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Doc/library/struct.rst
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Miss Islington (bot)
[3.15] gh-71086: Document negative offset support in struct.pack_into() and unpack_from() (GH-153650) (GH-153699)
15 июл 2026, 12:46
15 июл 2026, 12:4629e733a
:mod:`!struct` --- Interpret bytes as packed binary data
========================================================

.. testsetup:: *

   from struct import *

.. module:: struct
   :synopsis: Interpret bytes as packed binary data.

**Source code:** :source:`Lib/struct.py`

.. index::
   pair: C; structures
   triple: packing; binary; data

--------------

This module converts between Python values and C structs represented
as Python :class:`bytes` objects.  Compact :ref:`format strings <struct-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 :ref:`struct-alignment` for details.

Several :mod:`!struct` functions (and methods of :class:`Struct`) take a *buffer*
argument.  This refers to objects that implement the :ref:`bufferobjects` and
provide either a readable or read-writable buffer.  The most common types used
for that purpose are :class:`bytes` and :class:`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 :class:`bytes` object.


Functions and Exceptions
------------------------

The module defines the following exception and functions:


.. exception:: error

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


.. function:: pack(format, v1, v2, ...)

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


.. function:: pack_into(format, buffer, offset, v1, v2, ...)

   Pack the values *v1*, *v2*, ... according to the format string *format* and
   write the packed bytes into the writable buffer *buffer* starting at
   position *offset*.  Note that *offset* is a required argument.
   A negative *offset* counts from the end of *buffer*.


.. function:: unpack(format, buffer)

   Unpack from the buffer *buffer* (presumably packed by ``pack(format, ...)``)
   according to the format string *format*.  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 :func:`calcsize`.


.. function:: unpack_from(format, /, buffer, offset=0)

   Unpack from *buffer* starting at position *offset*, according to the format
   string *format*.  The result is a tuple even if it contains exactly one
   item.  The buffer's size in bytes, starting at position *offset*, must be at
   least the size required by the format, as reflected by :func:`calcsize`.
   A negative *offset* counts from the end of *buffer*.


.. function:: 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 :func:`calcsize`.

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

   .. versionadded:: 3.4


.. function:: calcsize(format)

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


.. _struct-format-strings:

Format Strings
--------------

Format strings describe the data layout when
packing and unpacking data.  They are built up from :ref:`format characters<format-characters>`,
which specify the type of data being packed/unpacked.  In addition,
special characters control the :ref:`byte order, size and alignment<struct-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.


.. _struct-alignment:

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).
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.

.. index::
   single: @ (at); in struct format strings
   single: = (equals); in struct format strings
   single: < (less); in struct format strings
   single: > (greater); in struct format strings
   single: ! (exclamation); in struct format strings

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.

.. 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 :data:`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 :ref:`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 ``'!'`` represents the network byte order which is always big-endian
as defined in `IETF RFC 1700 <IETF RFC 1700_>`_.

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 :ref:`struct-examples`.


.. _format-characters:

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           |                | \(7)       |
+--------+--------------------------+--------------------+----------------+------------+
| ``c``  | :c:expr:`char`           | bytes of length 1  | 1              |            |
+--------+--------------------------+--------------------+----------------+------------+
| ``b``  | :c:expr:`signed char`    | int                | 1              | \(2)       |
+--------+--------------------------+--------------------+----------------+------------+
| ``B``  | :c:expr:`unsigned char`  | int                | 1              | \(2)       |
+--------+--------------------------+--------------------+----------------+------------+
| ``?``  | :c:expr:`_Bool`          | bool               | 1              | \(1)       |
+--------+--------------------------+--------------------+----------------+------------+
| ``h``  | :c:expr:`short`          | int                | 2              | \(2)       |
+--------+--------------------------+--------------------+----------------+------------+
| ``H``  | :c:expr:`unsigned short` | int                | 2              | \(2)       |
+--------+--------------------------+--------------------+----------------+------------+
| ``i``  | :c:expr:`int`            | int                | 4              | \(2)       |
+--------+--------------------------+--------------------+----------------+------------+
| ``I``  | :c:expr:`unsigned int`   | int                | 4              | \(2)       |
+--------+--------------------------+--------------------+----------------+------------+
| ``l``  | :c:expr:`long`           | int                | 4              | \(2)       |
+--------+--------------------------+--------------------+----------------+------------+
| ``L``  | :c:expr:`unsigned long`  | int                | 4              | \(2)       |
+--------+--------------------------+--------------------+----------------+------------+
| ``q``  | :c:expr:`long long`      | int                | 8              | \(2)       |
+--------+--------------------------+--------------------+----------------+------------+
| ``Q``  | :c:expr:`unsigned long   | int                | 8              | \(2)       |
|        | long`                    |                    |                |            |
+--------+--------------------------+--------------------+----------------+------------+
| ``n``  | :c:type:`ssize_t`        | int                |                | \(2), \(3) |
+--------+--------------------------+--------------------+----------------+------------+
| ``N``  | :c:type:`size_t`         | int                |                | \(2), \(3) |
+--------+--------------------------+--------------------+----------------+------------+
| ``e``  | :c:expr:`_Float16`       | float              | 2              | \(4), \(6) |
+--------+--------------------------+--------------------+----------------+------------+
| ``f``  | :c:expr:`float`          | float              | 4              | \(4)       |
+--------+--------------------------+--------------------+----------------+------------+
| ``d``  | :c:expr:`double`         | float              | 8              | \(4)       |
+--------+--------------------------+--------------------+----------------+------------+
| ``Zf`` | :c:expr:`float complex`  | complex            | 8              | \(10)      |
+--------+--------------------------+--------------------+----------------+------------+
| ``Zd`` | :c:expr:`double complex` | complex            | 16             | \(10)      |
+--------+--------------------------+--------------------+----------------+------------+
| ``s``  | :c:expr:`char[]`         | bytes              |                | \(9)       |
+--------+--------------------------+--------------------+----------------+------------+
| ``p``  | :c:expr:`char[]`         | bytes              |                | \(8)       |
+--------+--------------------------+--------------------+----------------+------------+
| ``P``  | :c:expr:`void \*`        | int                |                | \(2), \(5) |
+--------+--------------------------+--------------------+----------------+------------+

