7.1. "struct" — Interpret bytes as packed binary data
*****************************************************

**Source code:** Lib/struct.py

======================================================================

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.

Several "struct" functions (and methods of "Struct") take a *buffer*
argument.  This refers to objects that implement the Buffer Protocol
and provide either a readable or read-writable buffer.  The most
common types used for that purpose are "bytes" and "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 "bytes" object.


7.1.1. 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 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’s size in bytes
   must match the size required by the format, as reflected by
   "calcsize()".

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.  The buffer’s size in bytes, minus *offset*, must
   be at least the size required by the format, as reflected by
   "calcsize()".

struct.iter_unpack(fmt, buffer)

   Iteratively unpack from the buffer *buffer* according to the format
   string *fmt*.  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 "calcsize()".

   Each iteration yields a tuple as specified by the format string.

   New in version 3.4.

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*.


7.1.2. 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.


7.1.2.1. 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.


7.1.2.2. 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"                     | bytes of length 1    | 1                |              |
+----------+----------------------------+----------------------+------------------+--------------+
| "b"      | "signed char"              | integer              | 1                | (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)     |
+----------+----------------------------+----------------------+------------------+--------------+
| "n"      | "ssize_t"                  | integer              |                  | (4)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "N"      | "size_t"                   | integer              |                  | (4)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "e"      | (7)                        | float                | 2                | (5)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "f"      | "float"                    | float                | 4                | (5)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "d"      | "double"                   | float                | 8                | (5)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "s"      | "char[]"                   | bytes                |                  |              |
+----------+----------------------------+----------------------+------------------+--------------+
| "p"      | "char[]"                   | bytes                |                  |              |
+----------+----------------------------+----------------------+------------------+--------------+
| "P"      | "void *"                   | integer              |                  | (6)          |
+----------+----------------------------+----------------------+------------------+--------------+

Changed in version 3.3: Added support for the "'n'" and "'N'" formats.

Changed in version 3.6: Added support for the "'e'" format.

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.

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.

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.

   Changed in version 3.2: Use of the "__index__()" method for non-
   integers is new in 3.2.

4. 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.

5. 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.

6. 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.

7. The IEEE 754 binary16 “half precision” type was introduced in the
   2008 revision of the 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 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 "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.


7.1.2.3. 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)
   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.

See also:

  Module "array"
     Packed binary storage of homogeneous data.

  Module "xdrlib"
     Packing and unpacking of XDR data.


7.1.3. 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.

   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 "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.
      The buffer’s size in bytes must equal "size".

   unpack_from(buffer, offset=0)

      Identical to the "unpack_from()" function, using the compiled
      format. The buffer’s size in bytes, minus *offset*, must be at
      least "size".

   iter_unpack(buffer)

      Identical to the "iter_unpack()" function, using the compiled
      format. The buffer’s size in bytes must be a multiple of "size".

      New in version 3.4.

   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".
