cp = set()
            cp |= self.pending
            for f in cp:
                f.cancel()

  File "/usr/local/lib/python3.2/concurrent/futures/_base.py", line 391, in result
    raise CancelledError()
concurrent.futures._base.CancelledError

AttributeError: 'list' object has no attribute 'add'
AttributeError: 'list' object has no attribute 'discard'

for f in set(self.pending):
    if not f.cancelled():
        f.cancel()