Python struct.pack explained

Python struct.pack Detailed Explanation

1. Introduction

struct.pack is a function in Python used to pack data into a specified format. The struct module provides powerful string formatting techniques for converting data to and from C structures. This article will detail how to use struct.pack and provide some example code.

2. Instruction Format

The basic format of the struct.pack function is as follows:

struct.pack(format, v1, v2, ...)

Where format is a format string that specifies how data is packed; v1, v2, ... are the data to be packed.

3. Format String

In the format parameter of the struct.pack function, you can use a format string to specify the type and order of the data to be packed.

A format string is a string consisting of format characters, each of which specifies a data type.

Common format characters are as follows:

• x: Represents a byte placeholder
• c: Packs a character into a byte
• b: Packs an integer into a byte
• B: Packs an integer into an unsigned byte
• h: Packs an integer into a short integer
• H: Packs an integer into an unsigned short integer
• i: Packs an integer into a Packs integers into an integer
• I: Packs integers into an unsigned integer
• l: Packs integers into a long integer
• L: Packs integers into an unsigned long integer
• f: Packs floating-point numbers into a single-precision floating-point number
• d: Packs floating-point numbers into a double-precision floating-point number
• s: Converts a string to a string (default length)
• p: s is similar, but a null character is appended to the string.
• q: Packs an integer into a long-long integer.
• Q: Packs an integer into an unsigned long-long integer.

In addition, special characters can be used in the format string to control the packing method:

• @: Local byte order
• =: Standard byte order
• <: Little-endian byte order
• >: Big-endian byte order
• !: Network byte order

4. Example Code

Below is some example code using the struct.pack function.

4.1 Packed Integers

import struct

i = 42
packed_data = struct.pack('i', i)
print(packed_data)

Running Result:

b'*x00x00x00'

4.2 Packed Floating-Point Numbers

import struct

f = 3.14
packed_data = struct.pack('f', f)
print(packed_data)

Running Result:

b'*x00x00x00'

line-numbers”>b’xdbx0fI@’

4.3 Packing Strings

import struct

s = 'Hello, world!'
packed_data = struct.pack('s', s.encode())
print(packed_data)

Running Result:

b'Hello, world!'

4.4 Packing Multiple Data

import struct

i = 42
f = 3.14
s = 'Hello, world!'

packed_data = struct.pack('if10s', i, f, s.encode())
print(packed_data)

Running result:

b'*x00x00x00x9ax99x99?Hello, worl'

In this example, the format string is 'if10s', which means to pack an integer first, then a floating-point number, and finally a string of length 10.

5. Conclusion

This article introduced the use of the struct.pack function. The format string allows you to flexibly specify the type and order of the data to be packed. In actual development, using the struct.pack function can conveniently pack and unpack data, making it particularly suitable for data exchange with other languages.