Simplify RowIterator constructor. Add test comments.

Use getattr instead of protecting with hasattr in the RowIterator
constructor.

Add comments about intentionally conflicting values for total_rows.

bigquery/google/cloud/bigquery/table.py

@@ -1298,17 +1298,13 @@ def __init__(
             page_start=_rows_page_start,
             next_token="pageToken",
         )
-        self._schema = schema
         self._field_to_index = _helpers._field_to_index_mapping(schema)
-
-        self._total_rows = None
-        if table is not None and hasattr(table, "num_rows"):
-            self._total_rows = table.num_rows
-
         self._page_size = page_size
-        self._table = table
-        self._selected_fields = selected_fields
         self._project = client.project
+        self._schema = schema
+        self._selected_fields = selected_fields
+        self._table = table
+        self._total_rows = getattr(table, "num_rows", None)
 
     def _get_next_page_response(self):
         """Requests the next page from the path provided.

bigquery/tests/unit/test_job.py

@@ -4021,6 +4021,8 @@ def test_result(self):
             "totalRows": "2",
         }
         tabledata_resource = {
+            # Explicitly set totalRows to be different from the query response.
+            # to test update during iteration.
             "totalRows": "1",
             "pageToken": None,
             "rows": [{"f": [{"v": "abc"}]}],
@@ -4038,6 +4040,8 @@ def test_result(self):
         rows = list(result)
         self.assertEqual(len(rows), 1)
         self.assertEqual(rows[0].col1, "abc")
+        # Test that the total_rows property has changed during iteration, based
+        # on the response from tabledata.list.
         self.assertEqual(result.total_rows, 1)
 
     def test_result_w_empty_schema(self):