Adding clustering_fields property to TableListItem (googleapis#7692)

* Added property clustering_fields to TableListItem along with test

* Updated tests for clustering fields.

bigquery/google/cloud/bigquery/table.py

@@ -1021,6 +1021,23 @@ def friendly_name(self):
 
     view_use_legacy_sql = property(_view_use_legacy_sql_getter)
 
+    @property
+    def clustering_fields(self):
+        """Union[List[str], None]: Fields defining clustering for the table
+
+        (Defaults to :data:`None`).
+
+        Clustering fields are immutable after table creation.
+
+        .. note::
+
+           As of 2018-06-29, clustering fields cannot be set on a table
+           which does not also have time partioning defined.
+        """
+        prop = self._properties.get("clustering")
+        if prop is not None:
+            return list(prop.get("fields", ()))
+
     @classmethod
     def from_string(cls, full_table_id):
         """Construct a table from fully-qualified table ID.

bigquery/tests/unit/test_table.py

@@ -1148,6 +1148,7 @@ def test_ctor(self):
                 "expirationMs": "10000",
             },
             "labels": {"some-stuff": "this-is-a-label"},
+            "clustering": {"fields": ["string"]},
         }
 
         table = self._make_one(resource)
@@ -1170,6 +1171,7 @@ def test_ctor(self):
         self.assertEqual(table.time_partitioning.field, "mycolumn")
         self.assertEqual(table.labels["some-stuff"], "this-is-a-label")
         self.assertIsNone(table.view_use_legacy_sql)
+        self.assertEqual(table.clustering_fields, ["string"])
 
         with warnings.catch_warnings(record=True) as warned:
             self.assertEqual(table.partitioning_type, "DAY")
@@ -1222,6 +1224,7 @@ def test_ctor_missing_properties(self):
         self.assertEqual(table.table_id, "testtable")
         self.assertIsNone(table.created)
         self.assertIsNone(table.expires)
+        self.assertIsNone(table.clustering_fields)
         self.assertIsNone(table.full_table_id)
         self.assertIsNone(table.friendly_name)
         self.assertIsNone(table.table_type)