Packing data with Python

Packing data with Python

06 Apr 2016

Defining how a sequence of bytes sits in a memory buffer or on disk can be challenging from time to time. Since everything that you’ll work with is a byte, it makes sense that we have an intuitive way to work with this information agnostic of the overlying type restrictions that the language will enforce on us.

In today’s post, I’m going to run through Python’s byte string packing and unpacking using the struct package.

Basics

From the Python documentation:

This module performs conversions between Python values and C structs represented as Python bytes objects. This can be used in handling binary data stored in files or from network connections, among other sources. It uses Format Strings as compact descriptions of the layout of the C structs and the intended conversion to/from Python values.

When working with a byte string in Python, you prefix your literals with b.

>>> b'Hello'
'Hello'

The ord function call is used to convert a text character into its character code representation.

>>> ord(b'H')
72
>>> ord(b'e')
101
>>> ord(b'l')
108

We can use list to convert a whole string of byte literals into an array.

>>> list(b'Hello')
[72, 101, 108, 108, 111]

The compliment to the ord call is chr, which converts the byte-value back into a character.

Packing

Using the struct module, we’re offered the pack function call. This function takes in a format of data and then the data itself. The first parameter defines how the data supplied in the second parameter should be laid out. We get started:

>>> import struct

If we pack the string 'Hello' as single bytes:

>>> list(b'Hello')
[72, 101, 108, 108, 111]
>>> struct.pack(b'BBBBB', 72, 101, 108, 108, 111)
b'Hello'

The format string b'BBBBB' tells pack to pack the values supplied into a string of 5 unsigned values. If we were to use a lower case b in our format string, pack would expect the byte value to be signed.

>>> struct.pack(b'bbbbb', 72, 101, 108, 108, 111)
b'Hello'

This only gets interesting once we send a value that would make the request overflow:

>>> struct.pack(b'bbbbb', 72, 101, 108, 129, 111)

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
struct.error: byte format requires -128 <= number <= 127

The following tables have been re-produced from the Python documentation.

Byte order, size and alignment

Character	Byte order	Size	Alignment
@	native	native	native
=	native	standard	none
<	little-endian	standard	none
>	big-endian	standard	none
!	network (= big-endian)	standard	none

Types

Format	C Type	Python type	Standard size	Notes
x	pad byte	no value
c	char	bytes of length 1	1
b	signed char	integer	1	(1),(3)
B	unsigned char	integer	1	(3)
?	_Bool	bool	1	(1)
h	short	integer	2	(3)
H	unsigned short	integer	2	(3)
i	int	integer	4	(3)
I	unsigned int	integer	4	(3)
l	long	integer	4	(3)
L	unsigned long	integer	4	(3)
q	long long	integer	8	(2), (3)
Q	unsigned long long	integer	8	(2), (3)
n	ssize_t	integer		(4)
N	size_t	integer		(4)
f	float	float	4	(5)
d	double	float	8	(5)
s	char[]	bytes
p	char[]	bytes
P	void *	integer		(6)

Unpacking

The direct reverse process of packing bytes into an array, is unpacking them again into usable variables inside of your python code.

>>> struct.unpack(b'BBBBB', struct.pack(b'BBBBB', 72, 101, 108, 108, 111))
(72, 101, 108, 108, 111)
>>> struct.unpack(b'5s', struct.pack(b'BBBBB', 72, 101, 108, 108, 111))
(b'Hello',)

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