mirror of https://github.com/python/cpython
Issue #7355: Various improvements to struct module documentation.
- note early on that the result of struct.pack includes padding bytes by default - add examples showing how order of struct fields can affect size (due to padding) - better headers and references; introduction to format strings - integrate packing notes into table Many thanks to Meador Inge for the patch.
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Mark Dickinson
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bbacb838c9
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@ -10,10 +10,18 @@
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triple: packing; binary; data
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This module performs conversions between Python values and C structs represented
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as Python strings. It uses :dfn:`format strings` (explained below) as compact
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descriptions of the lay-out of the C structs and the intended conversion to/from
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Python values. This can be used in handling binary data stored in files or from
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network connections, among other sources.
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as Python strings. This can be used in handling binary data stored in files or from network connections, among other sources. It uses
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:ref:`struct-format-strings` as compact descriptions of the layout of the C
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structs and the intended conversion to/from Python values.
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.. note::
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The string representation of a given C struct includes padding where
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necessary by default. This is the same behavior as provided by most
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C compilers. The padding may be disabled if desired.
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Functions and Exceptions
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------------------------
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The module defines the following exception and functions:
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@ -63,49 +71,62 @@ The module defines the following exception and functions:
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Return the size of the struct (and hence of the string) corresponding to the
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given format.
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.. _struct-format-strings:
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Format Strings
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--------------
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Format strings are the mechanism used to specify the expected layout when
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packing and unpacking data. They are built up from format characters, which
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specify the type of data being packed/unpacked. In addition, there are
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special characters for controlling the byte order, size, and alignment.
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Format Characters
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^^^^^^^^^^^^^^^^^
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Format characters have the following meaning; the conversion between C and
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Python values should be obvious given their types:
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+--------+-------------------------+--------------------+-------+
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| Format | C Type | Python | Notes |
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+========+=========================+====================+=======+
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| ``x`` | pad byte | no value | |
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+--------+-------------------------+--------------------+-------+
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| ``c`` | :ctype:`char` | string of length 1 | |
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+--------+-------------------------+--------------------+-------+
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| ``b`` | :ctype:`signed char` | integer | |
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+--------+-------------------------+--------------------+-------+
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| ``B`` | :ctype:`unsigned char` | integer | |
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+--------+-------------------------+--------------------+-------+
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| ``?`` | :ctype:`_Bool` | bool | \(1) |
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+--------+-------------------------+--------------------+-------+
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| ``h`` | :ctype:`short` | integer | |
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+--------+-------------------------+--------------------+-------+
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| ``H`` | :ctype:`unsigned short` | integer | |
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+--------+-------------------------+--------------------+-------+
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| ``i`` | :ctype:`int` | integer | |
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+--------+-------------------------+--------------------+-------+
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| ``I`` | :ctype:`unsigned int` | integer or long | |
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+--------+-------------------------+--------------------+-------+
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| ``l`` | :ctype:`long` | integer | |
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+--------+-------------------------+--------------------+-------+
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| ``L`` | :ctype:`unsigned long` | long | |
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+--------+-------------------------+--------------------+-------+
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| ``q`` | :ctype:`long long` | long | \(2) |
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+--------+-------------------------+--------------------+-------+
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| ``Q`` | :ctype:`unsigned long | long | \(2) |
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| | long` | | |
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+--------+-------------------------+--------------------+-------+
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| ``f`` | :ctype:`float` | float | |
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+--------+-------------------------+--------------------+-------+
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| ``d`` | :ctype:`double` | float | |
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+--------+-------------------------+--------------------+-------+
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| ``s`` | :ctype:`char[]` | string | |
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+--------+-------------------------+--------------------+-------+
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| ``p`` | :ctype:`char[]` | string | |
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+--------+-------------------------+--------------------+-------+
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| ``P`` | :ctype:`void \*` | long | |
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+--------+-------------------------+--------------------+-------+
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+--------+-------------------------+--------------------+------------+
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| Format | C Type | Python | Notes |
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+========+=========================+====================+============+
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| ``x`` | pad byte | no value | |
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+--------+-------------------------+--------------------+------------+
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| ``c`` | :ctype:`char` | string of length 1 | |
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+--------+-------------------------+--------------------+------------+
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| ``b`` | :ctype:`signed char` | integer | \(3) |
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+--------+-------------------------+--------------------+------------+
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| ``B`` | :ctype:`unsigned char` | integer | \(3) |
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+--------+-------------------------+--------------------+------------+
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| ``?`` | :ctype:`_Bool` | bool | \(1) |
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+--------+-------------------------+--------------------+------------+
