# `struct` --- Interpret bytes as packed binary data

Code source : Lib/struct.py

This module converts between Python values and C structs represented as Python `bytes` objects. Compact format strings describe the intended conversions to/from Python values. The module's functions and objects can be used for two largely distinct applications, data exchange with external sources (files or network connections), or data transfer between the Python application and the C layer.

Note

When no prefix character is given, native mode is the default. It packs or unpacks data based on the platform and compiler on which the Python interpreter was built. The result of packing a given C struct includes pad bytes which maintain proper alignment for the C types involved; similarly, alignment is taken into account when unpacking. In contrast, when communicating data between external sources, the programmer is responsible for defining byte ordering and padding between elements. See Boutisme, taille et alignement for details.

Several `struct` functions (and methods of `Struct`) take a buffer argument. This refers to objects that implement the Protocole tampon 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.

## Fonctions et exceptions

Le module définit les exceptions et fonctions suivantes :

exception struct.error

  Exception levée à plusieurs occasions ; l'argument est une chaîne qui décrit ce qui ne va pas.

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

  Renvoie un objet bytes contenant les valeurs v1, v2… agrégées conformément à la chaîne de format format. Les arguments doivent correspondre exactement aux valeurs requises par le format.

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

  Agrège les valeurs v1, v2… conformément à la chaîne de format format et écrit les octets agrégés dans le tampon buffer, en commençant à la position offset. Notez que offset est un argument obligatoire.

struct.unpack(format, buffer)

  Dissocie depuis le tampon buffer (en supposant que celui-ci a été agrégé avec `pack(format, …)`) à l'aide de la chaîne de format format. Le résultat est un n-uplet, qui peut éventuellement ne contenir qu'un seul élément. La taille de buffer en octets doit correspondre à la taille requise par le format, telle que calculée par `calcsize()`.

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

  Dissocie les éléments du tampon buffer, en commençant à la position offset, conformément à la chaîne de format format. Le résultat est un n-uplet, qui peut éventuellement ne contenir qu'un seul élément. La taille du tampon en octets, en commençant à la position offset, doit être au moins égale à la taille requise par le format, telle que calculée par `calcsize()`.

struct.iter_unpack(format, buffer)

  Iteratively unpack from the buffer buffer according to the format string format. 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()`.

  Chaque itération produit un n-uplet tel que spécifié par la chaîne de format.

  Ajouté dans la version 3.4.

struct.calcsize(format)

  Renvoie la taille de la structure (et donc celle de l'objet bytes produit par `pack(format, ...)`) correspondant à la chaîne de format format.

## Chaînes de spécification du format

Format strings describe the data layout when packing and unpacking data. They are built up from type codes, which specify the type of data being packed/unpacked. In addition, special characters control the byte order, size and alignment. Each format string consists of an optional prefix character which describes the overall properties of the data and one or more format characters which describe the actual data values and padding.

### Boutisme, taille et alignement

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). This behavior is chosen so that the bytes of a packed struct correspond exactly to the memory layout of the corresponding C struct. Whether to use native byte ordering and padding or standard formats depends on the application.

Cependant, le premier caractère de la chaîne de format peut être utilisé pour indiquer le boutisme, la taille et l'alignement des données agrégées, conformément à la table suivante :

Caractère | Boutisme | Taille | Alignement
--- | --- | --- | ---
`@` | natif | natif | natif
`=` | natif | standard | aucun
`<` | petit-boutiste | standard | aucun
`>` | gros-boutiste | standard | aucun
`!` | réseau (= gros-boutiste) | standard | aucun

Si le premier caractère n'est pas dans cette liste, le module se comporte comme si `'@'` avait été indiqué.

Note

The number 1023 (`0x3ff` in hexadecimal) has the following byte representations:

- `03 ff` in big-endian (`>`)

- `ff 03` in little-endian (`<`)

Python example:

    >>> import struct
    >>> struct.pack('>h', 1023)
    b'\x03\xff'
    >>> struct.pack('<h', 1023)
    b'\xff\x03'

Native byte order is big-endian or little-endian, depending on the host system. For example, Intel x86, AMD64 (x86-64), and Apple M1 are little-endian; IBM z and many legacy architectures are big-endian. Use `sys.byteorder` to check the endianness of your system.

