Question

Below Are Struct from C,and i m trying Convert to Python,
and use Socket to sending out the struct

C:

        struct CommandReq
        {
            char    sMark[6];  //start flag\r\n*KW
            short   nPackLen;  //packet length
            short   nFlag;     //command ID 0x0002
            int     nGisIp;    //GIS port
            short   nPort;     //GIS Port
            char    sData[50]; //command string
            char    sEnd[2];   //end flag "\r\n"
        };
        //source code
        CommandReq stResq;
        memset(&stResq, 0, sizeof(stResq));
        sprintf(stResq.sMark, "\r\n%s", "*KW");
        stResq.nFlag = 0x0002;
        stResq.nPackLen = sizeof(stResq);
        stResq.nGisIp = 0;
        stResq.nPort = 0;
        strcpy(stResq.sData, "*KW,CC09C00001,015,080756,#");
        strncpy(stResq.sEnd, "\r\n", 2);

I have created Python struct using namedtuple,and use socket to send this struct.
but unfortunately failed.

Python:

from collections import namedtuple
MyStruct = namedtuple("MyStruct", "sMark nPackLen nFlag nGisIp nPort sData sEnd")

Edit, in reply to answer below

after figure out, python 3.x must be added string .encode(“ascii”)

      format_ = "6shhih50s2s"
      MyStruct = namedtuple("MyStruct", "sMark nPackLen nFlag nGisIp nPort sData sEnd")
      tuple_to_send = MyStruct(sMark="\r\n{}".format("*KW").encode("ascii"), 
                 nPackLen=struct.calcsize(format_),
                 nFlag=0x0002,
                 nGisIp=0,
                 nPort=0,
                 sData= "*KW,NR09G05133,015,080756,#".encode("ascii"),
                 sEnd="\r\n".encode("ascii"))
      string_to_send = struct.pack(format_, *tuple_to_send._asdict().values()
      socket.sendto(string_to_send, self.client_address)

Additional Question

below is the packet are sending out with the struct

format_ = "6shhih50s2s" //total bytes should be 68 bytes?
    0d0a2a4b5700 //6 char \r\n*KW
    4600 // packet length 70 bytes
    0200 //command ID 0x0002,short 2 bytes
    00000000 //GIS port is integer 4 bytes
    0000 //GIS Port short 2 bytes
    0000 //unknown??what is this?
    2a4b572c4e5230394730353133332c3031352c3038303735362c230000000000000000000000000000000000000000000000 //sData[50] char 50 bytes
    0d0a //sEnd 2 char,2 bytes

how come the 2 unknown bytes??

Answer 1

namedtuple isn’t really comparable to a c-struct. If you are used to using structs, you can have a look at the struct-module to convert structured information to a string.

In general, Pythonistas prefer using the pickle-module for serialization.

from collections import namedtuple
import pickle # or cPickle, it's faster
MyStruct = namedtuple("MyStruct", "sMark nPackLen nFlag nGisIp nPort sData sEnd")

tuple_to_send = MyStruct(sMark="abcdef", nPackLen=...)
string_to_send = pickle.dumps(tuple_to_send)

There are two flavors of pickle, pickle and cPickle. The latter is faster, but is only available in CPython (which most programmers use), while pickle is also available in Jython, IronPython, …

If you want to stick to struct (e.g., because the other side expects this format), your format string will be

format_ = "6shhih50s2s"

So you can do:

import struct
from collections import namedtuple

format_ = "6shhih50s2s"

MyStruct = namedtuple("MyStruct", "sMark nPackLen nFlag nGisIp nPort sData sEnd")
tuple_to_send = MyStruct(sMark="\r\n{}".format("*KW"), 
                     nPackLen=struct.calcsize(format_),
                     nFlag=0x0002,
                     nGisIp=0,
                     nPort=0,
                     sData= "*KW,NR09G05133,015,080756,#",
                     sEnd="\r\n")


string_to_send = struct.pack(format_, *tuple_to_send._asdict().values())

BTW: namedtuples are immutable, i.e., you cannot do

tuple_to_send.sMark = "bcdefg"

to change some property. If you need such behavior, you must create a class.

Edit

For Python 3, string-treatment has changed. Conversion from unicode to bytes has to be performed, e.g.

import struct
from collections import namedtuple

format_ = "6shhih50s2s"

MyStruct = namedtuple("MyStruct", "sMark nPackLen nFlag nGisIp nPort sData sEnd")
tuple_to_send = MyStruct(sMark="\r\n{}".format("*KW").encode("ascii"), 
                     nPackLen=struct.calcsize(format_),
                     nFlag=0x0002,
                     nGisIp=0,
                     nPort=0,
                     sData= "*KW,NR09G05133,015,080756,#".encode("ascii"),
                     sEnd=b"\r\n")


string_to_send = struct.pack(format_, *tuple_to_send._asdict().values())