7.1. struct — Interpret bytes as packed binary data

Source code: Lib/struct.py

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.

Note

By default, the result of packing a given C struct includes pad bytes in order to maintain proper alignment for the C types involved; similarly, alignment is taken into account when unpacking. This behavior is chosen so that the bytes of a packed struct correspond exactly to the layout in memory of the corresponding C struct. To handle platform-independent data formats or omit implicit pad bytes, use standard size and alignment instead of native size and alignment: see Byte Order, Size, and Alignment for details.

Several struct functions (and methods of Struct) take a buffer argument. This refers to objects that implement the Buffer Protocol and provide either a readable or read-writable buffer. The most common types used for that purpose are bytes and bytearray, but many other types that can be viewed as an array of bytes implement the buffer protocol, so that they can be read/filled without additional copying from a bytes object.

7.1.1. Functions and Exceptions

The module defines the following exception and functions:

exception struct.error

Exception raised on various occasions; argument is a string describing what is wrong.

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

Return a bytes object containing the values v1, v2, … packed according to the format string fmt. The arguments must match the values required by the format exactly.

struct.pack_into(fmt, buffer, offset, v1, v2, ...)

Pack the values v1, v2, … according to the format string fmt and write the packed bytes into the writable buffer buffer starting at position offset. Note that offset is a required argument.

struct.unpack(fmt, buffer)

Unpack from the buffer buffer (presumably packed by pack(fmt, ...)) according to the format string fmt. The result is a tuple even if it contains exactly one item. The buffer’s size in bytes must match the size required by the format, as reflected by calcsize().

struct.unpack_from(fmt, buffer, offset=0)

Unpack from buffer starting at position offset, according to the format string fmt. The result is a tuple even if it contains exactly one item. The buffer’s size in bytes, minus offset, must be at least the size required by the format, as reflected by calcsize().

struct.iter_unpack(fmt, buffer)

Iteratively unpack from the buffer buffer according to the format string fmt. This function returns an iterator which will read equally-sized chunks from the buffer until all its contents have been consumed. The buffer’s size in bytes must be a multiple of the size required by the format, as reflected by calcsize().

Each iteration yields a tuple as specified by the format string.

New in version 3.4.

struct.calcsize(fmt)

Return the size of the struct (and hence of the bytes object produced by pack(fmt, ...)) corresponding to the format string fmt.

7.1.2. Format Strings

Format strings are the mechanism used to specify the expected layout when packing and unpacking data. They are built up from Format Characters, which specify the type of data being packed/unpacked. In addition, there are special characters for controlling the Byte Order, Size, and Alignment.

7.1.2.1. Byte Order, Size, and Alignment

By default, C types are represented in the machine’s native format and byte order, and properly aligned by skipping pad bytes if necessary (according to the rules used by the C compiler).

Alternatively, the first character of the format string can be used to indicate the byte order, size and alignment of the packed data, according to the following table:

Character

Byte order

Size

Alignment

@

native

native

native

=

native

standard

none

<

little-endian

standard

none

>

big-endian

standard

none

!

network (= big-endian)

standard

none

If the first character is not one of these, '@' is assumed.

Native byte order is big-endian or little-endian, depending on the host system. For example, Intel x86 and AMD64 (x86-64) are little-endian; Motorola 68000 and PowerPC G5 are big-endian; ARM and Intel Itanium feature switchable endianness (bi-endian). Use sys.byteorder to check the endianness of your system.

Native size and alignment are determined using the C compiler’s sizeof expression. This is always combined with native byte order.

Standard size depends only on the format character; see the table in the Format Characters section.

Note the difference between '@' and '=': both use native byte order, but the size and alignment of the latter is standardized.

The form '!' is available for those poor souls who claim they can’t remember whether network byte order is big-endian or little-endian.

There is no way to indicate non-native byte order (force byte-swapping); use the appropriate choice of '<' or '>'.

Notes:

  1. Padding is only automatically added between successive structure members. No padding is added at the beginning or the end of the encoded struct.

  2. No padding is added when using non-native size and alignment, e.g. with ‘<’, ‘>’, ‘=’, and ‘!’.

  3. To align the end of a structure to the alignment requirement of a particular type, end the format with the code for that type with a repeat count of zero. See Examples.

7.1.2.2. Format Characters

Format characters have the following meaning; the conversion between C and Python values should be obvious given their types. The ‘Standard size’ column refers to the size of the packed value in bytes when using standard size; that is, when the format string starts with one of '<', '>', '!' or '='. When using native size, the size of the packed value is platform-dependent.

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)

e

(7)

float

2

(5)

f

float

float

4

(5)

d

double

float

8

(5)

s

char[]

bytes

p

char[]

bytes

P

void *

integer

(6)

Changed in version 3.3: Added support for the 'n' and 'N' formats.

Changed in version 3.6: Added support for the 'e' format.

Notes:

  1. The '?' conversion code corresponds to the _Bool type defined by C99. If this type is not available, it is simulated using a char. In standard mode, it is always represented by one byte.

  2. The 'q' and 'Q' conversion codes are available in native mode only if the platform C compiler supports C long long, or, on Windows, __int64. They are always available in standard modes.

  3. When attempting to pack a non-integer using any of the integer conversion codes, if the non-integer has a __index__() method then that method is called to convert the argument to an integer before packing.

    Changed in version 3.2: Use of the __index__() method for non-integers is new in 3.2.

