[SPARK-13842] [PYSPARK] pyspark.sql.types.StructType accessor enhancements

## What changes were proposed in this pull request?

Expand the possible ways to interact with the contents of a `pyspark.sql.types.StructType` instance.
  - Iterating a `StructType` will iterate its fields
    - `[field.name for field in my_structtype]`
  - Indexing with a string will return a field by name
    - `my_structtype['my_field_name']`
  - Indexing with an integer will return a field by position
    - `my_structtype[0]`
  - Indexing with a slice will return a new `StructType` with just the chosen fields:
    - `my_structtype[1:3]`
  - The length is the number of fields (should also provide "truthiness" for free)
    - `len(my_structtype) == 2`

## How was this patch tested?

Extended the unit test coverage in the accompanying `tests.py`.

Author: Sheamus K. Parkes <shea.parkes@milliman.com>

Closes #12251 from skparkes/pyspark-structtype-enhance.

1 files changed, 19 insertions, 4 deletions

diff --git a/python/pyspark/sql/tests.py b/python/pyspark/sql/tests.py
index 1e864b4cd1..3b1b2948e9 100644
--- a/python/pyspark/sql/tests.py
+++ b/python/pyspark/sql/tests.py
@@ -802,11 +802,26 @@ class SQLTests(ReusedPySparkTestCase):
         self.assertNotEqual(struct1, struct2)
 
         # Catch exception raised during improper construction
-        try:
+        with self.assertRaises(ValueError):
             struct1 = StructType().add("name")
-            self.assertEqual(1, 0)
-        except ValueError:
-            self.assertEqual(1, 1)
+
+        struct1 = StructType().add("f1", StringType(), True).add("f2", StringType(), True, None)
+        for field in struct1:
+            self.assertIsInstance(field, StructField)
+
+        struct1 = StructType().add("f1", StringType(), True).add("f2", StringType(), True, None)
+        self.assertEqual(len(struct1), 2)
+
+        struct1 = StructType().add("f1", StringType(), True).add("f2", StringType(), True, None)
+        self.assertIs(struct1["f1"], struct1.fields[0])
+        self.assertIs(struct1[0], struct1.fields[0])
+        self.assertEqual(struct1[0:1], StructType(struct1.fields[0:1]))
+        with self.assertRaises(KeyError):
+            not_a_field = struct1["f9"]
+        with self.assertRaises(IndexError):
+            not_a_field = struct1[9]
+        with self.assertRaises(TypeError):
+            not_a_field = struct1[9.9]
 
     def test_metadata_null(self):
         from pyspark.sql.types import StructType, StringType, StructField