[SPARK-19427][PYTHON][SQL] Support data type string as a returnType argument of UDF

## What changes were proposed in this pull request?

Add support for data type string as a return type argument of `UserDefinedFunction`:

```python
f = udf(lambda x: x, "integer")
 f.returnType

## IntegerType
```

## How was this patch tested?

Existing unit tests, additional unit tests covering new feature.

Author: zero323 <[email protected]>

Closes apache#16769 from zero323/SPARK-19427.

Author: zero323

python/pyspark/sql/functions.py

@@ -27,7 +27,7 @@
 from pyspark import since, SparkContext
 from pyspark.rdd import _prepare_for_python_RDD, ignore_unicode_prefix
 from pyspark.serializers import PickleSerializer, AutoBatchedSerializer
-from pyspark.sql.types import StringType
+from pyspark.sql.types import StringType, DataType, _parse_datatype_string
 from pyspark.sql.column import Column, _to_java_column, _to_seq
 from pyspark.sql.dataframe import DataFrame
 
@@ -1865,7 +1865,9 @@ class UserDefinedFunction(object):
     """
     def __init__(self, func, returnType, name=None):
         self.func = func
-        self.returnType = returnType
+        self.returnType = (
+            returnType if isinstance(returnType, DataType)
+            else _parse_datatype_string(returnType))
         # Stores UserDefinedPythonFunctions jobj, once initialized
         self._judf_placeholder = None
         self._name = name or (
@@ -1909,7 +1911,7 @@ def udf(f, returnType=StringType()):
         it is present in the query.
 
     :param f: python function
-    :param returnType: a :class:`pyspark.sql.types.DataType` object
+    :param returnType: a :class:`pyspark.sql.types.DataType` object or data type string.
 
     >>> from pyspark.sql.types import IntegerType
     >>> slen = udf(lambda s: len(s), IntegerType())

python/pyspark/sql/tests.py

@@ -489,6 +489,21 @@ def test_udf_defers_judf_initalization(self):
             "judf should be initialized after UDF has been called."
         )
 
+    def test_udf_with_string_return_type(self):
+        from pyspark.sql.functions import UserDefinedFunction
+
+        add_one = UserDefinedFunction(lambda x: x + 1, "integer")
+        make_pair = UserDefinedFunction(lambda x: (-x, x), "struct<x:integer,y:integer>")
+        make_array = UserDefinedFunction(
+            lambda x: [float(x) for x in range(x, x + 3)], "array<double>")
+
+        expected = (2, Row(x=-1, y=1), [1.0, 2.0, 3.0])
+        actual = (self.spark.range(1, 2).toDF("x")
+                  .select(add_one("x"), make_pair("x"), make_array("x"))
+                  .first())
+
+        self.assertTupleEqual(expected, actual)
+
     def test_basic_functions(self):
         rdd = self.sc.parallelize(['{"foo":"bar"}', '{"foo":"baz"}'])
         df = self.spark.read.json(rdd)