Concurrently converts a generator object of Requests to
    a generator of Responses.

    :param requests: a generator of Request objects.
    :param stream: If True, the content will not be downloaded immediately.
    :param size: Specifies the number of requests to make at a time. default is 2
    :param exception_handler: Callback function, called when exception occured. Params: Request, Exception

def imap(requests, stream=False, size=2, exception_handler=None):
    """Concurrently converts a generator object of Requests to
    a generator of Responses.

    :param requests: a generator of Request objects.
    :param stream: If True, the content will not be downloaded immediately.
    :param size: Specifies the number of requests to make at a time. default is 2
    :param exception_handler: Callback function, called when exception occured. Params: Request, Exception
    """

    pool = Pool(size)

    def send(r):
        return r.send(stream=stream)

    for request in pool.imap_unordered(send, requests):
        if request.response is not None:
            yield request.response
        elif exception_handler:
            ex_result = exception_handler(request, request.exception)
            if ex_result is not None:
                yield ex_result

    pool.join()