La taille et l'alignement natifs sont déterminés en utilisant l'expression `sizeof` du compilateur C. Leur valeur est toujours combinée au boutisme natif.

Standard size depends only on the type code; see the table in the Type Codes section.

Notez la différence entre `'@'` et `'='` : les deux utilisent le boutisme natif mais la taille et l'alignement du dernier sont standards.

The form `'!'` represents the network byte order which is always big-endian as defined in IETF RFC 1700.

Il n'y a pas de moyen de spécifier le boutisme contraire au boutisme natif (c'est-à-dire forcer la permutation des octets) ; utilisez le bon caractère entre `'<'` et `'>'`.

Notes :

1. Le bourrage (padding en anglais) n'est automatiquement ajouté qu'entre les membres successifs de la structure. Il n'y a pas de bourrage au début ou à la fin de la structure agrégée.

2. Il n'y a pas d'ajout de bourrage lorsque vous utilisez une taille et un alignement non-natifs, par exemple avec `'<'`, `'>'`, `'='` ou `'!'`.

3. Pour aligner la fin d'une structure à l'alignement requis par un type particulier, terminez le format avec le code du type voulu et une valeur de répétition à zéro. Référez-vous à Exemples.

### Type Codes

Type codes (or format codes) 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 | Type C | Type Python | Taille standard | Notes
--- | --- | --- | --- | ---
`x` | octet de bourrage | pas de valeur |  | (7)
`c` | char | bytes (suite d'octets) de taille 1 | 1 |
`b` | signed char | int | 1 | (2)
`B` | unsigned char | int | 1 | (2)
`?` | _Bool | bool (booléen) | 1 | (1)
`h` | short | int | 2 | (2)
`H` | unsigned short | int | 2 | (2)
`i` | int | int | 4 | (2)
`I` | unsigned int | int | 4 | (2)
`l` | long | int | 4 | (2)
`L` | unsigned long | int | 4 | (2)
`q` | long long | int | 8 | (2)
`Q` | unsigned long long | int | 8 | (2)
`n` | `ssize_t` | int |  | (2), (3)
`N` | `size_t` | int |  | (2), (3)
`e` | _Float16 | float (nombre à virgule flottante) | 2 | (4), (6)
`f` | float | float (nombre à virgule flottante) | 4 | (4)
`d` | double | float (nombre à virgule flottante) | 8 | (4)
`F` | float complex | complex | 8 | (10)
`D` | double complex | complex | 16 | (10)
`Zf` | float complex | complex | 8 | (10)
`Zd` | double complex | complex | 16 | (10)
`s` | char[] | bytes (séquence d'octets) |  | (9)
`p` | char[] | bytes (séquence d'octets) |  | (8)
`P` | void* | int |  | (2), (5)

Modifié dans la version 3.3: ajouté la gestion des formats `'n'` et `'N'`.

Modifié dans la version 3.6: ajouté la gestion du format `'e'`.

Modifié dans la version 3.14: Added support for the `'F'` and `'D'` formats.

Modifié dans la version 3.15: Added support for the `'Zf'` and `'Zd'` formats.

Voir aussi

The `array` and ctypes modules, as well as third-party modules like numpy, use similar -- but slightly different -- type codes.

Notes :

1. The `'?'` conversion code corresponds to the _Bool type defined by C standards since C99. In standard mode, it is represented by one byte.

2. 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.

   Modifié dans la version 3.2: Added use of the `__index__()` method for non-integers.

3. Les codes de conversion `'n'` et `'N'` ne sont disponibles que pour la taille native (choisie par défaut ou à l'aide du caractère de boutisme `'@'`). Pour la taille standard, vous pouvez utiliser n'importe quel format d'entier qui convient à votre application.

