Issue #11777: Executor.map does not submit futures until iter.next() is called

Lib/concurrent/futures/_base.py

@@ -536,6 +536,9 @@ class Executor(object):
 
         fs = [self.submit(fn, *args) for args in zip(*iterables)]
 
+        # Yield must be hidden in closure so that the futures are submitted
+        # before the first iterator value is required.
+        def result_iterator():
             try:
                 for future in fs:
                     if timeout is None:
@@ -545,6 +548,7 @@ class Executor(object):
             finally:
                 for future in fs:
                     future.cancel()
+        return result_iterator()
 
     def shutdown(self, wait=True):
         """Clean-up the resources associated with the Executor.

Lib/test/test_concurrent_futures.py

@@ -369,7 +369,15 @@ class ExecutorTest(unittest.TestCase):
 
 
 class ThreadPoolExecutorTest(ThreadPoolMixin, ExecutorTest):
-    pass
+    def test_map_submits_without_iteration(self):
+        """Tests verifying issue 11777."""
+        finished = []
+        def record_finished(n):
+            finished.append(n)
+
+        self.executor.map(record_finished, range(10))
+        self.executor.shutdown(wait=True)
+        self.assertCountEqual(finished, range(10))
 
 
 class ProcessPoolExecutorTest(ProcessPoolMixin, ExecutorTest):