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cpython/Doc/library/struct.rst
Stargirl Flowers fb27187203
[doc] struct: update note about network byte order form to be more helpful (GH-22201) 
Update the sentence to provide some context on why network byte order is defined as big endian.
2020-09-11 08:20:12 -07:00

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:mod:`struct` --- Interpret bytes as packed binary data
=======================================================
.. 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 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.
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.
.. 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`.
.. 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 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).
.. 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.
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 ``'!'`` 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 | | |
+--------+--------------------------+--------------------+----------------+------------+
| ``c`` | :c:type:`char` | bytes of length 1 | 1 | |
+--------+--------------------------+--------------------+----------------+------------+
| ``b`` | :c:type:`signed char` | integer | 1 | \(1), \(2) |
+--------+--------------------------+--------------------+----------------+------------+
| ``B`` | :c:type:`unsigned char` | integer | 1 | \(2) |
+--------+--------------------------+--------------------+----------------+------------+
| ``?`` | :c:type:`_Bool` | bool | 1 | \(1) |
+--------+--------------------------+--------------------+----------------+------------+
| ``h`` | :c:type:`short` | integer | 2 | \(2) |
+--------+--------------------------+--------------------+----------------+------------+
| ``H`` | :c:type:`unsigned short` | integer | 2 | \(2) |
+--------+--------------------------+--------------------+----------------+------------+
| ``i`` | :c:type:`int` | integer | 4 | \(2) |
+--------+--------------------------+--------------------+----------------+------------+
| ``I`` | :c:type:`unsigned int` | integer | 4 | \(2) |
+--------+--------------------------+--------------------+----------------+------------+
| ``l`` | :c:type:`long` | integer | 4 | \(2) |
+--------+--------------------------+--------------------+----------------+------------+
| ``L`` | :c:type:`unsigned long` | integer | 4 | \(2) |
+--------+--------------------------+--------------------+----------------+------------+
| ``q`` | :c:type:`long long` | integer | 8 | \(2) |
+--------+--------------------------+--------------------+----------------+------------+
| ``Q`` | :c:type:`unsigned long | integer | 8 | \(2) |
| | long` | | | |
+--------+--------------------------+--------------------+----------------+------------+
| ``n`` | :c:type:`ssize_t` | integer | | \(3) |
+--------+--------------------------+--------------------+----------------+------------+
| ``N`` | :c:type:`size_t` | integer | | \(3) |
+--------+--------------------------+--------------------+----------------+------------+
| ``e`` | \(6) | float | 2 | \(4) |
+--------+--------------------------+--------------------+----------------+------------+
| ``f`` | :c:type:`float` | float | 4 | \(4) |
+--------+--------------------------+--------------------+----------------+------------+
| ``d`` | :c:type:`double` | float | 8 | \(4) |
+--------+--------------------------+--------------------+----------------+------------+
| ``s`` | :c:type:`char[]` | bytes | | |
+--------+--------------------------+--------------------+----------------+------------+
| ``p`` | :c:type:`char[]` | bytes | | |
+--------+--------------------------+--------------------+----------------+------------+
| ``P`` | :c:type:`void \*` | integer | | \(5) |
+--------+--------------------------+--------------------+----------------+------------+
.. versionchanged:: 3.3
Added support for the ``'n'`` and ``'N'`` formats.
.. versionchanged:: 3.6
Added support for the ``'e'`` format.
Notes:
(1)
.. index:: single: ? (question mark); in struct format strings
The ``'?'`` conversion code corresponds to the :c:type:`_Bool` type defined by
C99. If this type is not available, it is simulated using a :c:type:`char`. In
standard mode, it is always 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:`__index__` method then that method is
called to convert the argument to an integer before packing.
.. versionchanged:: 3.2
Added use of the :meth:`__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: 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.
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.
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 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
Previously, 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*, 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
: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.
.. 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::
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', b'*', 0x12131415)
b'*\x00\x00\x00\x12\x13\x14\x15'
>>> pack('ic', 0x12131415, b'*')
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.
.. note::
The compiled versions of the most recent format strings passed to
:class:`Struct` and 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`.
.. _half precision format: https://en.wikipedia.org/wiki/Half-precision_floating-point_format
.. _ieee 754 standard: https://en.wikipedia.org/wiki/IEEE_floating_point#IEEE_754-2008
.. _IETF RFC 1700: https://tools.ietf.org/html/rfc1700
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