Pack list of ints in Python

Question

I have got a list that I am packing as bytes using struct module in Python. Here is my list:

[39, 39, 126, 126, 256, 258, 260, 259, 257, 126]

I am packing my list as:

encoded = struct.pack(">{}H".format(len(list)), *list)

where I pass number of elements in list as a format.

Now, I need to unpack the packed struct. For that I will need a format where I again pass number of elements. For now I am doing it like so:

struct.unpack(">{}H".format(10), encoded)

However, I can't pass it as a simple parameter to function format because that struct is then written to file that I am using for compressing image. How can I add a number of elements to file, and unpack it after?

P.S. I would like to get that 10 (in unpacking) from file itself that is packed as bytes.

Answer 1

Form what I understood from the comments and questions. Maybe this will be helpful.

import struct
data = [39, 39, 126, 126, 256, 258, 260, 259, 257, 126]

encoded = struct.pack(">{}H".format(len(data)), *data)
tmp = struct.pack(">H", len(data))
encoded = tmp + encoded #appending at the start
begin = 2
try:
    size = struct.unpack(">H", encoded[0:begin])[0]
    print(size)
    print(struct.unpack(">{}H".format(size), encoded[begin:]))
except Exception as e:
    print(e)

Let me know if it helps.

Answer 2

Here is my approach of adding that [number of elements] to the file:

file.write(len(compressed_list).to_bytes(3,'big'))

I allocate 3 bytes of memory for the length of compressed_list, convert it to bytes, and add it to the beginning of the file. Further, write other left parts.

---

Next, when I need that number, I get it from the file like so:

sz = int.from_bytes(encoded[0:3],'big')

which means that I take first three bytes from byte array read from the file, and typecast that bytes to int.

That solved my problem.