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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_role_arn():
 """ Return role arn from X-Role-ARN header, lookup role arn from source IP, or fall back to command line default. """

    role_arn = bottle.request.headers.get('X-Role-ARN')
    if not role_arn:
        role_arn = _lookup_ip_role_arn(bottle.request.environ.get('REMOTE_ADDR'))
    if not role_arn:
        role_arn = _role_arn
    return role_arn