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AttributeError in /usr/lib/python3.4/asyncio/tasks.py feels like it might be a concurrency issue. I can't reproduce it, and my python isn't good enough to know how to simulate raising the exception at a random time during the execution of the program. Here's the PoC:
import asyncio
import sys
from asyncio import async
import time
import random
asyncio.tasks._DEBUG = True
loop = asyncio.get_event_loop()
def read_something():
 print(input())
@asyncio.coroutine
def func(arg):
 while True:
 sys.stdout.write("\rtest"+str(arg))
 yield from asyncio.sleep(0)
loop.add_reader(sys.stdin, read_something)
loop.call_soon(async, func(1))
loop.call_soon(async, func(2))
loop.call_soon(async, func(3))
loop.call_soon(async, func(4))
time.sleep(1)
try:
 loop.run_forever()
except KeyboardInterrupt:
 print("handled\n")
 pass
finally:
 pass
and here is the stack trace:
ktn:~/ $ python asynctest.py [11:55:03]
test3^Chandled
Future/Task exception was never retrieved
future: Task(<func>)<exception=KeyboardInterrupt()>
Traceback (most recent call last):
 File "asynctest.py", line 29, in <module>
 loop.run_forever()
 File "/usr/lib/python3.4/asyncio/base_events.py", line 184, in run_forever
 self._run_once()
 File "/usr/lib/python3.4/asyncio/base_events.py", line 800, in _run_once
 handle._run()
 File "/usr/lib/python3.4/asyncio/events.py", line 39, in _run
 self._callback(*self._args)
 File "/usr/lib/python3.4/asyncio/tasks.py", line 337, in _wakeup
 self._step(value, None)
 File "/usr/lib/python3.4/asyncio/tasks.py", line 283, in _step
 result = next(coro)
 File "/usr/lib/python3.4/asyncio/tasks.py", line 50, in __next__
 return next(self.gen)
 File "asynctest.py", line 18, in func
 yield from asyncio.sleep(0)
 File "/usr/lib/python3.4/asyncio/tasks.py", line 94, in wrapper
 w = CoroWrapper(coro(*args, **kwds), func)
 File "/usr/lib/python3.4/asyncio/tasks.py", line 42, in __init__
 assert inspect.isgenerator(gen), gen
KeyboardInterrupt
Exception ignored in: <bound method CoroWrapper.__del__ of <asyncio.tasks.CoroWrapper object at 0x7f0dedaf79d8>>
Traceback (most recent call last):
 File "/usr/lib/python3.4/asyncio/tasks.py", line 62, in __del__
 frame = self.gen.gi_frame
AttributeError: gen

Looks like there is a bug in CoroWrapper -- when the assert in __init__ fails, __del__ gets called imediately after and that triggers this traceback.
However I'm not sure what causes the assert to fail -- it looks like this is coming from sleep(), which does not make sense to me.

Oh wait, it looks like the assert failed because KeyboardInterrupt hit right at that point. I ran the program a few times and when I hit ^C I get a traceback at a different point in the code each time. This is as expected. You must have hit the rare case where it hit right at the assert -- then the behavior I described can happen.
Anyway, I think this would fix it:
--- a/asyncio/tasks.py Fri Apr 18 09:51:35 2014 -0700
+++ b/asyncio/tasks.py Thu Apr 24 23:44:57 2014 -0700
@@ -76,7 +76,8 @@
 return self.gen.gi_code
 def __del__(self):
- frame = self.gen.gi_frame
+ gen = getattr(self, 'gen', None)
+ frame = getattr(gen, 'gi_frame', None)
 if frame is not None and frame.f_lasti == -1:
 func = self.func
 code = func.__code__

New changeset d42d3d3f9c41 by Guido van Rossum in branch '3.4':
asyncio: Be careful accessing instance variables in __del__ (closes #21340).
http://hg.python.org/cpython/rev/d42d3d3f9c41
New changeset 0cb436c6f082 by Guido van Rossum in branch 'default':
Merge 3.4 -> default: asyncio: Be careful accessing instance variables in __del__ (closes #21340).
http://hg.python.org/cpython/rev/0cb436c6f082 

Why not using try/except AttributeError?

Sorry, should have let someone review it. I'm a bit out of practice. :-)
But in this case I think three-arg getattr() is better; less code, less indentation, and the final question (is the frame not None?) must still be asked.