Insure concurrent execution of zero tasks returns immediately.

fixes #85

PiperOrigin-RevId: 678658517
Change-Id: If1f6616706b8daece557d302d41ed7e3ff5eb9ba

concordia/utils/concurrency.py

@@ -85,6 +85,8 @@ def _as_completed(
   Raises:
     TimeoutError: If all the results are not generated before the timeout.
   """
+  if not tasks:
+    return
   if max_workers is None:
     max_workers = len(tasks)
   with _executor(max_workers=max_workers) as executor_:

concordia/utils/concurrency_test.py

@@ -101,9 +101,13 @@ def test_run_tasks_in_background(self):
     with self.subTest('errors'):
       self.assertEqual(
           {key: type(error) for key, error in errors.items()},
-          {'error': ExpectedError, 'wait': TimeoutError}
+          {'error': ExpectedError, 'wait': TimeoutError},
       )
 
+  def test_run_tasks_empty(self):
+    results = concurrency.run_tasks({})
+    self.assertEmpty(results)
+
   def test_map_parallel(self):
     results = concurrency.map_parallel(
         return_after, [1, 0.5, 0.1], ['a', 'b', 'c']