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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 interface(enode, portlbl, addr=None, up=None, shell=None):
 """ Configure a interface. All parameters left as ``None`` are ignored and thus no configuration action is taken for that parameter (left "as-is"). :param enode: Engine node to communicate with. :type enode: topology.platforms.base.BaseNode :param str portlbl: Port label to configure. Port label will be mapped to real port automatically. :param str addr: IPv4 or IPv6 address to add to the interface: - IPv4 address and netmask to assign to the interface in the form ``'192.168.20.20/24'``. - IPv6 address and subnets to assign to the interface in the form ``'2001::1/120'``. :param bool up: Bring up or down the interface. :param str shell: Shell name to execute commands. If ``None``, use the Engine Node default shell. """

    assert portlbl
    port = enode.ports[portlbl]

    if addr is not None:
        assert ip_interface(addr)
        cmd = 'ip addr add {addr} dev {port}'.format(addr=addr, port=port)
        response = enode(cmd, shell=shell)
        assert not response

    if up is not None:
        cmd = 'ip link set dev {port} {state}'.format(
            port=port, state='up' if up else 'down'
        )
        response = enode(cmd, shell=shell)
        assert not response