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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 _get_v4_signed_headers(self):
 """Returns V4 signed get-caller-identity request headers"""

        if self.aws_session is None:
            boto_session = session.Session()
            creds = boto_session.get_credentials()
        else:
            creds = self.aws_session.get_credentials()
        if creds is None:
            raise CerberusClientException("Unable to locate AWS credentials")
        readonly_credentials = creds.get_frozen_credentials()

        # hardcode get-caller-identity request
        data = OrderedDict((('Action','GetCallerIdentity'), ('Version', '2011-06-15')))
        url = 'https://sts.{}.amazonaws.com/'.format(self.region)
        request_object = awsrequest.AWSRequest(method='POST', url=url, data=data)

        signer = auth.SigV4Auth(readonly_credentials, 'sts', self.region)
        signer.add_auth(request_object)
        return request_object.headers