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

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

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

This module converts between Python values and C structs represented
as Python "bytes" objects.  Compact format strings describe the
intended conversions to/from Python values. The module’s functions and
objects can be used for two largely distinct applications, data
exchange with external sources (files or network connections), or data
transfer between the Python application and the C layer.

Note:

  When no prefix character is given, native mode is the default. It
  packs or unpacks data based on the platform and compiler on which
  the Python interpreter was built. The result of packing a given C
  struct includes pad bytes which maintain proper alignment for the C
  types involved; similarly, alignment is taken into account when
  unpacking.  In contrast, when communicating data between external
  sources, the programmer is responsible for defining byte ordering
  and padding between elements. 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.


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(format, v1, v2, ...)

   Return a bytes object containing the values *v1*, *v2*, … packed
   according to the format string *format*.  The arguments must match
   the values required by the format exactly.

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

   Pack the values *v1*, *v2*, … according to the format string
   *format* and write the packed bytes into the writable buffer
   *buffer* starting at position *offset*.  Note that *offset* is a
   required argument.

struct.unpack(format, buffer)

   Unpack from the buffer *buffer* (presumably packed by "pack(format,
   ...)") according to the format string *format*.  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(format, /, buffer, offset=0)

   Unpack from *buffer* starting at position *offset*, according to
   the format string *format*.  The result is a tuple even if it
   contains exactly one item.  The buffer’s size in bytes, starting at
   position *offset*, must be at least the size required by the
   format, as reflected by "calcsize()".

struct.iter_unpack(format, buffer)

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

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


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

Format strings describe the data layout when packing and unpacking
data.  They are built up from format characters, which specify the
type of data being packed/unpacked.  In addition, special characters
control the byte order, size and alignment. Each format string
consists of an optional prefix character which describes the overall
properties of the data and one or more format characters which
describe the actual data values and padding.


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). This behavior is
chosen so that the bytes of a packed struct correspond exactly to the
memory layout of the corresponding C struct. Whether to use native
byte ordering and padding or standard formats depends on the
application.

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, AMD64 (x86-64), and Apple M1 are
little-endian; IBM z and many legacy architectures are big-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 "'!'" represents the network byte order which is always big-
endian as defined in IETF RFC 1700.

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             |                  | (7)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "c"      | "char"                     | bytes of length 1    | 1                |              |
+----------+----------------------------+----------------------+------------------+--------------+
| "b"      | "signed char"              | integer              | 1                | (1), (2)     |
+----------+----------------------------+----------------------+------------------+--------------+
| "B"      | "unsigned char"            | integer              | 1                | (2)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "?"      | "_Bool"                    | bool                 | 1                | (1)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "h"      | "short"                    | integer              | 2                | (2)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "H"      | "unsigned short"           | integer              | 2                | (2)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "i"      | "int"                      | integer              | 4                | (2)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "I"      | "unsigned int"             | integer              | 4                | (2)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "l"      | "long"                     | integer              | 4                | (2)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "L"      | "unsigned long"            | integer              | 4                | (2)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "q"      | "long long"                | integer              | 8                | (2)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "Q"      | "unsigned long long"       | integer              | 8                | (2)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "n"      | "ssize_t"                  | integer              |                  | (3)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "N"      | "size_t"                   | integer              |                  | (3)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "e"      | (6)                        | float                | 2                | (4)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "f"      | "float"                    | float                | 4                | (4)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "d"      | "double"                   | float                | 8                | (4)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "s"      | "char[]"                   | bytes                |                  | (9)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "p"      | "char[]"                   | bytes                |                  | (8)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "P"      | "void*"                    | integer              |                  | (5)          |
+----------+----------------------------+----------------------+------------------+--------------+

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. 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: Added use of the "__index__()" method for
   non-integers.

3. 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.

4. 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.

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.

6. 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.

7. When packing, "'x'" inserts one NUL byte.

8. 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.

9. 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
   mapping to or from a single Python byte string, while "'10c'" means
   10 separate one byte character elements (e.g., "cccccccccc")
   mapping to or from ten different Python byte objects. (See Examples
   for a concrete demonstration of the difference.) 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).

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.

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: Previously, some of the integer formats
wrapped out-of-range values and raised "DeprecationWarning" instead of
"struct.error".

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:

  Native byte order examples (designated by the "'@'" format prefix or
  lack of any prefix character) may not match what the reader’s
  machine produces as that depends on the platform and compiler.

