Class ConditionalField

scapy/fields.py:381–433

class ConditionalField(_FieldContainer):
    __slots__ = ["fld", "cond"]

    def __init__(self,
                 fld,  # type: AnyField
                 cond  # type: Callable[[Packet], bool]
                 ):
        # type: (...) -> None
        self.fld = fld
        self.cond = cond

    def _evalcond(self, pkt):
        # type: (Packet) -> bool
        return bool(self.cond(pkt))

    def any2i(self, pkt, x):
        # type: (Optional[Packet], Any) -> Any
        # BACKWARD COMPATIBILITY
        # Note: we shouldn't need this function. (it's not correct)
        # However, having i2h implemented (#2364), it changes the default
        # behavior and broke all packets that wrongly use two ConditionalField
        # with the same name. Those packets are the problem: they are wrongly
        # built (they should either be reusing the same conditional field, or
        # using a MultipleTypeField).
        # But I don't want to dive into fixing all of them just yet,
        # so for now, let's keep this this way, even though it's not correct.
        if type(self.fld) is Field:
            return x
        return self.fld.any2i(pkt, x)

    def i2h(self, pkt, val):
        # type: (Optional[Packet], Any) -> Any
        if pkt and not self._evalcond(pkt):
            return None
        return self.fld.i2h(pkt, val)

    def getfield(self, pkt, s):
        # type: (Packet, bytes) -> Tuple[bytes, Any]
        if self._evalcond(pkt):
            return self.fld.getfield(pkt, s)
        else:
            return s, None

    def addfield(self, pkt, s, val):
        # type: (Packet, bytes, Any) -> bytes
        if self._evalcond(pkt):
            return self.fld.addfield(pkt, s, val)
        else:
            return s

    def __getattr__(self, attr):
        # type: (str) -> Any
        return getattr(self.fld, attr)


class MultipleTypeField(_FieldContainer):