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Packing a structure

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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’

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Binary Data:
* Binary Data
* Format a list of values into a byte object
* Unpack a byte object according to a format string
* Packing a structure
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0 Getting Started
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88 Pickle data serialization
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98 PostgreSQL
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159 base64 module
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165 hashlib
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177 Functional Programming
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181 virtualenvwrapper on Windows
182 Python Requests Post
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184 Python Lex-Yacc
185 pyaudio
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187 pip and PyPI Package Manager
188 Writing to CSV from String or List
189 Raise Custom Errors Exceptions
190 Using loops within functions
191 Contributors