Pack and unpack integers of three different sizes, using big endian
ordering:

   >>> from struct import *
   >>> pack(">bhl", 1, 2, 3)
   b'\x01\x00\x02\x00\x00\x00\x03'
   >>> unpack('>bhl', b'\x01\x00\x02\x00\x00\x00\x03'
   (1, 2, 3)
   >>> calcsize('>bhl')
   7

Attempt to pack an integer which is too large for the defined field:

   >>> pack(">h", 99999)
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   struct.error: 'h' format requires -32768 <= number <= 32767

Demonstrate the difference between "'s'" and "'c'" format characters:

   >>> pack("@ccc", b'1', b'2', b'3')
   b'123'
   >>> pack("@3s", b'123')
   b'123'

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 in native
mode since padding is implicit. In standard mode, the user is
responsible for inserting any desired padding. Note in the first
"pack" call below that three NUL bytes were added after the packed
"'#'" to align the following integer on a four-byte boundary. In this
example, the output was produced on a little endian machine:

   >>> pack('@ci', b'#', 0x12131415)
   b'#\x00\x00\x00\x15\x14\x13\x12'
   >>> pack('@ic', 0x12131415, b'#')
   b'\x15\x14\x13\x12#'
   >>> calcsize('@ci')
   8
   >>> calcsize('@ic')
   5

The following format "'llh0l'" results in two pad bytes being added at
the end, assuming the platform’s 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'

See also:

  Module "array"
     Packed binary storage of homogeneous data.

  Module "json"
     JSON encoder and decoder.

  Module "pickle"
     Python object serialization.


Applications
============

Two main applications for the "struct" module exist, data interchange
between Python and C code within an application or another application
compiled using the same compiler (native formats), and data
interchange between applications using agreed upon data layout
(standard formats).  Generally speaking, the format strings
constructed for these two domains are distinct.


Native Formats
--------------

When constructing format strings which mimic native layouts, the
compiler and machine architecture determine byte ordering and padding.
In such cases, the "@" format character should be used to specify
native byte ordering and data sizes.  Internal pad bytes are normally
inserted automatically.  It is possible that a zero-repeat format code
will be needed at the end of a format string to round up to the
correct byte boundary for proper alignment of consective chunks of
data.

Consider these two simple examples (on a 64-bit, little-endian
machine):

   >>> calcsize('@lhl')
   24
   >>> calcsize('@llh')
   18

Data is not padded to an 8-byte boundary at the end of the second
format string without the use of extra padding.  A zero-repeat format
code solves that problem:

   >>> calcsize('@llh0l')
   24

The "'x'" format code can be used to specify the repeat, but for
native formats it is better to use a zero-repeat format like "'0l'".

By default, native byte ordering and alignment is used, but it is
better to be explicit and use the "'@'" prefix character.


Standard Formats
----------------

When exchanging data beyond your process such as networking or
storage, be precise.  Specify the exact byte order, size, and
alignment.  Do not assume they match the native order of a particular
machine. For example, network byte order is big-endian, while many
popular CPUs are little-endian.  By defining this explicitly, the user
need not care about the specifics of the platform their code is
running on. The first character should typically be "<" or ">" (or
"!").  Padding is the responsibility of the programmer.  The zero-
repeat format character won’t work.  Instead, the user must explicitly
add "'x'" pad bytes where needed.  Revisiting the examples from the
previous section, we have:

   >>> calcsize('<qh6xq')
   24
   >>> pack('<qh6xq', 1, 2, 3) == pack('@lhl', 1, 2, 3)
   True
   >>> calcsize('@llh')
   18
   >>> pack('@llh', 1, 2, 3) == pack('<qqh', 1, 2, 3)
   True
   >>> calcsize('<qqh6x')
   24
   >>> calcsize('@llh0l')
   24
   >>> pack('@llh0l', 1, 2, 3) == pack('<qqh6x', 1, 2, 3)
   True

The above results (executed on a 64-bit machine) aren’t guaranteed to
match when executed on different machines.  For example, the examples
below were executed on a 32-bit machine:

   >>> calcsize('<qqh6x')
   24
   >>> calcsize('@llh0l')
   12
   >>> pack('@llh0l', 1, 2, 3) == pack('<qqh6x', 1, 2, 3)
   False


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
   module-level functions with the same format since the format string
   is only compiled once.

   Note:

     The compiled versions of the most recent format strings passed to
     "Struct" and the module-level functions are cached, so programs
     that use only a few format strings needn’t worry about reusing a
     single "Struct" instance.

   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, starting at position
      *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.

      Changed in version 3.7: The format string type is now "str"
      instead of "bytes".

   size

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