4. Pour les codes de conversion `'f'`, `'d'` et `'e'`, la représentation agrégée utilise respectivement le format IEEE 754 binaire32, binaire64 ou binaire16 (pour `'f'`, `'d'` ou `'e'` respectivement), quel que soit le format des nombres à virgule flottante de la plateforme.

5. The `'P'` type code 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.

6. 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: it's available as _Float16 type, if the compiler supports the Annex H of the C23 standard. 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.

7. When packing, `'x'` inserts one NUL byte.

8. The `'p'` type code 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 byte 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 byte 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'` type code consumes `count` bytes, but that the `bytes` object returned can never contain more than 255 bytes. When packing, arguments of types `bytes` and `bytearray` are accepted.

9. For the `'s'` type code, the count is interpreted as the length of the byte string, not a repeat count like for the other type codes; for example, `'10s'` means a single 10-byte string mapping to or from a single Python byte string, while `'10c'` means 10 separate one byte character elements (e.g., `cccccccccc`) mapping to or from ten different Python byte objects. (See Exemples for a concrete demonstration of the difference.) If a count is not given, it defaults to 1. For packing, the byte 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 byte string (while `'0c'` means 0 characters). When packing, arguments of types `bytes` and `bytearray` are accepted.

10. For the `'F'` and `'D'` type codes, the packed representation uses the IEEE 754 binary32 and binary64 format for components of the complex number, regardless of the floating-point format used by the platform. Note that complex types (`F`/`Zf` and `D`/`Zd`) are available unconditionally, despite complex types being an optional feature in C. As specified in the C11 standard, each complex type is represented by a two-element C array containing, respectively, the real and imaginary parts.

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

Les caractères d'espacement entre les indications de format sont ignorés ; cependant, le nombre de répétitions et le format associé ne doivent pas être séparés par des caractères d'espacement.

Lors de l'agrégation d'une valeur `x` en utilisant l'un des formats pour les entiers (`'b'`, `'B'`, `'h'`, `'H'`, `'i'`, `'I'`, `'l'`, `'L'`, `'q'`, `'Q'`), si `x` est en dehors de l'intervalle du format spécifié, une `struct.error` est levée.

Modifié dans la version 3.1: Previously, some of the integer formats wrapped out-of-range values and raised `DeprecationWarning` instead of `struct.error`.

For the `'?'` type code, 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.

### Exemples

Note

Native byte order examples (designated by the `'@'` format prefix or lack of any prefix character) may not match what the reader's machine produces as that depends on the platform and compiler.

Pack and unpack integers of three different sizes, using big endian ordering:

    >>> from struct import *
    >>> pack(">bhl", 1, 2, 3)
    b'\x01\x00\x02\x00\x00\x00\x03'
    >>> unpack('>bhl', b'\x01\x00\x02\x00\x00\x00\x03')
    (1, 2, 3)
    >>> calcsize('>bhl')
    7

Attempt to pack an integer which is too large for the defined field:

    >>> pack(">h", 99999)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    struct.error: 'h' format requires -32768 <= number <= 32767

Demonstrate the difference between `'s'` and `'c'` format characters:

    >>> pack("@ccc", b'1', b'2', b'3')
    b'123'
    >>> pack("@3s", b'123')
    b'123'

Les champs dissociés peuvent être nommés en leur assignant des variables ou en encapsulant le résultat dans un n-uplet nommé :

    >>> 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 type codes may have an impact on size in native mode since padding is implicit. In standard mode, the user is responsible for inserting any desired padding. Note in the first `pack` call below that three NUL bytes were added after the packed `'#'` to align the following integer on a four-byte boundary. In this example, the output was produced on a little endian machine:

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

The following format `'llh0l'` results in two pad bytes being added at the end, assuming the platform's 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'

Voir aussi

Module `array`

  Stockage agrégé binaire de données homogènes.

Module `json`

  JSON encoder and decoder.

Module `pickle`

  Python object serialization.

