bpo-31033: Improve the traceback for cancelled asyncio tasks (GH-19951)
When an asyncio.Task is cancelled, the exception traceback now
starts with where the task was first interrupted. Previously,
the traceback only had "depth one."
diff --git a/Lib/asyncio/futures.py b/Lib/asyncio/futures.py
index 889f3e6..bed4da5 100644
--- a/Lib/asyncio/futures.py
+++ b/Lib/asyncio/futures.py
@@ -52,6 +52,8 @@
_loop = None
_source_traceback = None
_cancel_message = None
+ # A saved CancelledError for later chaining as an exception context.
+ _cancelled_exc = None
# This field is used for a dual purpose:
# - Its presence is a marker to declare that a class implements
@@ -124,6 +126,21 @@
raise RuntimeError("Future object is not initialized.")
return loop
+ def _make_cancelled_error(self):
+ """Create the CancelledError to raise if the Future is cancelled.
+
+ This should only be called once when handling a cancellation since
+ it erases the saved context exception value.
+ """
+ if self._cancel_message is None:
+ exc = exceptions.CancelledError()
+ else:
+ exc = exceptions.CancelledError(self._cancel_message)
+ exc.__context__ = self._cancelled_exc
+ # Remove the reference since we don't need this anymore.
+ self._cancelled_exc = None
+ return exc
+
def cancel(self, msg=None):
"""Cancel the future and schedule callbacks.
@@ -175,9 +192,8 @@
the future is done and has an exception set, this exception is raised.
"""
if self._state == _CANCELLED:
- raise exceptions.CancelledError(
- '' if self._cancel_message is None else self._cancel_message)
-
+ exc = self._make_cancelled_error()
+ raise exc
if self._state != _FINISHED:
raise exceptions.InvalidStateError('Result is not ready.')
self.__log_traceback = False
@@ -194,8 +210,8 @@
InvalidStateError.
"""
if self._state == _CANCELLED:
- raise exceptions.CancelledError(
- '' if self._cancel_message is None else self._cancel_message)
+ exc = self._make_cancelled_error()
+ raise exc
if self._state != _FINISHED:
raise exceptions.InvalidStateError('Exception is not set.')
self.__log_traceback = False