Make None msg result in no arguments.

Author: cjerdonek

Lib/asyncio/futures.py

@@ -175,7 +175,7 @@ def result(self):
         the future is done and has an exception set, this exception is raised.
         """
         if self._state == _CANCELLED:
-            raise exceptions.CancelledError(self._cancel_message)
+            raise _create_cancelled_error(self._cancel_message)
         if self._state != _FINISHED:
             raise exceptions.InvalidStateError('Result is not ready.')
         self.__log_traceback = False
@@ -192,7 +192,7 @@ def exception(self):
         InvalidStateError.
         """
         if self._state == _CANCELLED:
-            raise exceptions.CancelledError(self._cancel_message)
+            raise _create_cancelled_error(self._cancel_message)
         if self._state != _FINISHED:
             raise exceptions.InvalidStateError('Exception is not set.')
         self.__log_traceback = False
@@ -293,6 +293,15 @@ def _set_result_unless_cancelled(fut, result):
     fut.set_result(result)
 
 
+# This provides a nicer-looking traceback when no msg is passed, which
+# has been asyncio's default behavior.
+def _create_cancelled_error(msg=None):
+    if msg is None:
+        return exceptions.CancelledError()
+
+    return exceptions.CancelledError(msg)
+
+
 def _convert_future_exc(exc):
     exc_class = type(exc)
     if exc_class is concurrent.futures.CancelledError:

Lib/asyncio/tasks.py

@@ -270,7 +270,7 @@ def __step(self, exc=None):
                 f'_step(): already done: {self!r}, {exc!r}')
         if self._must_cancel:
             if not isinstance(exc, exceptions.CancelledError):
-                exc = exceptions.CancelledError(self._cancel_message)
+                exc = futures._create_cancelled_error(self._cancel_message)
             self._must_cancel = False
         coro = self._coro
         self._fut_waiter = None
@@ -778,7 +778,7 @@ def _done_callback(fut):
                 # Check if 'fut' is cancelled first, as
                 # 'fut.exception()' will *raise* a CancelledError
                 # instead of returning it.
-                exc = exceptions.CancelledError(fut._cancel_message)
+                exc = futures._create_cancelled_error(fut._cancel_message)
                 outer.set_exception(exc)
                 return
             else:
@@ -797,7 +797,7 @@ def _done_callback(fut):
                     # Check if 'fut' is cancelled first, as
                     # 'fut.exception()' will *raise* a CancelledError
                     # instead of returning it.
-                    res = exceptions.CancelledError(fut._cancel_message)
+                    res = futures._create_cancelled_error(fut._cancel_message)
                 else:
                     res = fut.exception()
                     if res is None:
@@ -809,7 +809,7 @@ def _done_callback(fut):
                 # cancellation regardless of *return_exceptions* argument.
                 # See issue 32684.
                 outer.set_exception(
-                    exceptions.CancelledError(fut._cancel_message))
+                    futures._create_cancelled_error(fut._cancel_message))
             else:
                 outer.set_result(results)
 

Lib/test/test_asyncio/test_tasks.py

@@ -517,8 +517,8 @@ async def task():
     def test_cancel_with_message_then_future_result(self):
         # Test Future.result() after calling cancel() with a message.
         cases = [
-            ((), (None,)),
-            ((None,), (None,)),
+            ((), ()),
+            ((None,), ()),
             (('my message',), ('my message',)),
             # Non-string values should roundtrip.
             ((5,), (5,)),
@@ -547,8 +547,8 @@ async def coro():
     def test_cancel_with_message_then_future_exception(self):
         # Test Future.exception() after calling cancel() with a message.
         cases = [
-            ((), (None,)),
-            ((None,), (None,)),
+            ((), ()),
+            ((None,), ()),
             (('my message',), ('my message',)),
             # Non-string values should roundtrip.
             ((5,), (5,)),
@@ -2319,8 +2319,8 @@ def cancelling_callback(_):
 
     def test_cancel_gather_2(self):
         cases = [
-            ((), (None,)),
-            ((None,), (None,)),
+            ((), ()),
+            ((None,), ()),
             (('my message',), ('my message',)),
             # Non-string values should roundtrip.
             ((5,), (5,)),

Modules/_asynciomodule.c

@@ -599,11 +599,12 @@ future_set_exception(FutureObj *fut, PyObject *exc)
 static PyObject *
 create_cancelled_error(PyObject *msg)
 {
-    if (msg == NULL) {
-        msg = Py_None;
+    PyObject *exc;
+    if (msg == NULL || msg == Py_None) {
+        exc = PyObject_CallNoArgs(asyncio_CancelledError);
+    } else {
+        exc = PyObject_CallOneArg(asyncio_CancelledError, msg);
     }
-    PyObject *exc = PyObject_CallOneArg(asyncio_CancelledError, msg);
-
     return exc;
 }