"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.

Nota:

  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.

   Added 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 type codes, 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.

Nota:

  The number 1023 ("0x3ff" in hexadecimal) has the following byte
  representations:

  * "03 ff" in big-endian (">")

  * "ff 03" in little-endian ("<")

  Python example:

  >>> import struct
  >>> struct.pack('>h', 1023)
  b'\x03\xff'
  >>> struct.pack('<h', 1023)
  b'\xff\x03'

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 type code;  see the table in the
Type Codes 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.


Type Codes
----------

Type codes (or format codes) 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                | int                  | 1                | (2)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "B"      | unsigned char              | int                  | 1                | (2)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "?"      | _Bool                      | bool                 | 1                | (1)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "h"      | short                      | int                  | 2                | (2)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "H"      | unsigned short             | int                  | 2                | (2)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "i"      | int                        | int                  | 4                | (2)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "I"      | unsigned int               | int                  | 4                | (2)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "l"      | long                       | int                  | 4                | (2)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "L"      | unsigned long              | int                  | 4                | (2)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "q"      | long long                  | int                  | 8                | (2)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "Q"      | unsigned long long         | int                  | 8                | (2)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "n"      | "ssize_t"                  | int                  |                  | (2), (3)     |
+----------+----------------------------+----------------------+------------------+--------------+
| "N"      | "size_t"                   | int                  |                  | (2), (3)     |
+----------+----------------------------+----------------------+------------------+--------------+
| "e"      | _Float16                   | float                | 2                | (4), (6)     |
+----------+----------------------------+----------------------+------------------+--------------+
| "f"      | float                      | float                | 4                | (4)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "d"      | double                     | float                | 8                | (4)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "F"      | float complex              | complex              | 8                | (10)         |
+----------+----------------------------+----------------------+------------------+--------------+
| "D"      | double complex             | complex              | 16               | (10)         |
+----------+----------------------------+----------------------+------------------+--------------+
| "Zf"     | float complex              | complex              | 8                | (10)         |
+----------+----------------------------+----------------------+------------------+--------------+
| "Zd"     | double complex             | complex              | 16               | (10)         |
+----------+----------------------------+----------------------+------------------+--------------+
| "s"      | char[]                     | bytes                |                  | (9)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "p"      | char[]                     | bytes                |                  | (8)          |
+----------+----------------------------+----------------------+------------------+--------------+
| "P"      | void*                      | int                  |                  | (2), (5)     |
+----------+----------------------------+----------------------+------------------+--------------+

Cambiato nella versione 3.3: Added support for the "'n'" and "'N'"
formats.

Cambiato nella versione 3.6: Added support for the "'e'" format.

Cambiato nella versione 3.14: Added support for the "'F'" and "'D'"
formats.

Cambiato nella versione 3.15: Added support for the "'Zf'" and "'Zd'"
formats.

Vedi anche:

  The "array" and ctypes modules, as well as third-party modules like
  numpy, use similar -- but slightly different -- type codes.

Notes:

1. The "'?'" conversion code corresponds to the _Bool type defined by
   C standards since C99.  In standard mode, it is 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.

   Cambiato nella versione 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'" type code 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: it's available as _Float16 type, if the compiler
   supports the Annex H of the C23 standard.  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'" type code 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
   byte 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 byte 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'" type code consumes "count" bytes,
   but that the "bytes" object returned can never contain more than
   255 bytes. When packing, arguments of types "bytes" and "bytearray"
   are accepted.

9. For the "'s'" type code, the count is interpreted as the length of
   the byte string, not a repeat count like for the other type codes;
   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 byte 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 byte string
   (while "'0c'" means 0 characters). When packing, arguments of types
   "bytes" and "bytearray" are accepted.

10. For the "'F'" and "'D'" type codes, the packed representation uses
    the IEEE 754 binary32 and binary64 format for components of the
    complex number, regardless of the floating-point format used by
    the platform. Note that complex types ("F"/"Zf" and "D"/"Zd") are
    available unconditionally, despite complex types being an optional
    feature in C. As specified in the C11 standard, each complex type
    is represented by a two-element C array containing, respectively,
    the real and imaginary parts.

A type code 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.

Cambiato nella versione 3.1: Previously, some of the integer formats
wrapped out-of-range values and raised "DeprecationWarning" instead of
"struct.error".

For the "'?'" type code, 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
--------

Nota:

  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 type codes 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'

Vedi anche:

  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 type code
will be needed at the end of a format string to round up to the
correct byte boundary for proper alignment of consecutive 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'" type 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.

   Nota:

     The compiled versions of the most recent format strings passed to
     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".

      Added in version 3.4.

   format

      The format string used to construct this Struct object.

      Cambiato nella versione 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".

   Cambiato nella versione 3.13: The *repr()* of structs has changed.
   It is now:

   >>> Struct('i')
   Struct('i')
