struct — Interpret bytes as packed binary data

Source code:Lib/struct.py

This module performs conversions between Python values and C structs representedas Pythonbytes objects. This can be used in handling binary datastored in files or from network connections, among other sources. It usesFormat Strings as compact descriptions of the layout of the Cstructs and the intended conversion to/from Python values.

Note

By default, the result of packing a given C struct includes pad bytes inorder to maintain proper alignment for the C types involved; similarly,alignment is taken into account when unpacking. This behavior is chosen sothat the bytes of a packed struct correspond exactly to the layout in memoryof the corresponding C struct. To handle platform-independent data formatsor omit implicit pad bytes, usestandard size and alignment instead ofnative size and alignment: seeByte Order, Size, and Alignment for details.

Severalstruct functions (and methods ofStruct) take abufferargument. This refers to objects that implement theBuffer Protocol andprovide either a readable or read-writable buffer. The most common types usedfor that purpose arebytes andbytearray, but many other typesthat can be viewed as an array of bytes implement the buffer protocol, so thatthey can be read/filled without additional copying from abytes object.

Functions and Exceptions

The module defines the following exception and functions:

exceptionstruct.error

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

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

Return a bytes object containing the valuesv1,v2, … packed accordingto the format stringformat. The arguments must match the values required bythe format exactly.

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

Pack the valuesv1,v2, … according to the format stringformat andwrite the packed bytes into the writable bufferbuffer starting atpositionoffset. Note thatoffset is a required argument.

struct.unpack(format,buffer)

Unpack from the bufferbuffer (presumably packed bypack(format,...))according to the format stringformat. The result is a tuple even if itcontains exactly one item. The buffer’s size in bytes must match thesize required by the format, as reflected bycalcsize().

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

Unpack frombuffer starting at positionoffset, according to the formatstringformat. The result is a tuple even if it contains exactly oneitem. The buffer’s size in bytes, starting at positionoffset, must be atleast the size required by the format, as reflected bycalcsize().

struct.iter_unpack(format,buffer)

Iteratively unpack from the bufferbuffer according to the formatstringformat. This function returns an iterator which will readequally-sized chunks from the buffer until all its contents have beenconsumed. The buffer’s size in bytes must be a multiple of the sizerequired by the format, as reflected bycalcsize().

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

New in version 3.4.

struct.calcsize(format)

Return the size of the struct (and hence of the bytes object produced bypack(format,...)) corresponding to the format stringformat.

Format Strings

Format strings are the mechanism used to specify the expected layout whenpacking and unpacking data. They are built up fromFormat Characters,which specify the type of data being packed/unpacked. In addition, there arespecial characters for controlling theByte Order, Size, and Alignment.

Byte Order, Size, and Alignment

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

Alternatively, the first character of the format string can be used to indicatethe byte order, size and alignment of the packed data, according to thefollowing 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 hostsystem. For example, Intel x86 and AMD64 (x86-64) are little-endian;Motorola 68000 and PowerPC G5 are big-endian; ARM and Intel Itanium featureswitchable endianness (bi-endian). Usesys.byteorder to check theendianness of your system.

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

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

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

The form'!' represents the network byte order which is always big-endianas defined inIETF RFC 1700.

There is no way to indicate non-native byte order (force byte-swapping); use theappropriate 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 aparticular type, end the format with the code for that type with a repeatcount of zero. SeeExamples.

Format Characters

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

Format

C Type

Python type

Standard size

Notes

x

pad byte

no value

c

char

bytes of length 1

1

b

signedchar

integer

1

(1), (2)

B

unsignedchar

integer

1

(2)

?

