
``struct`` --- Interpret bytes as packed binary data
****************************************************

This module performs conversions between Python values and C structs
represented as Python ``bytes`` objects.  This can be used in handling
binary data stored in files or from network connections, among other
sources.  It uses *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 *Byte Order, Size, and Alignment*
  for details.


Functions and Exceptions
========================

The module defines the following exception and functions:

exception exception struct.error

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

struct.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.

struct.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.

struct.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)``).

struct.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)``).

struct.calcsize(fmt)

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


Format Strings
==============

Format strings are the mechanism used to specify the expected layout
when packing and unpacking data.  They are built up from *Format
Characters*, which specify the type of data being packed/unpacked.  In
addition, there are special characters for controlling the *Byte
Order, Size, and 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 *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 *Examples*.


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

Notes:

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

2. The ``'?'`` conversion code corresponds to the ``_Bool`` type
   defined by C99. If this type is not available, it is simulated
   using a ``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 ``long long``, or,
   on Windows, ``__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 ``struct.error`` is raised.

Changed in version 3.1: In 3.0, some of the integer formats wrapped
out-of-range values and raised ``DeprecationWarning`` instead of
``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 ``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
``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 ``True``
or ``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.


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.

See also:

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

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


Classes
=======

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

class class struct.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
   ``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:

   pack(v1, v2, ...)

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

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

      Identical to the ``pack_into()`` function, using the compiled
      format.

   unpack(buffer)

      Identical to the ``unpack()`` function, using the compiled
      format. (``len(buffer)`` must equal ``self.size``).

   unpack_from(buffer, offset=0)

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

   format

      The format string used to construct this Struct object.

   size

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