bpo-20369: concurrent.futures.wait() now deduplicates futures given a… (GH-30168)

* bpo-20369: concurrent.futures.wait() now deduplicates futures given as arg.

* 📜🤖 Added by blurb_it.

Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>

1 files changed, 7 insertions, 6 deletions

diff --git a/Lib/concurrent/futures/_base.py b/Lib/concurrent/futures/_base.py
index b033739..c5912c2 100644
--- a/Lib/concurrent/futures/_base.py
+++ b/Lib/concurrent/futures/_base.py
@@ -282,13 +282,14 @@ def wait(fs, timeout=None, return_when=ALL_COMPLETED):
         A named 2-tuple of sets. The first set, named 'done', contains the
         futures that completed (is finished or cancelled) before the wait
         completed. The second set, named 'not_done', contains uncompleted
-        futures.
+        futures. Duplicate futures given to *fs* are removed and will be 
+        returned only once.
     """
+    fs = set(fs)
     with _AcquireFutures(fs):
-        done = set(f for f in fs
-                   if f._state in [CANCELLED_AND_NOTIFIED, FINISHED])
-        not_done = set(fs) - done
-
+        done = {f for f in fs
+                   if f._state in [CANCELLED_AND_NOTIFIED, FINISHED]}
+        not_done = fs - done
         if (return_when == FIRST_COMPLETED) and done:
             return DoneAndNotDoneFutures(done, not_done)
         elif (return_when == FIRST_EXCEPTION) and done:
@@ -307,7 +308,7 @@ def wait(fs, timeout=None, return_when=ALL_COMPLETED):
             f._waiters.remove(waiter)
 
     done.update(waiter.finished_futures)
-    return DoneAndNotDoneFutures(done, set(fs) - done)
+    return DoneAndNotDoneFutures(done, fs - done)
 
 class Future(object):
     """Represents the result of an asynchronous computation."""