def deferToThreadPool(reactor, threadpool, f, *args, **kwargs):
    """
    Call the function C{f} using a thread from the given threadpool and return
    the result as a Deferred.

    This function is only used by client code which is maintaining its own
    threadpool.  To run a function in the reactor's threadpool, use
    C{deferToThread}.

    @param reactor: The reactor in whose main thread the Deferred will be
        invoked.

    @param threadpool: An object which supports the C{callInThreadWithCallback}
        method of C{twisted.python.threadpool.ThreadPool}.

    @param f: The function to call.
    @param *args: positional arguments to pass to f.
    @param **kwargs: keyword arguments to pass to f.

    @return: A Deferred which fires a callback with the result of f, or an
        errback with a L{twisted.python.failure.Failure} if f throws an
        exception.
    """
    d = defer.Deferred()

    def onResult(success, result):
        if success:
            reactor.callFromThread(d.callback, result)
        else:
            reactor.callFromThread(d.errback, result)

    threadpool.callInThreadWithCallback(onResult, f, *args, **kwargs)

    return d

    Call the function C{f} using a thread from the given threadpool and return
    the result as a Deferred.

    This function is only used by client code which is maintaining its own
    threadpool.  To run a function in the reactor's threadpool, use
    C{deferToThread}.

    @param reactor: The reactor in whose main thread the Deferred will be
        invoked.

    @param threadpool: An object which supports the C{callInThreadWithCallback}
        method of C{twisted.python.threadpool.ThreadPool}.

    @param f: The function to call.
    @param *args: positional arguments to pass to f.
    @param **kwargs: keyword arguments to pass to f.

    @return: A Deferred which fires a callback with the result of f, or an
        errback with a L{twisted.python.failure.Failure} if f throws an
        exception.
    
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/threads.py