.7 --modules=pyspark-ml

Will test against the following Python executables: ['python2.7']
Will test the following Python modules: ['pyspark-ml']
Finished test(python2.7): pyspark.ml.evaluation (11s)
Finished test(python2.7): pyspark.ml.clustering (16s)
Finished test(python2.7): pyspark.ml.classification (24s)
Finished test(python2.7): pyspark.ml.recommendation (24s)
Finished test(python2.7): pyspark.ml.feature (39s)
Finished test(python2.7): pyspark.ml.regression (26s)
Finished test(python2.7): pyspark.ml.tuning (15s)
Finished test(python2.7): pyspark.ml.tests (30s)
Tests passed in 55 seconds

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

Closes #11945 from wangmiao1981/fix_property.

python/pyspark/ml/tests.py

@@ -51,6 +51,7 @@ from pyspark.ml.param.shared import HasMaxIter, HasInputCol, HasSeed
 from pyspark.ml.regression import LinearRegression, DecisionTreeRegressor
 from pyspark.ml.tuning import *
 from pyspark.ml.util import keyword_only
+from pyspark.ml.util import MLWritable, MLWriter
 from pyspark.ml.wrapper import JavaWrapper
 from pyspark.mllib.linalg import DenseVector, SparseVector
 from pyspark.sql import DataFrame, SQLContext, Row
@@ -655,6 +656,10 @@ class PersistenceTest(PySparkTestCase):
             except OSError:
                 pass
 
+    def test_write_property(self):
+        lr = LinearRegression(maxIter=1)
+        self.assertTrue(isinstance(lr.write, MLWriter))
+
     def test_decisiontree_classifier(self):
         dt = DecisionTreeClassifier(maxDepth=1)
         path = tempfile.mkdtemp()

python/pyspark/ml/util.py

@@ -134,13 +134,14 @@ class MLWritable(object):
     .. versionadded:: 2.0.0
     """
 
+    @property
     def write(self):
         """Returns an JavaMLWriter instance for this ML instance."""
         raise NotImplementedError("MLWritable is not yet implemented for type: %r" % type(self))
 
     def save(self, path):
         """Save this ML instance to the given path, a shortcut of `write().save(path)`."""
-        self.write().save(path)
+        self.write.save(path)
 
 
 @inherit_doc
@@ -149,6 +150,7 @@ class JavaMLWritable(MLWritable):
     (Private) Mixin for ML instances that provide :py:class:`JavaMLWriter`.
     """
 
+    @property
     def write(self):
         """Returns an JavaMLWriter instance for this ML instance."""
         return JavaMLWriter(self)