Question

I’m trying to use the pack function in the struct module to encode data into formats required by a network protocol. I’ve run into a problem in that I don’t see any way to encode arrays of anything other than 8-bit characters.

For example, to encode “TEST”, I can use format specifier “4s”. But how do I encode an array or list of 32-bit integers or other non-string types?

Here is a concrete example. Suppose I have a function doEncode which takes an array of 32-bit values. The protocol requires a 32-bit length field, followed by the array itself. Here is what I have been able to come up with so far.

from array import *
from struct import *

def doEncode(arr):
    bin=pack('>i'+len(arr)*'I',len(arr), ???)

arr=array('I',[1,2,3])
doEncode(arr)

The best I have been able to come up with is generating a format to the pack string dynamically from the length of the array. Is there some way of specifying that I have an array so I don’t need to do this, like there is with a string (which e.g. would be pack('>i'+len(arr)+'s')?

Even with the above approach, I’m not sure how I would go about actually passing the elements in the array in a similar dynamic way, i.e. I can’t just say , arr[0], arr[1], ... because I don’t know ahead of time what the length will be.

I suppose I could just pack each individual integer in the array in a loop, and then join all the results together, but this seems like a hack. Is there some better way to do this? The array and struct modules each seem to do their own thing, but in this case what I’m trying to do is a combination of both, which neither wants to do.

Answer 1

data = pack('>i', len(arr)) + arr.tostring()