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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:
 def create_signed_url(self, url, keypair_id, expire_time=None, valid_after_time=None, ip_address=None, policy_url=None, private_key_file=None, private_key_string=None):
 """ Creates a signed CloudFront URL that is only valid within the specified parameters. :type url: str :param url: The URL of the protected object. :type keypair_id: str :param keypair_id: The keypair ID of the Amazon KeyPair used to sign theURL. This ID MUST correspond to the private key specified with private_key_file or private_key_string. :type expire_time: int :param expire_time: The expiry time of the URL. If provided, the URL will expire after the time has passed. If not provided the URL will never expire. Format is a unix epoch. Use time.time() + duration_in_sec. :type valid_after_time: int :param valid_after_time: If provided, the URL will not be valid until after valid_after_time. Format is a unix epoch. Use time.time() + secs_until_valid. :type ip_address: str :param ip_address: If provided, only allows access from the specified IP address. Use '192.168.0.10' for a single IP or use '192.168.0.0/24' CIDR notation for a subnet. :type policy_url: str :param policy_url: If provided, allows the signature to contain wildcard globs in the URL. For example, you could provide: 'http://example.com/media/\*' and the policy and signature would allow access to all contents of the media subdirectory. If not specified, only allow access to the exact url provided in 'url'. :type private_key_file: str or file object. :param private_key_file: If provided, contains the filename of the private key file used for signing or an open file object containing the private key contents. Only one of private_key_file or private_key_string can be provided. :type private_key_string: str :param private_key_string: If provided, contains the private key string used for signing. Only one of private_key_file or private_key_string can be provided. :rtype: str :return: The signed URL. """

        # Get the required parameters
        params = self._create_signing_params(
                     url=url, keypair_id=keypair_id, expire_time=expire_time,
                     valid_after_time=valid_after_time, ip_address=ip_address,
                     policy_url=policy_url, private_key_file=private_key_file,
                     private_key_string=private_key_string)

        #combine these into a full url
        if "?" in url:
            sep = "&"
        else:
            sep = "?"
        signed_url_params = []
        for key in ["Expires", "Policy", "Signature", "Key-Pair-Id"]:
            if key in params:
                param = "%s=%s" % (key, params[key])
                signed_url_params.append(param)
        signed_url = url + sep + "&".join(signed_url_params)
        return signed_url