7.3. "struct" — Interpret strings as packed 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 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.


7.3.1. Functions and Exceptions
===============================

The module defines the following exception and functions:

exception struct.error

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

struct.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.

struct.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.

   New in version 2.5.

struct.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)").

struct.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)").

   New in version 2.5.

struct.calcsize(fmt)

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


7.3.2. 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.


7.3.2.1. 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.


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

Notes:

1. 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.

   New in version 2.6.

2. 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.

   New in version 2.2.

3. When attempting to pack a non-integer using any of the integer
   conversion codes, if the non-integer has a "__index__()" method
   then that method is called to convert the argument to an integer
   before packing.  If no "__index__()" method exists, or the call to
   "__index__()" raises "TypeError", then the "__int__()" method is
   tried.  However, the use of "__int__()" is deprecated, and will
   raise "DeprecationWarning".

   Changed in version 2.7: Use of the "__index__()" method for non-
   integers is new in 2.7.

   Changed in version 2.7: Prior to version 2.7, not all integer
   conversion codes would use the "__int__()" method to convert, and
   "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 "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 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 "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.


7.3.2.3. 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.

See also:

  Module "array"
     Packed binary storage of homogeneous data.

  Module "xdrlib"
     Packing and unpacking of XDR data.


7.3.3. Classes
==============

The "struct" module also defines the following type:

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.

   New in version 2.5.

   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(string)

      Identical to the "unpack()" function, using the compiled format.
      ("len(string)" 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 string)
      corresponding to "format".
