7.3. struct — Interpret strings as packed binary data

This module performs conversions between Python values and C structs represented as Python strings. 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.

7.3.1. 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 string containing the values v1, v2, ... packed according to the given format. 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 given format, write the packed bytes into the writable buffer starting at offset. Note that the offset is a required argument.

New in version 2.5.

struct.unpack(fmt, string)

Unpack the string (presumably packed by pack(fmt, ...)) according to the given 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 equal calcsize(fmt)).

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

Unpack the buffer according to the given format. The result is a tuple even if it contains exactly one item. The buffer must contain at least the amount of data required by the format (len(buffer[offset:]) must be at least calcsize(fmt)).

New in version 2.5.

struct.calcsize(fmt)

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

7.3.2. 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.

7.3.2.1. 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.

7.3.2.2. 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 string of length 1 1  
b signed char integer 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)
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 by C99. If this type is not available, it is simulated using a char. In standard 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 if the platform C compiler supports C long long, 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 conversion codes, if the non-integer has a __index__() method then that method is called to convert the argument to an integer before packing. If no __index__() method exists, or the call to __index__() raises TypeError, then the __int__() method is tried. However, the use of __int__() is deprecated, and will raise DeprecationWarning.

    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, and DeprecationWarning was raised only for float arguments.

  4. For the 'f' and 'd' conversion codes, the packed representation uses the 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 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.

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 size of the string, 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 string always has exactly the specified number of bytes. 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 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 characters.

For the 'P' format character, the return value is a Python integer or long integer, depending on the size needed to hold a pointer when it has been cast to an integer type. A NULL pointer will always be returned as the Python integer 0. When packing pointer-sized values, Python integer or long integer objects may be used. For example, the Alpha and Merced processors use 64-bit pointer values, meaning a Python long integer will be used to hold the pointer; other platforms use 32-bit pointers and will use a Python integer.

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.

7.3.2.3. 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)
'\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 wrapping the result in a named tuple:

>>> record = '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='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', '*', 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, assuming longs 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 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.

7.3.3. 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.

New in version 2.5.

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 self.size.)

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

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

unpack(string)

Identical to the unpack() function, using the compiled format. (len(string) must equal self.size).

unpack_from(buffer, offset=0)

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

format

The format string used to construct this Struct object.

size

The calculated size of the struct (and hence of the string) corresponding to format.