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gh-99146 struct module documentation should have more predictable exa…
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…mples/warnings (GH-99141)

* nail down a couple examples to have more predictable output

* update a number of things, but this is really just a stash...

* added an applications section to describe typical uses for native and machine-independent formats

* make sure all format strings use a format prefix character

* responding to comments from @gpshead. Not likely finished yet.

* This got more involved than I expected...

* respond to several PR comments
* a lot of wordsmithing
* try and be more consistent in use of ``x`` vs ``'x'``
* expand examples a bit
* update the "see also" to be more up-to-date
* original examples relied on import * so present all examples as if

* reformat based on @gpshead comment (missed before)

* responding to comments

* missed this

* one more suggested edit

* wordsmithing
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@smontanaro
smontanaro authored Nov 22, 2022
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282 changes: 206 additions & 76 deletions Doc/library/struct.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -12,21 +12,25 @@

--------------

This module performs conversions between Python values and C structs represented
as Python :class:`bytes` objects. This can be used in handling binary data
stored in files or from network connections, among other sources. It uses
:ref:`struct-format-strings` as compact descriptions of the layout of the C
structs and the intended conversion to/from Python values.
This module converts between Python values and C structs represented
as Python :class:`bytes` objects. Compact :ref:`format strings <struct-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::

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 :ref:`struct-alignment` for details.
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 :ref:`struct-alignment` for details.

Several :mod:`struct` functions (and methods of :class:`Struct`) take a *buffer*
argument. This refers to objects that implement the :ref:`bufferobjects` and
Expand Down Expand Up @@ -102,10 +106,13 @@ The module defines the following exception and functions:
Format Strings
--------------

Format strings are the mechanism used to specify the expected layout when
packing and unpacking data. They are built up from :ref:`format-characters`,
which specify the type of data being packed/unpacked. In addition, there are
special characters for controlling the :ref:`struct-alignment`.
Format strings describe the data layout when
packing and unpacking data. They are built up from :ref:`format characters<format-characters>`,
which specify the type of data being packed/unpacked. In addition,
special characters control the :ref:`byte order, size and alignment<struct-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.


.. _struct-alignment:
Expand All @@ -116,6 +123,11 @@ 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.

.. index::
single: @ (at); in struct format strings
Expand Down Expand Up @@ -144,12 +156,10 @@ following table:

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;
IBM z and most legacy architectures are big-endian;
and ARM, RISC-V and IBM Power feature switchable endianness
(bi-endian, though the former two are nearly always little-endian in practice).
Use ``sys.byteorder`` to check the endianness of your system.
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 :data:`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.
Expand Down Expand Up @@ -231,9 +241,9 @@ platform-dependent.
+--------+--------------------------+--------------------+----------------+------------+
| ``d`` | :c:expr:`double` | float | 8 | \(4) |
+--------+--------------------------+--------------------+----------------+------------+
| ``s`` | :c:expr:`char[]` | bytes | | |
| ``s`` | :c:expr:`char[]` | bytes | | \(9) |
+--------+--------------------------+--------------------+----------------+------------+
| ``p`` | :c:expr:`char[]` | bytes | | |
| ``p`` | :c:expr:`char[]` | bytes | | \(8) |
+--------+--------------------------+--------------------+----------------+------------+
| ``P`` | :c:expr:`void \*` | integer | | \(5) |
+--------+--------------------------+--------------------+----------------+------------+
Expand Down Expand Up @@ -292,24 +302,40 @@ Notes:
format <half precision format_>`_ for more information.

(7)
For padding, ``x`` inserts null bytes.

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
:func:`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 :func:`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 :ref:`struct-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.

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
Expand All @@ -319,17 +345,6 @@ then :exc:`struct.error` is raised.
Previously, some of the integer formats wrapped out-of-range values and
raised :exc:`DeprecationWarning` instead of :exc:`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
:func:`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 :func:`unpack`, the ``'p'`` format character consumes
``count`` bytes, but that the string returned can never contain more than 255
bytes.

.. index:: single: ? (question mark); in struct format strings

For the ``'?'`` format character, the return value is either :const:`True` or
Expand All @@ -345,18 +360,36 @@ Examples
^^^^^^^^

.. note::
All examples assume a native byte order, size, and alignment with a
big-endian machine.
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::

A basic example of packing/unpacking three integers::
>>> 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'

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@cbast

cbast Jan 3, 2023

The closing (at the end of line) parenthesis was unintentionally dropped.
The example fails because of it.
(1, 2, 3)
>>> calcsize('>bhl')
7

>>> 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
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::
Expand All @@ -369,35 +402,132 @@ the result in a named tuple::
>>> 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')
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')
>>> calcsize('@ic')
5

The following format ``'llh0l'`` specifies two pad bytes at the end, assuming
longs are aligned on 4-byte boundaries::
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)
>>> 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.


.. seealso::

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

Module :mod:`xdrlib`
Packing and unpacking of XDR data.
Module :mod:`json`
JSON encoder and decoder.

Module :mod:`pickle`
Python object serialization.


.. _applications:

Applications
------------

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


.. _struct-native-formats:

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.


.. _struct-standard-formats:

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


.. _struct-objects:
Expand All @@ -411,9 +541,9 @@ The :mod:`struct` module also defines the following type:
.. class:: 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 :mod:`struct` functions with the
same format since the format string only needs to be compiled once.
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::

Expand Down

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