Master Python’s struct Module: Binary Data Packing & Unpacking Explained

struct is a module that solves conversion between bytes and other binary data types, making stream operations simple and addressing Python's lack of a native byte data type.

Purpose

1. Convert Python data to a byte‑stream string for network transmission, e.g., converting int to bytes before sending.

2. Convert byte streams back to specified Python data types.

3. Handle binary data when reading/writing files in binary mode ('wb', 'rb').

Functions in the struct module

See the table below:

Format strings

When packing or unpacking, a format string specifies data types and controls byte order, size, and alignment.

Alignment

To exchange data with C structs, consider the compiler's byte alignment (commonly 4‑byte on 32‑bit systems). The first character of the format string can change alignment. Definitions:

Endianess: big‑endian stores the most significant byte at the highest address; little‑endian stores it at the lowest address.

Format specifiers

See the table below:

Notes:

_Bool is defined in C99; if unavailable, it is treated as a one‑byte char.

q and Q are for 64‑bit machines only.

A numeric prefix before a format indicates repetition (e.g., 4s for a 4‑character string, 4i for four ints).

P converts a pointer, size depends on the machine.

f and d sizes depend on the machine.

Examples

Typical packing converts any data type to bytes:

Unpacking bytes back to data types can cause overflow if the buffer length is incorrect; ensure the buffer length matches the expected size.

For image data, read the file first then unpack:

pack_into places multiple data objects into a buffer and converts them to a byte stream, allowing predefined packing ranges.

unpack_from converts a byte stream back to data objects, with optional offset.

calcsize computes the memory size of a format string:

Conclusion

This article introduced Python’s struct module for binary data handling, covering its purpose, functions, format strings, alignment, specifiers, and practical usage examples.

Reference:

https://blog.csdn.net/qq_30638831/article/details/80421019