[SPARK-19163][PYTHON][SQL] Delay _judf initialization to the __call__

## What changes were proposed in this pull request?

Defer `UserDefinedFunction._judf` initialization to the first call. This prevents unintended `SparkSession` initialization.  This allows users to define and import UDF without creating a context / session as a side effect.

[SPARK-19163](https://issues.apache.org/jira/browse/SPARK-19163)

## How was this patch tested?

Unit tests.

Author: zero323 <zero323@users.noreply.github.com>

Closes #16536 from zero323/SPARK-19163.

python/pyspark/sql/functions.py

@@ -1826,25 +1826,38 @@ class UserDefinedFunction(object):
     def __init__(self, func, returnType, name=None):
         self.func = func
         self.returnType = returnType
-        self._judf = self._create_judf(name)
+        # Stores UserDefinedPythonFunctions jobj, once initialized
+        self._judf_placeholder = None
+        self._name = name or (
+            func.__name__ if hasattr(func, '__name__')
+            else func.__class__.__name__)
 
-    def _create_judf(self, name):
+    @property
+    def _judf(self):
+        # It is possible that concurrent access, to newly created UDF,
+        # will initialize multiple UserDefinedPythonFunctions.
+        # This is unlikely, doesn't affect correctness,
+        # and should have a minimal performance impact.
+        if self._judf_placeholder is None:
+            self._judf_placeholder = self._create_judf()
+        return self._judf_placeholder
+
+    def _create_judf(self):
         from pyspark.sql import SparkSession
-        sc = SparkContext.getOrCreate()
-        wrapped_func = _wrap_function(sc, self.func, self.returnType)
+
         spark = SparkSession.builder.getOrCreate()
+        sc = spark.sparkContext
+
+        wrapped_func = _wrap_function(sc, self.func, self.returnType)
         jdt = spark._jsparkSession.parseDataType(self.returnType.json())
-        if name is None:
-            f = self.func
-            name = f.__name__ if hasattr(f, '__name__') else f.__class__.__name__
         judf = sc._jvm.org.apache.spark.sql.execution.python.UserDefinedPythonFunction(
-            name, wrapped_func, jdt)
+            self._name, wrapped_func, jdt)
         return judf
 
     def __call__(self, *cols):
+        judf = self._judf
         sc = SparkContext._active_spark_context
-        jc = self._judf.apply(_to_seq(sc, cols, _to_java_column))
-        return Column(jc)
+        return Column(judf.apply(_to_seq(sc, cols, _to_java_column)))
 
 
 @since(1.3)

python/pyspark/sql/tests.py

@@ -468,6 +468,27 @@ class SQLTests(ReusedPySparkTestCase):
         row2 = df2.select(sameText(df2['file'])).first()
         self.assertTrue(row2[0].find("people.json") != -1)
 
+    def test_udf_defers_judf_initalization(self):
+        # This is separate of  UDFInitializationTests
+        # to avoid context initialization
+        # when udf is called
+
+        from pyspark.sql.functions import UserDefinedFunction
+
+        f = UserDefinedFunction(lambda x: x, StringType())
+
+        self.assertIsNone(
+            f._judf_placeholder,
+            "judf should not be initialized before the first call."
+        )
+
+        self.assertIsInstance(f("foo"), Column, "UDF call should return a Column.")
+
+        self.assertIsNotNone(
+            f._judf_placeholder,
+            "judf should be initialized after UDF has been called."
+        )
+
     def test_basic_functions(self):
         rdd = self.sc.parallelize(['{"foo":"bar"}', '{"foo":"baz"}'])
         df = self.spark.read.json(rdd)
@@ -1947,6 +1968,29 @@ class SQLTests2(ReusedPySparkTestCase):
         df.collect()
 
 
+class UDFInitializationTests(unittest.TestCase):
+    def tearDown(self):
+        if SparkSession._instantiatedSession is not None:
+            SparkSession._instantiatedSession.stop()
+
+        if SparkContext._active_spark_context is not None:
+            SparkContext._active_spark_contex.stop()
+
+    def test_udf_init_shouldnt_initalize_context(self):
+        from pyspark.sql.functions import UserDefinedFunction
+
+        UserDefinedFunction(lambda x: x, StringType())
+
+        self.assertIsNone(
+            SparkContext._active_spark_context,
+            "SparkContext shouldn't be initialized when UserDefinedFunction is created."
+        )
+        self.assertIsNone(
+            SparkSession._instantiatedSession,
+            "SparkSession shouldn't be initialized when UserDefinedFunction is created."
+        )
+
+
 class HiveContextSQLTests(ReusedPySparkTestCase):
 
     @classmethod