Class FieldLenField

scapy/fields.py:2178–2212

class FieldLenField(Field[int, int]):
    __slots__ = ["length_of", "count_of", "adjust"]

    def __init__(
            self,
            name,  # type: str
            default,  # type: Optional[Any]
            length_of=None,  # type: Optional[str]
            fmt="H",  # type: str
            count_of=None,  # type: Optional[str]
            adjust=lambda pkt, x: x,  # type: Callable[[Packet, int], int]
    ):
        # type: (...) -> None
        Field.__init__(self, name, default, fmt)
        self.length_of = length_of
        self.count_of = count_of
        self.adjust = adjust

    def i2m(self, pkt, x):
        # type: (Optional[Packet], Optional[int]) -> int
        if x is None and pkt is not None:
            if self.length_of is not None:
                fld, fval = pkt.getfield_and_val(self.length_of)
                f = fld.i2len(pkt, fval)
            elif self.count_of is not None:
                fld, fval = pkt.getfield_and_val(self.count_of)
                f = fld.i2count(pkt, fval)
            else:
                raise ValueError(
                    "Field should have either length_of or count_of"
                )
            x = self.adjust(pkt, f)
        elif x is None:
            x = 0
        return x


class StrNullField(StrField):