_Bool

bool

1

(1)

h

short

integer

2

(2)

H

unsignedshort

integer

2

(2)

i

int

integer

4

(2)

I

unsignedint

integer

4

(2)

l

long

integer

4

(2)

L

unsignedlong

integer

4

(2)

q

longlong

integer

8

(2)

Q

unsignedlonglong

integer

8

(2)

n

ssize_t

integer

(3)

N

size_t

integer

(3)

e

(6)

float

2

(4)

f

float

float

4

(4)

d

double

float

8

(4)

s

char[]

bytes

p

char[]

bytes

P

void*

integer

(5)

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 byC99. If this type is not available, it is simulated using achar. Instandard mode, it is always represented by one byte.

  2. When attempting to pack a non-integer using any of the integer conversioncodes, if the non-integer has a__index__() method then that method iscalled 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.

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

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

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

  6. The IEEE 754 binary16 “half precision” type was introduced in the 2008revision of theIEEE 754 standard. It has a signbit, a 5-bit exponent and 11-bit precision (with 10 bits explicitly stored),and can represent numbers between approximately6.1e-05 and6.5e+04at full precision. This type is not widely supported by C compilers: on atypical machine, an unsigned short can be used for storage, but not for mathoperations. See the Wikipedia page on thehalf-precision floating-pointformat 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 mustnot contain whitespace though.

For the's' format character, the count is interpreted as the length of thebytes, 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 istruncated or padded with null bytes as appropriate to make it fit. Forunpacking, the resulting bytes object always has exactly the specified numberof bytes. As a special case,'0s' means a single, empty string (while'0c' means 0 characters).

When packing a valuex using one of the integer formats ('b','B','h','H','i','I','l','L','q','Q'), ifx is outside the valid range for that formatthenstruct.error is raised.

Changed in version 3.1:In 3.0, some of the integer formats wrapped out-of-range values andraisedDeprecationWarning instead ofstruct.error.

The'p' format character encodes a “Pascal string”, meaning a shortvariable-length string stored in afixed number of bytes, given by the count.The first byte stored is the length of the string, or 255, whichever issmaller. The bytes of the string follow. If the string passed in topack() is too long (longer than the count minus 1), only the leadingcount-1 bytes of the string are stored. If the string is shorter thancount-1, it is padded with null bytes so that exactly count bytes in allare used. Note that forunpack(), the'p' format character consumescount bytes, but that the string returned can never contain more than 255bytes.

For the'?' format character, the return value is eitherTrue orFalse. 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, andany non-zero value will beTrue when unpacking.

Examples

Note

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

A basic example of packing/unpacking three integers:

>>>fromstructimport*>>>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 wrappingthe result in a named tuple:

>>>record=b'raymond\x32\x12\x08\x01\x08'>>>name,serialnum,school,gradelevel=unpack('<10sHHb',record)>>>fromcollectionsimportnamedtuple>>>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 paddingneeded 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, assuminglongs 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 andalignment does not enforce any alignment.

See also

Modulearray

Packed binary storage of homogeneous data.

Modulexdrlib

Packing and unpacking of XDR data.

Classes

Thestruct module also defines the following type:

classstruct.Struct(format)

Return a new Struct object which writes and reads binary data according tothe format stringformat. Creating a Struct object once and calling itsmethods is more efficient than calling thestruct functions with thesame format since the format string only needs to be compiled once.

Note

The compiled versions of the most recent format strings passed toStruct and the module-level functions are cached, so programsthat use only a few format strings needn’t worry about reusing a singleStruct instance.

Compiled Struct objects support the following methods and attributes:

pack(v1,v2,...)

Identical to thepack() function, using the compiled format.(len(result) will equalsize.)

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

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

unpack(buffer)

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

unpack_from(buffer,offset=0)

Identical to theunpack_from() function, using the compiled format.The buffer’s size in bytes, starting at positionoffset, must be at leastsize.

iter_unpack(buffer)

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

New in version 3.4.

format

The format string used to construct this Struct object.

Changed in version 3.7:The format string type is nowstr instead ofbytes.

size

The calculated size of the struct (and hence of the bytes object producedby thepack() method) corresponding toformat.