Python Language Binary Data Packing a structure
Example
The module "struct" provides facility to pack python objects as contiguous chunk of bytes or dissemble a chunk of bytes to python structures.
The pack function takes a format string and one or more arguments, and returns a binary string. This looks very much like you are formatting a string except that the output is not a string but a chunk of bytes.
import struct
import sys
print "Native byteorder: ", sys.byteorder
# If no byteorder is specified, native byteorder is used
buffer = struct.pack("ihb", 3, 4, 5)
print "Byte chunk: ", repr(buffer)
print "Byte chunk unpacked: ", struct.unpack("ihb", buffer)
# Last element as unsigned short instead of unsigned char ( 2 Bytes)
buffer = struct.pack("ihh", 3, 4, 5)
print "Byte chunk: ", repr(buffer)
Output:
Native byteorder: little Byte chunk: '\x03\x00\x00\x00\x04\x00\x05' Byte chunk unpacked: (3, 4, 5) Byte chunk: '\x03\x00\x00\x00\x04\x00\x05\x00'
You could use network byte order with data received from network or pack data to send it to network.
import struct
# If no byteorder is specified, native byteorder is used
buffer = struct.pack("hhh", 3, 4, 5)
print "Byte chunk native byte order: ", repr(buffer)
buffer = struct.pack("!hhh", 3, 4, 5)
print "Byte chunk network byte order: ", repr(buffer)
Output:
Byte chunk native byte order: '\x03\x00\x04\x00\x05\x00'
Byte chunk network byte order: '\x00\x03\x00\x04\x00\x05'
You can optimize by avoiding the overhead of allocating a new buffer by providing a buffer that was created earlier.
import struct
from ctypes import create_string_buffer
bufferVar = create_string_buffer(8)
bufferVar2 = create_string_buffer(8)
# We use a buffer that has already been created
# provide format, buffer, offset and data
struct.pack_into("hhh", bufferVar, 0, 3, 4, 5)
print "Byte chunk: ", repr(bufferVar.raw)
struct.pack_into("hhh", bufferVar2, 2, 3, 4, 5)
print "Byte chunk: ", repr(bufferVar2.raw)
Output:
Byte chunk: '\x03\x00\x04\x00\x05\x00\x00\x00'
Byte chunk: '\x00\x00\x03\x00\x04\x00\x05\x00'
