Fix bigquery bulk complete failing (#2441)

* bugfix bigquery bulk complete failing when argument is a generator

luigi/contrib/bigquery.py

@@ -420,11 +420,10 @@ class MixinBigQueryBulkComplete(object):
 
     @classmethod
     def bulk_complete(cls, parameter_tuples):
-        if len(parameter_tuples) < 1:
-            return
-
         # Instantiate the tasks to inspect them
         tasks_with_params = [(cls(p), p) for p in parameter_tuples]
+        if not tasks_with_params:
+            return
 
         # Grab the set of BigQuery datasets we are interested in
         datasets = {t.output().table.dataset for t, p in tasks_with_params}

test/contrib/bigquery_test.py

@@ -135,6 +135,14 @@ def test_bulk_complete(self):
         complete = list(TestRunQueryTask.bulk_complete(parameters))
         self.assertEqual(complete, ['table2'])
 
+        # Test that bulk_complete accepts lazy sequences in addition to lists
+        def parameters_gen():
+            yield 'table1'
+            yield 'table2'
+
+        complete = list(TestRunQueryTask.bulk_complete(parameters_gen()))
+        self.assertEqual(complete, ['table2'])
+
     def test_dataset_doesnt_exist(self):
         client = MagicMock()
         client.dataset_exists.return_value = False