struct — pack and unpack primitive data types#

This module performs conversions between Python values and C-style structs represented as Python bytes objects, using format strings to describe the layout of the data.

The following byte orders are supported:

Character

Byte order

Size

Alignment

@

native

native

native

<

little-endian

standard

none

>

big-endian

standard

none

!

network (= big-endian)

standard

none

The following data types are supported:

Format

C Type

Python type

Standard size

b

signed char

integer

1

B

unsigned char

integer

1

h

short

integer

2

H

unsigned short

integer

2

i

int

integer (1)

4

I

unsigned int

integer (1)

4

l

long

integer (1)

4

L

unsigned long

integer (1)

4

q

long long

integer (1)

8

Q

unsigned long long

integer (1)

8

e

n/a (half-float)

float (2)

2

f

float

float (2)

4

d

double

float (2)

8

s

char[]

bytes

P

void *

integer

  1. Requires long support when used with values larger than 30 bits.

  2. Requires floating point support.

Difference to CPython

Whitespace is not supported in format strings.

Examples#

Pack and unpack little-endian values. The < prefix selects little-endian byte order with standard sizes and no alignment:

import struct

# Pack an unsigned short (H, 2 bytes) then an unsigned int (I, 4 bytes).
data = struct.pack("<HI", 7, 1000)
# data == b'\x07\x00\xe8\x03\x00\x00'

# Unpack returns a tuple of the values, in order.
struct.unpack("<HI", data)
# (7, 1000)

# calcsize() reports how many bytes the format needs.
struct.calcsize("<HI")
# 6

Pack into and unpack from an existing buffer at a byte offset:

buf = bytearray(8)

# Write a little-endian signed int (i) at offset 2.
struct.pack_into("<i", buf, 2, -12345)
# buf == bytearray(b'\x00\x00\xc7\xcf\xff\xff\x00\x00')

# Read it back from the same offset.
struct.unpack_from("<i", buf, 2)
# (-12345,)

Functions#

struct.calcsize(fmt: str) → int#

Return the number of bytes needed to store the given fmt.

struct.pack(fmt: str, *values: Any) → bytes#

Pack the values according to the format string fmt. The return value is a bytes object encoding the values.

struct.pack_into(fmt: str, buffer: Any, offset: int, *values: Any) → None#

Pack the values according to the format string fmt into a buffer starting at offset. offset may be negative to count from the end of buffer.

struct.unpack(fmt: str, data: bytes) → Tuple#

Unpack from the data according to the format string fmt. The return value is a tuple of the unpacked values.

struct.unpack_from(fmt: str, data: bytes, offset: int = 0, /) → Tuple#

Unpack from the data starting at offset according to the format string fmt. offset may be negative to count from the end of data. The return value is a tuple of the unpacked values.