# Python struct pack, unpack

Published on August 3, 2022

Shubham

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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.

## Python Struct 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)
``````

When we run this script, we get the following representation: Note that ‘b’ in the Output stands for binary.

### 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))
``````

When we run this script, we get back our original representation: Clearly, we must tell the Python interpreter the format we need to unpack the values into.

### 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')))
``````

When we run this script, we get the following representation:

### 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))
``````

When we run this script, we get the following representation: That’s all for a short introduction of python `struct` module.

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Shubham

author

#### Still looking for an answer?

Ask a questionSearch for more help

JournalDev
DigitalOcean Employee
June 9, 2021

I agree, the way the struct var prints is ridiculous. How did it get past the checking process?

- Pete

JournalDev
DigitalOcean Employee
April 2, 2021

is it possible to switch off this feature and get it ‘uneasy’ ?

- wally

JournalDev
DigitalOcean Employee
April 8, 2019

why 10 and 13 return b’\n\x00\r\x00’

- Richard Ng

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