Class Field

scapy/fields.py:145–306

For more information on how this works, please refer to the 'Adding new protocols' chapter in the online documentation: https://scapy.readthedocs.io/en/stable/build_dissect.html

class Field(Generic[I, M], metaclass=Field_metaclass):
    """
    For more information on how this works, please refer to the
    'Adding new protocols' chapter in the online documentation:

    https://scapy.readthedocs.io/en/stable/build_dissect.html
    """
    __slots__ = [
        "name",
        "fmt",
        "default",
        "sz",
        "owners",
        "struct"
    ]
    islist = 0
    ismutable = False
    holds_packets = 0

    def __init__(self, name, default, fmt="H"):
        # type: (str, Any, str) -> None
        if not isinstance(name, str):
            raise ValueError("name should be a string")
        self.name = name
        if fmt[0] in "@=<>!":
            self.fmt = fmt
        else:
            self.fmt = "!" + fmt
        self.struct = struct.Struct(self.fmt)
        self.default = self.any2i(None, default)
        self.sz = struct.calcsize(self.fmt)  # type: int
        self.owners = []  # type: List[Type[Packet]]

    def register_owner(self, cls):
        # type: (Type[Packet]) -> None
        self.owners.append(cls)

    def i2len(self,
              pkt,  # type: Packet
              x,  # type: Any
              ):
        # type: (...) -> int
        """Convert internal value to a length usable by a FieldLenField"""
        if isinstance(x, RawVal):
            return len(x)
        return self.sz

    def i2count(self, pkt, x):
        # type: (Optional[Packet], I) -> int
        """Convert internal value to a number of elements usable by a FieldLenField.
        Always 1 except for list fields"""
        return 1

    def h2i(self, pkt, x):
        # type: (Optional[Packet], Any) -> I
        """Convert human value to internal value"""
        return cast(I, x)