7.3.struct — Interpret strings as packed binary data

This module performs conversions between Python values and C structs representedas Python strings. This can be used in handling binary data stored in files orfrom 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.

7.3.1.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(fmt,v1,v2,...)

Return a string containing the valuesv1,v2,... packed according to thegiven format. The arguments must match the values required by the formatexactly.

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

Pack the valuesv1,v2,... according to the given format, write thepacked bytes into the writablebuffer starting atoffset. Note that theoffset is a required argument.

New in version 2.5.

struct.unpack(fmt,string)

Unpack the string (presumably packed bypack(fmt,...)) according to thegiven format. The result is a tuple even if it contains exactly one item.The string must contain exactly the amount of data required by the format(len(string) must equalcalcsize(fmt)).

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

Unpack thebuffer according to the given format. The result is a tuple evenif it contains exactly one item. Thebuffer must contain at least theamount of data required by the format (len(buffer[offset:]) must be atleastcalcsize(fmt)).

New in version 2.5.

struct.calcsize(fmt)

Return the size of the struct (and hence of the string) corresponding to thegiven format.

7.3.2.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.

7.3.2.1.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'!' is available for those poor souls who claim they can’t rememberwhether network byte order is big-endian or little-endian.

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.

7.3.2.2.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

string of length 1

1

b

signedchar

integer

1

(3)

B

unsignedchar

integer

1

(3)

?

_Bool

bool

1

(1)

h

short

integer

2

(3)

H

unsignedshort

integer

2

(3)

i

int

integer

4

(3)

I

unsignedint

integer

4

(3)

l

long

integer

4

(3)

L

unsignedlong

integer

4

(3)

q

longlong

integer

8

(2), (3)

Q

unsignedlonglong

integer

8

(2), (3)

f

float

float

4

(4)

d

double

float

8

(4)

s

char[]

string

p

char[]

string

P

void*

integer

(5), (3)

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.

    New in version 2.6.

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

    New in version 2.2.

  3. 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. If no__index__() method exists, or the call to__index__() raisesTypeError, then the__int__() method is tried. However, the useof__int__() is deprecated, and will raiseDeprecationWarning.

    Changed in version 2.7:Use of the__index__() method for non-integers is new in 2.7.

    Changed in version 2.7:Prior to version 2.7, not all integer conversion codes would use the__int__() method to convert, andDeprecationWarning wasraised only for float arguments.

  4. For the'f' and'd' conversion codes, the packed representation usesthe IEEE 754 binary32 (for'f') or binary64 (for'd') format,regardless of the floating-point format 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.

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 size of thestring, 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 string always has exactly the specified number ofbytes. As a special case,'0s' means a single, empty string (while'0c' means 0 characters).

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 is smaller.The bytes of the string follow. If the string passed in topack() is toolong (longer than the count minus 1), only the leadingcount-1 bytes of thestring are stored. If the string is shorter thancount-1, it is padded withnull bytes so that exactly count bytes in all are used. Note that forunpack(), the'p' format character consumes count bytes, but that thestring returned can never contain more than 255 characters.

For the'P' format character, the return value is a Python integer or longinteger, depending on the size needed to hold a pointer when it has been cast toan integer type. ANULL pointer will always be returned as the Python integer0. When packing pointer-sized values, Python integer or long integer objectsmay be used. For example, the Alpha and Merced processors use 64-bit pointervalues, meaning a Python long integer will be used to hold the pointer; otherplatforms use 32-bit pointers and will use a Python integer.

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.

7.3.2.3.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)'\x00\x01\x00\x02\x00\x00\x00\x03'>>>unpack('hhl','\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='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='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','*',0x12131415)'*\x00\x00\x00\x12\x13\x14\x15'>>>pack('ic',0x12131415,'*')'\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)'\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.

7.3.3.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.

New in version 2.5.

Compiled Struct objects support the following methods and attributes:

pack(v1,v2,...)

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

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

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

unpack(string)

Identical to theunpack() function, using the compiled format.(len(string) must equalself.size).

unpack_from(buffer,offset=0)

Identical to theunpack_from() function, using the compiled format.(len(buffer[offset:]) must be at leastself.size).

format

The format string used to construct this Struct object.

size

The calculated size of the struct (and hence of the string) correspondingtoformat.