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| ``h`` | :ctype:`short` | integer | \(3) |
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+--------+-------------------------+--------------------+------------+
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| ``H`` | :ctype:`unsigned short` | integer | \(3) |
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+--------+-------------------------+--------------------+------------+
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| ``i`` | :ctype:`int` | integer | \(3) |
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+--------+-------------------------+--------------------+------------+
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| ``I`` | :ctype:`unsigned int` | integer or long | \(3) |
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+--------+-------------------------+--------------------+------------+
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| ``l`` | :ctype:`long` | integer | \(3) |
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+--------+-------------------------+--------------------+------------+
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| ``L`` | :ctype:`unsigned long` | long | \(3) |
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+--------+-------------------------+--------------------+------------+
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| ``q`` | :ctype:`long long` | long | \(2),\(3) |
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+--------+-------------------------+--------------------+------------+
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| ``Q`` | :ctype:`unsigned long | long | \(2),\(3) |
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| | long` | | |
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+--------+-------------------------+--------------------+------------+
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| ``f`` | :ctype:`float` | float | |
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+--------+-------------------------+--------------------+------------+
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| ``d`` | :ctype:`double` | float | |
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+--------+-------------------------+--------------------+------------+
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| ``s`` | :ctype:`char[]` | string | |
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+--------+-------------------------+--------------------+------------+
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| ``p`` | :ctype:`char[]` | string | |
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+--------+-------------------------+--------------------+------------+
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| ``P`` | :ctype:`void \*` | long | \(3) |
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+--------+-------------------------+--------------------+------------+
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Notes:
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@ -180,6 +201,9 @@ For the ``'?'`` format character, the return value is either :const:`True` or
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Either 0 or 1 in the native or standard bool representation will be packed, and
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any non-zero value will be True when unpacking.
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Byte Order, Size, and Alignment
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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By default, C numbers are represented in the machine's native format and byte
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order, and properly aligned by skipping pad bytes if necessary (according to the
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rules used by the C compiler).
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@ -234,8 +258,29 @@ order character ``'='`` chooses to use little- or big-endian ordering based on
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the host system. The struct module does not interpret this as native ordering,
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so the ``'P'`` format is not available.
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Examples (all using native byte order, size and alignment, on a big-endian
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machine)::
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Notes:
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(1) Padding is only automatically added between successive structure members.
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Never at the beginning of the string encoding and never at the end.
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(2) Padding is disabled when using non-native size and alignment, e.g.
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with '<', '>', '=', and '!'.
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(3) To align the end of a structure to the alignment requirement of a
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particular type, end the format with the code for that type with a repeat
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count of zero. See :ref:`struct-examples`.
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.. _struct-examples:
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Examples
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^^^^^^^^
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.. note::
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All examples assume a native byte order, size, and alignment with a
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big-endian machine.
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A basic example of packing/unpacking three integers::
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>>> from struct import *
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>>> pack('hhl', 1, 2, 3)
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@ -245,13 +290,6 @@ machine)::
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>>> calcsize('hhl')
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8
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Hint: to align the end of a structure to the alignment requirement of a
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particular type, end the format with the code for that type with a repeat count
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of zero. For example, the format ``'llh0l'`` specifies two pad bytes at the
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end, assuming longs are aligned on 4-byte boundaries. This only works when
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native size and alignment are in effect; standard size and alignment does not
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enforce any alignment.
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Unpacked fields can be named by assigning them to variables or by wrapping
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the result in a named tuple::
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@ -263,6 +301,27 @@ the result in a named tuple::
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>>> Student._make(unpack('<10sHHb', s))
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Student(name='raymond ', serialnum=4658, school=264, gradelevel=8)
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The ordering of format characters may have an impact on size since the padding
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needed to satisfy alignment requirements is different::
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>>> pack('ci', '*', 0x12131415)
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'*\x00\x00\x00\x12\x13\x14\x15'
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>>> pack('ic', 0x12131415, '*')
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'\x12\x13\x14\x15*'
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>>> calcsize('ci')
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8
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>>> calcsize('ic')
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5
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The following format ``'llh0l'`` specifies two pad bytes at the end, assuming longs are aligned on 4-byte boundaries::
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>>> pack('llh0l', 1, 2, 3)
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'\x00\x00\x00\x01\x00\x00\x00\x02\x00\x03\x00\x00'
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This only works when native size and alignment are in effect; standard size and
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alignment does not enforce any alignment.
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.. seealso::
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Module :mod:`array`
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@ -274,8 +333,8 @@ the result in a named tuple::
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.. _struct-objects:
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Struct Objects
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--------------
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Objects
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-------
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The :mod:`struct` module also defines the following type:
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