Doc/library/struct.rst - platform/external/python/cpython2 - Git at Google


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

 .. module:: struct
    :synopsis: Interpret strings 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 strings.  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 string containing the values ``v1, v2, ...`` packed according to the
    given format.  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 given format, write the
    packed bytes into the writable *buffer* starting at *offset*. Note that the
    offset is a required argument.

    .. versionadded:: 2.5


 .. function:: unpack(fmt, string)

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


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

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

    .. versionadded:: 2.5


 .. function:: calcsize(fmt)

    Return the size of the struct (and hence of the string) corresponding to the
    given 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).

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

 Notes:

 (1)
    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.

    .. versionadded:: 2.6

 (2)
    The ``'q'`` and ``'Q'`` conversion codes are available in native mode only if
    the platform C compiler supports C :c:type:`long long`, or, on Windows,
    :c:type:`__int64`.  They are always available in standard modes.

    .. versionadded:: 2.2

 (3)
    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.  If no
    :meth:`__index__` method exists, or the call to :meth:`__index__` raises
    :exc:`TypeError`, then the :meth:`__int__` method is tried.  However, the use
    of :meth:`__int__` is deprecated, and will raise :exc:`DeprecationWarning`.

    .. versionchanged:: 2.7
       Use of the :meth:`__index__` method for non-integers is new in 2.7.

    .. versionchanged:: 2.7
       Prior to version 2.7, not all integer conversion codes would use the
       :meth:`__int__` method to convert, and :exc:`DeprecationWarning` was
       raised only for float arguments.

 (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 size of the
 string, 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 string always has exactly the specified number of
 bytes.  As a special case, ``'0s'`` means a single, empty string (while
 ``'0c'`` means 0 characters).

 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 characters.

 For the ``'P'`` format character, the return value is a Python integer or long
 integer, depending on the size needed to hold a pointer when it has been cast to
 an integer type.  A *NULL* pointer will always be returned as the Python integer
 ``0``. When packing pointer-sized values, Python integer or long integer objects
 may be used.  For example, the Alpha and Merced processors use 64-bit pointer
 values, meaning a Python long integer will be used to hold the pointer; other
 platforms use 32-bit pointers and will use a Python integer.

 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)
    '\x00\x01\x00\x02\x00\x00\x00\x03'
    >>> unpack('hhl', '\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 = '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='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)
     '*\x00\x00\x00\x12\x13\x14\x15'
     >>> pack('ic', 0x12131415, '*')
     '\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)
     '\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.

    .. versionadded:: 2.5

    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(string)

       Identical to the :func:`unpack` function, using the compiled format.
       (``len(string)`` 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 string) corresponding
       to :attr:`format`.

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

	.. module:: struct
	:synopsis: Interpret strings 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 strings. 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 string containing the values ``v1, v2, ...`` packed according to the
	given format. 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 given format, write the
	packed bytes into the writable buffer starting at offset. Note that the
	offset is a required argument.

	.. versionadded:: 2.5


	.. function:: unpack(fmt, string)

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


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

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

	.. versionadded:: 2.5


	.. function:: calcsize(fmt)

	Return the size of the struct (and hence of the string) corresponding to the
	given 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).

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

	Notes:

	(1)
	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.

	.. versionadded:: 2.6

	(2)
	The ``'q'`` and ``'Q'`` conversion codes are available in native mode only if
	the platform C compiler supports C :c:type:`long long`, or, on Windows,
	:c:type:`__int64`. They are always available in standard modes.

	.. versionadded:: 2.2

	(3)
	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. If no
	:meth:`__index__` method exists, or the call to :meth:`__index__` raises
	:exc:`TypeError`, then the :meth:`__int__` method is tried. However, the use
	of :meth:`__int__` is deprecated, and will raise :exc:`DeprecationWarning`.

	.. versionchanged:: 2.7
	Use of the :meth:`__index__` method for non-integers is new in 2.7.

	.. versionchanged:: 2.7
	Prior to version 2.7, not all integer conversion codes would use the
	:meth:`__int__` method to convert, and :exc:`DeprecationWarning` was
	raised only for float arguments.

	(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 size of the
	string, 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 string always has exactly the specified number of
	bytes. As a special case, ``'0s'`` means a single, empty string (while
	``'0c'`` means 0 characters).

	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 characters.

	For the ``'P'`` format character, the return value is a Python integer or long
	integer, depending on the size needed to hold a pointer when it has been cast to
	an integer type. A NULL pointer will always be returned as the Python integer
	``0``. When packing pointer-sized values, Python integer or long integer objects
	may be used. For example, the Alpha and Merced processors use 64-bit pointer
	values, meaning a Python long integer will be used to hold the pointer; other
	platforms use 32-bit pointers and will use a Python integer.

	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)
	'\x00\x01\x00\x02\x00\x00\x00\x03'
	>>> unpack('hhl', '\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 = '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='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)
	'*\x00\x00\x00\x12\x13\x14\x15'
	>>> pack('ic', 0x12131415, '*')
	'\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)
	'\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.

	.. versionadded:: 2.5

	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(string)

	Identical to the :func:`unpack` function, using the compiled format.
	(``len(string)`` 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 string) corresponding
	to :attr:`format`.