## Applications

Two main applications for the `struct` module exist, data interchange between Python and C code within an application or another application compiled using the same compiler (native formats), and data interchange between applications using agreed upon data layout (standard formats). Generally speaking, the format strings constructed for these two domains are distinct.

### Native Formats

When constructing format strings which mimic native layouts, the compiler and machine architecture determine byte ordering and padding. In such cases, the `@` format character should be used to specify native byte ordering and data sizes. Internal pad bytes are normally inserted automatically. It is possible that a zero-repeat type code will be needed at the end of a format string to round up to the correct byte boundary for proper alignment of consecutive chunks of data.

Consider these two simple examples (on a 64-bit, little-endian machine):

    >>> calcsize('@lhl')
    24
    >>> calcsize('@llh')
    18

Data is not padded to an 8-byte boundary at the end of the second format string without the use of extra padding. A zero-repeat format code solves that problem:

    >>> calcsize('@llh0l')
    24

The `'x'` type code can be used to specify the repeat, but for native formats it is better to use a zero-repeat format like `'0l'`.

By default, native byte ordering and alignment is used, but it is better to be explicit and use the `'@'` prefix character.

### Standard Formats

When exchanging data beyond your process such as networking or storage, be precise. Specify the exact byte order, size, and alignment. Do not assume they match the native order of a particular machine. For example, network byte order is big-endian, while many popular CPUs are little-endian. By defining this explicitly, the user need not care about the specifics of the platform their code is running on. The first character should typically be `<` or `>` (or `!`). Padding is the responsibility of the programmer. The zero-repeat format character won't work. Instead, the user must explicitly add `'x'` pad bytes where needed. Revisiting the examples from the previous section, we have:

    >>> calcsize('<qh6xq')
    24
    >>> pack('<qh6xq', 1, 2, 3) == pack('@lhl', 1, 2, 3)
    True
    >>> calcsize('@llh')
    18
    >>> pack('@llh', 1, 2, 3) == pack('<qqh', 1, 2, 3)
    True
    >>> calcsize('<qqh6x')
    24
    >>> calcsize('@llh0l')
    24
    >>> pack('@llh0l', 1, 2, 3) == pack('<qqh6x', 1, 2, 3)
    True

The above results (executed on a 64-bit machine) aren't guaranteed to match when executed on different machines. For example, the examples below were executed on a 32-bit machine:

    >>> calcsize('<qqh6x')
    24
    >>> calcsize('@llh0l')
    12
    >>> pack('@llh0l', 1, 2, 3) == pack('<qqh6x', 1, 2, 3)
    False

## 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 module-level functions with the same format since the format string is only compiled once.

  Note

  The compiled versions of the most recent format strings passed to the module-level functions are cached, so programs that use only a few format strings needn't worry about reusing a single `Struct` instance.

  Les objets `Struct` compilés gèrent les méthodes et attributs suivants :

  pack(v1, v2, ...)

    Identique à la fonction `pack()`, en utilisant le format compilé (`len(result)` vaut `size`).

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

    Identique à la fonction `pack_into()`, en utilisant le format compilé.

  unpack(buffer)

    Identique à la fonction `unpack()`, en utilisant le format compilé. La taille du tampon buffer en octets doit valoir `size`.

  unpack_from(buffer, offset=0)

    Identique à la fonction `unpack_from()`, en utilisant le format compilé. La taille du tampon buffer en octets, en commençant à la position offset, doit valoir au moins `size`.

  iter_unpack(buffer)

    Identique à la fonction `iter_unpack()`, en utilisant le format compilé. La taille du tampon buffer en octets doit être un multiple de `size`.

    Ajouté dans la version 3.4.

  format

    La chaîne de format utilisée pour construire l'objet `Struct`.

    Modifié dans la version 3.7: la chaîne de format est maintenant de type `str` au lieu de `bytes`.

  size

    La taille calculée de la structure agrégée (et donc de l'objet `bytes` produit par la méthode `pack()`) correspondante à `format`.

  Modifié dans la version 3.13: The repr() of structs has changed. It is now:

      >>> Struct('i')
      Struct('i')