Optional exception argument to Future.cancel()

asyncio/futures.py

Index: asyncio/futures.py 
=================================================================== 
--- a/asyncio/futures.py 
+++ b/asyncio/futures.py 
@@ -183,7 +183,7 @@ 
 } 
 self._loop.call_exception_handler(context) 
 
- def cancel(self): 
+ def cancel(self, exc=None): 
 """Cancel the future and schedule callbacks. 
 
 If the future is already done or cancelled, return False. Otherwise, 
@@ -192,6 +192,13 @@ 
 """ 
 if self._state != _PENDING: 
 return False 
+ if exc is None: 
+ exc = CancelledError 
+ elif (not isinstance(exc, CancelledError) and 
+ not isinstance(exc, type) and 
+ not issubclass(exc, CancelledError)): 
+ raise ValueError('exc must subclass CancelledError') 
+ self._exception = exc 
 self._state = _CANCELLED 
 self._schedule_callbacks() 
 return True 
@@ -232,7 +239,7 @@ 
 the future is done and has an exception set, this exception is raised. 
 """ 
 if self._state == _CANCELLED: 
- raise CancelledError 
+ raise self._exception 
 if self._state != _FINISHED: 
 raise InvalidStateError('Result is not ready.') 
 self._log_traceback = False 
@@ -252,7 +259,7 @@ 
 InvalidStateError. 
 """ 
 if self._state == _CANCELLED: 
- raise CancelledError 
+ raise self._exception 
 if self._state != _FINISHED: 
 raise InvalidStateError('Exception is not set.') 
 self._log_traceback = False 
@@ -333,7 +340,7 @@ 
 return 
 assert not self.done() 
 if other.cancelled(): 
- self.cancel() 
+ self.cancel(self._exception) 
 else: 
 exception = other.exception() 
 if exception is not None: 
@@ -362,6 +369,8 @@ 
 
 def _check_cancel_other(f): 
 if f.cancelled(): 
+ # concurrent.futures.Future.cancel() doesn't support custom 
+ # exceptions so we can't pass f._exception here. 
 fut.cancel() 
 
 new_future.add_done_callback(_check_cancel_other)