Issue #11777: Executor.map does not submit futures until iter.next() is called
diff --git a/Lib/concurrent/futures/_base.py b/Lib/concurrent/futures/_base.py
index 79b91d4..6cfded3 100644
--- a/Lib/concurrent/futures/_base.py
+++ b/Lib/concurrent/futures/_base.py
@@ -536,15 +536,19 @@
fs = [self.submit(fn, *args) for args in zip(*iterables)]
- try:
- for future in fs:
- if timeout is None:
- yield future.result()
- else:
- yield future.result(end_time - time.time())
- finally:
- for future in fs:
- future.cancel()
+ # 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:
+ yield future.result()
+ else:
+ yield future.result(end_time - time.time())
+ finally:
+ for future in fs:
+ future.cancel()
+ return result_iterator()
def shutdown(self, wait=True):
"""Clean-up the resources associated with the Executor.