.. versionchanged:: 3.3
   Added support for the ``'n'`` and ``'N'`` formats.

.. versionchanged:: 3.6
   Added support for the ``'e'`` format.

.. versionchanged:: 3.14
   Added support for the ``'F'`` and ``'D'`` formats.

.. versionchanged:: 3.15
   Added support for the ``'Zf'`` and ``'Zd'`` formats.
   ``'F'`` and ``'D'`` formats are :term:`soft deprecated`.

.. seealso::

   The :mod:`array` and :ref:`ctypes <ctypes-fundamental-data-types>` modules,
   as well as third-party modules like `numpy <https://numpy.org/doc/stable/reference/arrays.interface.html#object.__array_interface__>`__,
   use similar -- but slightly different -- type codes.


Notes:

(1)
   .. index:: single: ? (question mark); in struct format strings

   The ``'?'`` conversion code corresponds to the :c:expr:`_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 :meth:`~object.__index__` method then that method is
   called to convert the argument to an integer before packing.

   .. versionchanged:: 3.2
      Added use of the :meth:`~object.__index__` method for non-integers.

(3)
   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.

(4)
   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.

(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.

(6)
   The IEEE 754 binary16 "half precision" type was introduced in the 2008
   revision of the `IEEE 754 standard <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 :c:expr:`_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 <half precision format_>`_ for more information.

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

(8)
   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 byte string passed in to
   :func:`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 :func:`unpack`, the ``'p'`` format character consumes
   ``count`` bytes, but that the :class:`!bytes` object returned can never contain more than 255
   bytes.
   When packing, arguments of types :class:`bytes` and :class:`bytearray`
   are accepted.

(9)
   For the ``'s'`` format character, the count is interpreted as the length of the
   byte string, not a repeat count like for the other format characters; 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 :ref:`struct-examples`
   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 :class:`!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 :class:`bytes` and :class:`bytearray`
   are accepted.

(10)
   For the ``'Zf'`` and ``'Zd'`` 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 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.
   The ``'F'`` and ``'D'`` (for ``'Zf'`` and ``'Zd'``, respectively) format
   characters are supported for compatibility.


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.

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 :exc:`struct.error` is raised.

.. versionchanged:: 3.1
   Previously, some of the integer formats wrapped out-of-range values and
   raised :exc:`DeprecationWarning` instead of :exc:`struct.error`.

.. index:: single: ? (question mark); in struct format strings

For the ``'?'`` format character, the return value is either :const:`True` or
:const:`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.



.. _struct-examples:

Examples
^^^^^^^^

.. 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'

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 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'


.. seealso::

   Module :mod:`array`
      Packed binary storage of homogeneous data.

   Module :mod:`json`
      JSON encoder and decoder.

   Module :mod:`pickle`
      Python object serialization.


.. _applications:

Applications
------------

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


.. _struct-native-formats:

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 format 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'`` format 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.


.. _struct-standard-formats:

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


.. _struct-objects:

Classes
-------

The :mod:`!struct` module also defines the following type:


.. class:: 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 :class:`Struct`
      instance.

   Compiled Struct objects support the following methods and attributes:

   .. method:: pack(v1, v2, ...)

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


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

      Identical to the :func:`pack_into` function, using the compiled format.


   .. method:: unpack(buffer)

      Identical to the :func:`unpack` function, using the compiled format.
      The buffer's size in bytes must equal :attr:`size`.


   .. method:: unpack_from(buffer, offset=0)

      Identical to the :func:`unpack_from` function, using the compiled format.
      The buffer's size in bytes, starting at position *offset*, must be at least
      :attr:`size`.


   .. method:: iter_unpack(buffer)

      Identical to the :func:`iter_unpack` function, using the compiled format.
      The buffer's size in bytes must be a multiple of :attr:`size`.

      .. versionadded:: 3.4

   .. attribute:: format

      The format string used to construct this Struct object.

      .. versionchanged:: 3.7
         The format string type is now :class:`str` instead of :class:`bytes`.

   .. attribute:: size

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

   .. versionchanged:: 3.13 The *repr()* of structs has changed.  It
      is now:

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

.. _half precision format: https://en.wikipedia.org/wiki/Half-precision_floating-point_format

.. _ieee 754 standard: https://en.wikipedia.org/wiki/IEEE_754-2008_revision

.. _IETF RFC 1700: https://datatracker.ietf.org/doc/html/rfc1700