python encode() and decode() issues

January 7, 2022

Can anyone help me with this issue please
encode method is not working and i cannot discover why

def encode_OctetString(A,flags,data):
    fs="!"+str(len(data))+"s"
    dbg="Encoding String format:",fs
    logging.debug(dbg)
    ret=struct.pack(fs,data).encode("hex")
    pktlen=8+len(ret)/2
    return encode_finish(A,flags,pktlen,ret

error code

 File "/home/ubuntu/diameter-test/libDiameter.py", line 434, in encode_OctetString
    ret=struct.pack(fs,data).encode("hex")
struct.error: argument for 's' must be a bytes object

>Solution :

Note that struct.pack() will normally return a bytes and bytes don’t have an .encode() method, just a .decode() one – were you trying to first decode and then re-encode as "hex"?

If so, you’ll need to import codecs.encode() and apply that to the bytes object:

import struct
import logging
import codecs


def encode_OctetString(A, flags, data):
    fs = "!" + str(len(data)) + "s"
    dbg = "Encoding String format:", fs
    logging.debug(dbg)
    ret = codecs.encode(struct.pack(fs, data), "hex")
    pktlen = 8 + len(ret) / 2
    return encode_finish(A, flags, pktlen, ret)

If you somehow intended encode_OctetString to work on strings instead of bytes objects, you’d use:

def encode_OctetString(A, flags, data):
    fs = "!" + str(len(data)) + "s"
    dbg = "Encoding String format:", fs
    logging.debug(dbg)
    ret = codecs.encode(struct.pack(fs, data.encode()), "hex")
    pktlen = 8 + len(ret) / 2
    return encode_finish(A, flags, pktlen, ret)

Given the link to the original code, that depends on the expected type for AVP_Value in encodeAVP – which is never called in the shared code, nor is its type documented and no type hint is provided, so it’s impossible to say.