I would like to know if it's possible to modify an already made struct.
What I want is to calculate the CRC of header and then write the CRC value to the 3rd position in header.
import struct
import crcmod
crcfunc = crcmod.mkCrcFun(0x112, 0x00, 0x00)
header = struct.pack('!hii', 1, 2, 0)
crcvalue = crcfunc(header)
Since the result of calling struct.pack is to create a byte string, it is immutable.
You could either splice the crc into the byte string by slicing it, or create a new byte string:
header2 = struct.pack('!hii', 1, 2, crcvalue)
Alternatively, if you want to create a mutable byte string, you could create a buffer, and use struct.pack_into instead of struct.pack.
pack_into is the way to go. +1You can not do it directly: struct.pack returns a bytes object, which like str, is immutable. However, you can very easily create the updated object:
header = header[:-4] + c.to_bytes(4, byteorder=sys.byteorder)
If you really meant to compute the CRC value on everything excluding the last element, you can do something like this instead:
header = struct.pack('!hi', 1, 2)
header += crcfunc(header).to_bytes(4, byteorder=sys.byteorder)