bpo-35930: Raising an exception raised in a "future" instance will create reference cycles (#24995)

Before: 0c14bc58/attachment-0002.png

After: 0c14bc58/attachment-0003.png

Lib/concurrent/futures/_base.py

@@ -386,7 +386,11 @@ class Future(object):
 
     def __get_result(self):
         if self._exception:
+            try:
                 raise self._exception
+            finally:
+                # Break a reference cycle with the exception in self._exception
+                self = None
         else:
             return self._result
 
@@ -426,6 +430,7 @@ class Future(object):
                 timeout.
             Exception: If the call raised then that exception will be raised.
         """
+        try:
             with self._condition:
                 if self._state in [CANCELLED, CANCELLED_AND_NOTIFIED]:
                     raise CancelledError()
@@ -440,6 +445,9 @@ class Future(object):
                     return self.__get_result()
                 else:
                     raise TimeoutError()
+        finally:
+            # Break a reference cycle with the exception in self._exception
+            self = None
 
     def exception(self, timeout=None):
         """Return the exception raised by the call that the future represents.

Misc/NEWS.d/next/Library/2021-03-23-17-18-56.bpo-35930.RZ51pM.rst

@@ -0,0 +1,2 @@
+Raising an exception raised in a "future" instance will create reference
+cycles.