Some helpers for PEP 3156 Futures.

gistfile1.py

def link(f, other):
    '''Resolve other with the current Future.'''

    def on_done(f):
        e = f.exception()
        if e:
            other.set_exception(e)
        else:
            other.set_result(f.result())

    f.add_done_callback(on_done)

def cancel_on_cond(f, cond, callback):
    ''' Cancel the future if condition is fulfilled else it
    processes the callback.
    '''
    ret_f = Future()

    def on_done(f):
        # sane exception handling ?!
        r = f.result()
        if cond(r):
            ret_f.cancel()
        else:
            inner_f = callback(r)
            link(inner_f, ret_f)

    f.add_done_callback(on_done)

    return ret_f

def next(f, callback):
    '''Filter the result of the Future and pass it to
    the callback.
    '''
    ret_f = Future()

    def on_done(f):
        r = f.result()
        inner_f = callback(r)
        link(inner_f, ret_f)

    return ret_f