gistfile1.py

VlanTagOrType = Struct("VlanTagOrType",
    Enum(UBInt16("type"),
        IPv4 = 0x0800,
        ARP = 0x0806,
        RARP = 0x8035,
        X25 = 0x0805,
        IPX = 0x8137,
        IPv6 = 0x86DD,
        VLAN_TAG = 0x8100,
        _default_ = Pass,
    ),
    If(lambda ctx: ctx.type == "VLAN_TAG",
        Struct("VlanTag",
            UBInt16("whatever_the_tag_is"),
            LazyBound(lambda: VlanTagOrType)
        )
    )
)

ethernet_header = Struct("ethernet_header",
    MacAddress("destination"),
    MacAddress("source"),
    VlanTagOrType,
)