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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 async def build_get_ddo_request(submitter_did: Optional[str], target_did: str) -> str: """ Builds a request to get a DDO. :param submitter_did: (Optional) DID of the read request sender (if not provided then default Libindy DID will be used). :param target_did: Id of Identity stored in secured Wallet. :return: Request result as json. """


    logger = logging.getLogger(__name__)
    logger.debug("build_get_ddo_request: >>> submitter_did: %r, target_did: %r",
                 submitter_did,
                 target_did)

    if not hasattr(build_get_ddo_request, "cb"):
        logger.debug("build_get_ddo_request: Creating callback")
        build_get_ddo_request.cb = create_cb(CFUNCTYPE(None, c_int32, c_int32, c_char_p))

    c_submitter_did = c_char_p(submitter_did.encode('utf-8')) if submitter_did is not None else None
    c_target_did = c_char_p(target_did.encode('utf-8'))

    request_json = await do_call('indy_build_get_ddo_request',
                                 c_submitter_did,
                                 c_target_did,
                                 build_get_ddo_request.cb)

    res = request_json.decode()
    logger.debug("build_get_ddo_request: <<< res: %r", res)
    return res