Python struct pack, unpack

Python struct module is capable of performing the conversions between the Python values and C structs, which are represented as Python Strings.

Python struct

• Python struct module can be used in handling binary data stored in files, database or from network connections etc.
• It uses format Strings as compact descriptions of the layout of the C structs and the intended conversion to/from Python values.

Functions

There are five important functions in struct module – pack(), unpack(), calcsize(), pack_into() and unpack_from(). In all these functions, we have to provide the format of the data to be converted into binary. Some of the popular format characters are:

• ?: boolean
• h: short
• l: long
• i: int
• f: float
• q: long long int

You can get the complete list of format characters here. Let’s start looking into struct module functions one by one.

Python struct.pack()

This function packs a list of values into a String representation of the specified type. The arguments must match the values required by the format exactly. Let’s quickly look at struct pack() example:

import struct

var = struct.pack('hhl', 5, 10, 15)
print(var)
 
var = struct.pack('iii', 10, 20, 30)
print(var)

Python struct.unpack()

This function unpacks the packed value into its original representation with the specified format. This function always returns a tuple, even if there is only one element. Let’s quickly look at struct unpack() function example:

import struct
var = struct.pack('hhl', 5, 10, 15)
print(var)
print(struct.unpack('hhl', var))

Python struct calcsize()

This function calculates and returns the size of the String representation of struct with a given format. Size is calculated in terms of bytes. Let’s quickly look at an example code snippet:

import struct
 
var = struct.pack('hhl', 5, 10, 15)
print(var)
print("Size of String representation is {}.".format(struct.calcsize('hhl')))

Python struct pack_into(), unpack_from()

These functions allow us to pack the values into string buffer and unpack from a string buffer. These functions are introduced in version 2.5.

import struct
# ctypes is imported to create a string buffer
import ctypes

# As shown in previous example
size = struct.calcsize('hhl')
print(size)

# Buffer 'buff' is created from ctypes
buff = ctypes.create_string_buffer(siz)

# struct.pack_into() packs data into buff and it doesn't return any value
# struct.unpack_from() unpacks data from buff, returns a tuple of values
print(struct.pack_into('hhl', buff, 0, 5, 10, 15))
print(struct.unpack_from('hhl', buff, 0))

Conclusion

That’s all for a short introduction of python struct module.