path: root/Doc/library/struct.rst

:mod:`struct` --- Interpret bytes as packed binary data
=======================================================

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

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

This module performs conversions between Python values and C structs represented
as Python :class:`bytes` objects.  This can be used in handling binary data
stored in files or from network connections, among other sources.  It uses
:ref:`struct-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 :ref:`struct-alignment` for details.

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(fmt, v1, v2, ...)

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


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

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


.. function:: unpack(fmt, buffer)

   Unpack from the buffer *buffer* (presumably packed by ``pack(fmt, ...)``)
   according to the format string *fmt*.  The result is a tuple even if it
   contains exactly one item.  The buffer must contain exactly the amount of
   data required by the format (``len(bytes)`` must equal ``calcsize(fmt)``).


.. function:: unpack_from(fmt, buffer, offset=0)

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


.. function:: calcsize(fmt)

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

.. _struct-format-strings:

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

Format strings are the mechanism used to specify the expected layout when
packing and unpacking data.  They are built up from :ref:`format-characters`,
which specify the type of data being packed/unpacked.  In addition, there are
special characters for controlling the :ref:`struct-alignment`.


.. _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).

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 :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 ``'!'`` 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 :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           |                |            |
+--------+-------------------------+--------------------+----------------+------------+
| ``c``  | :ctype:`char`           | bytes of length 1  | 1              |            |
+--------+-------------------------+--------------------+----------------+------------+
| ``b``  | :ctype:`signed char`    | integer            | 1              | \(1)       |
+--------+-------------------------+--------------------+----------------+------------+
| ``B``  | :ctype:`unsigned char`  | integer            | 1              |            |
+--------+-------------------------+--------------------+----------------+------------+
| ``?``  | :ctype:`_Bool`          | bool               | 1              | \(2)       |
+--------+-------------------------+--------------------+----------------+------------+
| ``h``  | :ctype:`short`          | integer            | 2              |            |
+--------+-------------------------+--------------------+----------------+------------+
| ``H``  | :ctype:`unsigned short` | integer            | 2              |            |
+--------+-------------------------+--------------------+----------------+------------+
| ``i``  | :ctype:`int`            | integer            | 4              |            |
+--------+-------------------------+--------------------+----------------+------------+
| ``I``  | :ctype:`unsigned int`   | integer            | 4              |            |
+--------+-------------------------+--------------------+----------------+------------+
| ``l``  | :ctype:`long`           | integer            | 4              |            |
+--------+-------------------------+--------------------+----------------+------------+
| ``L``  | :ctype:`unsigned long`  | integer            | 4              |            |
+--------+-------------------------+--------------------+----------------+------------+
| ``q``  | :ctype:`long long`      | integer            | 8              | \(3)       |
+--------+-------------------------+--------------------+----------------+------------+
| ``Q``  | :ctype:`unsigned long   | integer            | 8              | \(3)       |
|        | long`                   |                    |                |            |
+--------+-------------------------+--------------------+----------------+------------+
| ``f``  | :ctype:`float`          | float              | 4              | \(4)       |
+--------+-------------------------+--------------------+----------------+------------+
| ``d``  | :ctype:`double`         | float              | 8              | \(4)       |
+--------+-------------------------+--------------------+----------------+------------+
| ``s``  | :ctype:`char[]`         | bytes              |                | \(1)       |
+--------+-------------------------+--------------------+----------------+------------+
| ``p``  | :ctype:`char[]`         | bytes              |                | \(1)       |
+--------+-------------------------+--------------------+----------------+------------+
| ``P``  | :ctype:`void \*`        | integer            |                | \(5)       |
+--------+-------------------------+--------------------+----------------+------------+

Notes:

(1)
   The ``c``, ``s`` and ``p`` conversion codes operate on :class:`bytes`
   objects, but packing with such codes also supports :class:`str` objects,
   which are encoded using UTF-8.

(2)
   The ``'?'`` conversion code corresponds to the :ctype:`_Bool` type defined by
   C99. If this type is not available, it is simulated using a :ctype:`char`. In
   standard mode, it is always represented by one byte.

(3)
   The ``'q'`` and ``'Q'`` conversion codes are available in native mode only if
   the platform C compiler supports C :ctype:`long long`, or, on Windows,
   :ctype:`__int64`.  They are always available in standard modes.

(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 length of the
bytes, not a repeat count like for the other format characters; for example,
``'10s'`` means a single 10-byte string, while ``'10c'`` means 10 characters.
For packing, the 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
string (while ``'0c'`` means 0 characters).

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
   In 3.0, some of the integer formats wrapped out-of-range values and
   raised :exc:`DeprecationWarning` instead of :exc:`struct.error`.


The ``'p'`` format character encodes a "Pascal string", meaning a short
variable-length string stored in a fixed number of bytes. The count is the total
number of bytes stored.  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 :func:`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 :func:`unpack`, the ``'p'`` format character consumes count
bytes, but that the string returned can never contain more than 255 bytes.



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::
   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)
   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 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 since the padding
needed to satisfy alignment requirements is different::

    >>> pack('ci', '*', 0x12131415)
    b'*\x00\x00\x00\x12\x13\x14\x15'
    >>> pack('ic', 0x12131415, '*')
    b'\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)
    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 and
alignment does not enforce any alignment.


.. seealso::

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

   Module :mod:`xdrlib`
      Packing and unpacking of XDR data.


.. _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 the :mod:`struct` functions with the
   same format since the format string only needs to be compiled once.


   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:`self.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.
      (``len(buffer)`` must equal :attr:`self.size`).


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

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


   .. attribute:: format

      The format string used to construct this Struct object.

   .. attribute:: size

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