def join(sep, *args):
    if len(args) < 2:
        raise IntrinsicFuncInputError(join._errmsg_needinput)

    return {'Fn::Join': [sep, list(args)]}
The intrinsic function Fn::Join appends a set of values into a single     value, separated by the specified delimiter.
dataset/ETHPy150Open seandst/cfn-pyplates/cfn_pyplates/functions.py/join