feat: DB API cursors are now iterable (#618)

* feat: make DB API Cursors iterable

* Raise error if obtaining iterator of closed Cursor

google/cloud/bigquery/dbapi/_helpers.py

@@ -276,7 +276,7 @@ def decorate_public_methods(klass):
         """Apply ``_raise_on_closed()`` decorator to public instance methods.
         """
         for name in dir(klass):
-            if name.startswith("_"):
+            if name.startswith("_") and name != "__iter__":
                 continue
 
             member = getattr(klass, name)

google/cloud/bigquery/dbapi/cursor.py

@@ -365,6 +365,10 @@ def setinputsizes(self, sizes):
     def setoutputsize(self, size, column=None):
         """No-op, but for consistency raise an error if cursor is closed."""
 
+    def __iter__(self):
+        self._try_fetch()
+        return iter(self._query_data)
+
 
 def _format_operation_list(operation, parameters):
     """Formats parameters in operation in the way BigQuery expects.

tests/unit/test_dbapi_cursor.py

@@ -178,6 +178,7 @@ def test_raises_error_if_closed(self):
             "fetchone",
             "setinputsizes",
             "setoutputsize",
+            "__iter__",
         )
 
         for method in method_names:
@@ -611,6 +612,29 @@ def test_executemany_w_dml(self):
         self.assertIsNone(cursor.description)
         self.assertEqual(cursor.rowcount, 12)
 
+    def test_is_iterable(self):
+        from google.cloud.bigquery import dbapi
+
+        connection = dbapi.connect(
+            self._mock_client(rows=[("hello", "there", 7), ("good", "bye", -3)])
+        )
+        cursor = connection.cursor()
+        cursor.execute("SELECT foo, bar, baz FROM hello_world WHERE baz < 42;")
+
+        rows_iter = iter(cursor)
+
+        row = next(rows_iter)
+        self.assertEqual(row, ("hello", "there", 7))
+        row = next(rows_iter)
+        self.assertEqual(row, ("good", "bye", -3))
+        self.assertRaises(StopIteration, next, rows_iter)
+
+        self.assertEqual(
+            list(cursor),
+            [],
+            "Iterating again over the same results should produce no rows.",
+        )
+
     def test__format_operation_w_dict(self):
         from google.cloud.bigquery.dbapi import cursor