  4. The 'n' and 'N' conversion codes are only available for the native size (selected as the default or with the '@' byte order character). For the standard size, you can use whichever of the other integer formats fits your application.

  5. For the 'f', 'd' and 'e' conversion codes, the packed representation uses the IEEE 754 binary32, binary64 or binary16 format (for 'f', 'd' or 'e' respectively), regardless of the floating-point format used by the platform.

  6. The 'P' format character is only available for the native byte ordering (selected as the default or with the '@' byte order character). The byte order character '=' chooses to use little- or big-endian ordering based on the host system. The struct module does not interpret this as native ordering, so the 'P' format is not available.

  7. The IEEE 754 binary16 “half precision” type was introduced in the 2008 revision of the IEEE 754 standard. It has a sign bit, a 5-bit exponent and 11-bit precision (with 10 bits explicitly stored), and can represent numbers between approximately 6.1e-05 and 6.5e+04 at full precision. This type is not widely supported by C compilers: on a typical machine, an unsigned short can be used for storage, but not for math operations. See the Wikipedia page on the half-precision floating-point format for more information.

A format character may be preceded by an integral repeat count. For example, the format string '4h' means exactly the same as 'hhhh'.

Whitespace characters between formats are ignored; a count and its format must not contain whitespace though.

For the 's' format character, the count is interpreted as the length of the bytes, not a repeat count like for the other format characters; for example, '10s' means a single 10-byte string, while '10c' means 10 characters. If a count is not given, it defaults to 1. For packing, the string is truncated or padded with null bytes as appropriate to make it fit. For unpacking, the resulting bytes object always has exactly the specified number of bytes. As a special case, '0s' means a single, empty string (while '0c' means 0 characters).

When packing a value x using one of the integer formats ('b', 'B', 'h', 'H', 'i', 'I', 'l', 'L', 'q', 'Q'), if x is outside the valid range for that format then struct.error is raised.

Changed in version 3.1: In 3.0, some of the integer formats wrapped out-of-range values and raised DeprecationWarning instead of struct.error.

The 'p' format character encodes a “Pascal string”, meaning a short variable-length string stored in a fixed number of bytes, given by the count. The first byte stored is the length of the string, or 255, whichever is smaller. The bytes of the string follow. If the string passed in to pack() is too long (longer than the count minus 1), only the leading count-1 bytes of the string are stored. If the string is shorter than count-1, it is padded with null bytes so that exactly count bytes in all are used. Note that for unpack(), the 'p' format character consumes count bytes, but that the string returned can never contain more than 255 bytes.

For the '?' format character, the return value is either True or False. When packing, the truth value of the argument object is used. Either 0 or 1 in the native or standard bool representation will be packed, and any non-zero value will be True when unpacking.

7.1.2.3. Examples

Note

All examples assume a native byte order, size, and alignment with a big-endian machine.

A basic example of packing/unpacking three integers:

>>> from struct import *
>>> pack('hhl', 1, 2, 3)
b'\x00\x01\x00\x02\x00\x00\x00\x03'
>>> unpack('hhl', b'\x00\x01\x00\x02\x00\x00\x00\x03')
(1, 2, 3)
>>> calcsize('hhl')
8

Unpacked fields can be named by assigning them to variables or by wrapping the result in a named tuple:

>>> record = b'raymond   \x32\x12\x08\x01\x08'
>>> name, serialnum, school, gradelevel = unpack('<10sHHb', record)

>>> from collections import namedtuple
>>> Student = namedtuple('Student', 'name serialnum school gradelevel')
>>> Student._make(unpack('<10sHHb', record))
Student(name=b'raymond   ', serialnum=4658, school=264, gradelevel=8)

The ordering of format characters may have an impact on size since the padding needed to satisfy alignment requirements is different:

>>> pack('ci', b'*', 0x12131415)
b'*\x00\x00\x00\x12\x13\x14\x15'
>>> pack('ic', 0x12131415, b'*')
b'\x12\x13\x14\x15*'
>>> calcsize('ci')
8
>>> calcsize('ic')
5

The following format 'llh0l' specifies two pad bytes at the end, assuming longs are aligned on 4-byte boundaries:

>>> pack('llh0l', 1, 2, 3)
b'\x00\x00\x00\x01\x00\x00\x00\x02\x00\x03\x00\x00'

This only works when native size and alignment are in effect; standard size and alignment does not enforce any alignment.

See also

Module array

Packed binary storage of homogeneous data.

Module xdrlib

Packing and unpacking of XDR data.

7.1.3. Classes

The struct module also defines the following type:

class struct.Struct(format)

Return a new Struct object which writes and reads binary data according to the format string format. Creating a Struct object once and calling its methods is more efficient than calling the struct functions with the same format since the format string only needs to be compiled once.

Compiled Struct objects support the following methods and attributes:

pack(v1, v2, ...)

Identical to the pack() function, using the compiled format. (len(result) will equal size.)

pack_into(buffer, offset, v1, v2, ...)

Identical to the pack_into() function, using the compiled format.

unpack(buffer)

Identical to the unpack() function, using the compiled format. The buffer’s size in bytes must equal size.

unpack_from(buffer, offset=0)

Identical to the unpack_from() function, using the compiled format. The buffer’s size in bytes, minus offset, must be at least size.

iter_unpack(buffer)

Identical to the iter_unpack() function, using the compiled format. The buffer’s size in bytes must be a multiple of size.

New in version 3.4.

format

The format string used to construct this Struct object.

size

The calculated size of the struct (and hence of the bytes object produced by the pack() method) corresponding to format.