struct — pack/unpack binary data

import struct

Convert between Python values and C-style binary representations. Useful for binary file formats, network protocols, and reading DOS-era data structures.

Format characters

Code	C type	Python	Size (bytes)
b	signed char	int	1
B	unsigned char	int	1
h	short	int	2
H	unsigned short	int	2
i	int	int	4
I	unsigned int	int	4
l	long	int	4
L	unsigned long	int	4
q	long long	int	8
Q	unsigned long long	int	8
f	float	float	4
d	double	float	8
s	char[N]	bytes	N
p	pascal string	bytes	N
x	pad byte	—	1

Endianness / alignment prefix:

Prefix	Byte order	Size
<	little-endian	packed
>	big-endian	packed
=	native	packed
@	native	aligned
!	network (= >)	packed

Default (no prefix) is native + aligned (@).

Functions

struct.pack(fmt, *vals) → bytes

struct.pack('<I', 0xCAFEBABE)             # b'\xbe\xba\xfe\xca'
struct.pack('>HH', 1, 256)                # b'\x00\x01\x01\x00'
struct.pack('<BBBB', 1, 2, 3, 4)          # b'\x01\x02\x03\x04'
struct.pack('5s', b'hello')               # b'hello'
struct.pack('<If', 42, 3.14)              # 8 bytes

struct.unpack(fmt, buf) → tuple

struct.unpack('<I', b'\xbe\xba\xfe\xca')  # (3405691582,)
x, = struct.unpack('<I', buf)             # comma → unpack 1-tuple

struct.pack_into(fmt, buf, offset, *vals) / struct.unpack_from(fmt, buf, offset=0)

Write/read in-place into a bytearray / memoryview:

buf = bytearray(16)
struct.pack_into('<II', buf, 0, 0xDEADBEEF, 0xCAFEBABE)
struct.unpack_from('<II', buf, 0)         # (3735928559, 3405691582)

struct.calcsize(fmt) → int

Size in bytes of one record of fmt:

struct.calcsize('<I')                     # 4
struct.calcsize('<IIH')                   # 10

Example: parse a BMP header

import struct

with open('IMAGE.BMP', 'rb') as f:
    hdr = f.read(14)

magic, size, _, _, off = struct.unpack('<2sIHHI', hdr)
print('BMP magic:', magic, 'file size:', size, 'pixel offset:', off)

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Credit: MicroPython struct docs (MIT).