cpython/Doc/library/struct.rst

386 lines
16 KiB
ReStructuredText
Raw Normal View History

:mod:`struct` --- Interpret bytes as packed binary data
=======================================================
2007-08-15 11:28:22 -03:00
.. module:: struct
:synopsis: Interpret bytes as packed binary data.
2007-08-15 11:28:22 -03:00
.. index::
pair: C; structures
triple: packing; binary; data
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.
.. 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.
Functions and Exceptions
------------------------
2007-08-15 11:28:22 -03:00
The module defines the following exception and functions:
.. exception:: error
Exception raised on various occasions; argument is a string describing what
is wrong.
2007-08-15 11:28:22 -03:00
.. function:: 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.
2007-08-15 11:28:22 -03:00
.. function:: 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.
2007-08-15 11:28:22 -03:00
.. function:: unpack(fmt, buffer)
2007-08-15 11:28:22 -03:00
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 must contain exactly the amount of
data required by the format (``len(bytes)`` must equal ``calcsize(fmt)``).
2007-08-15 11:28:22 -03:00
.. function:: unpack_from(fmt, buffer, offset=0)
2007-08-15 11:28:22 -03:00
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. *buffer* must contain at least the amount of data required by the
format (``len(buffer[offset:])`` must be at least ``calcsize(fmt)``).
2007-08-15 11:28:22 -03:00
.. function:: calcsize(fmt)
Return the size of the struct (and hence of the bytes object produced by
``pack(fmt, ...)``) corresponding to the format string *fmt*.
2007-08-15 11:28:22 -03:00
.. _struct-format-strings:
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`.
.. _struct-alignment:
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 :ref:`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 :ref:`struct-examples`.
.. _format-characters:
Format Characters
^^^^^^^^^^^^^^^^^
2007-08-15 11:28:22 -03:00
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.
2007-08-15 11:28:22 -03:00
+--------+--------------------------+--------------------+----------------+------------+
| Format | C Type | Python type | Standard size | Notes |
+========+==========================+====================+================+============+
| ``x`` | pad byte | no value | | |
+--------+--------------------------+--------------------+----------------+------------+
| ``c`` | :c:type:`char` | bytes of length 1 | 1 | |
+--------+--------------------------+--------------------+----------------+------------+
| ``b`` | :c:type:`signed char` | integer | 1 | \(1),\(3) |
+--------+--------------------------+--------------------+----------------+------------+
| ``B`` | :c:type:`unsigned char` | integer | 1 | \(3) |
+--------+--------------------------+--------------------+----------------+------------+
| ``?`` | :c:type:`_Bool` | bool | 1 | \(1) |
+--------+--------------------------+--------------------+----------------+------------+
| ``h`` | :c:type:`short` | integer | 2 | \(3) |
+--------+--------------------------+--------------------+----------------+------------+
| ``H`` | :c:type:`unsigned short` | integer | 2 | \(3) |
+--------+--------------------------+--------------------+----------------+------------+
| ``i`` | :c:type:`int` | integer | 4 | \(3) |
+--------+--------------------------+--------------------+----------------+------------+
| ``I`` | :c:type:`unsigned int` | integer | 4 | \(3) |
+--------+--------------------------+--------------------+----------------+------------+
| ``l`` | :c:type:`long` | integer | 4 | \(3) |
+--------+--------------------------+--------------------+----------------+------------+
| ``L`` | :c:type:`unsigned long` | integer | 4 | \(3) |
+--------+--------------------------+--------------------+----------------+------------+
| ``q`` | :c:type:`long long` | integer | 8 | \(2), \(3) |
+--------+--------------------------+--------------------+----------------+------------+
| ``Q`` | :c:type:`unsigned long | integer | 8 | \(2), \(3) |
| | long` | | | |
+--------+--------------------------+--------------------+----------------+------------+
| ``f`` | :c:type:`float` | float | 4 | \(4) |
+--------+--------------------------+--------------------+----------------+------------+
| ``d`` | :c:type:`double` | float | 8 | \(4) |
+--------+--------------------------+--------------------+----------------+------------+
| ``s`` | :c:type:`char[]` | bytes | | |
+--------+--------------------------+--------------------+----------------+------------+
| ``p`` | :c:type:`char[]` | bytes | | |
+--------+--------------------------+--------------------+----------------+------------+
| ``P`` | :c:type:`void \*` | integer | | \(5) |
+--------+--------------------------+--------------------+----------------+------------+
2007-08-15 11:28:22 -03:00
Notes:
(1)
The ``'?'`` conversion code corresponds to the :c:type:`_Bool` type defined by
C99. If this type is not available, it is simulated using a :c:type:`char`. In
2007-08-15 11:28:22 -03:00
standard mode, it is always represented by one byte.
(2)
2007-08-15 11:28:22 -03:00
The ``'q'`` and ``'Q'`` conversion codes are available in native mode only if
the platform C compiler supports C :c:type:`long long`, or, on Windows,
:c:type:`__int64`. They are always available in standard modes.
2007-08-15 11:28:22 -03:00
(3)
When attempting to pack a non-integer using any of the integer conversion
codes, if the non-integer has a :meth:`__index__` method then that method is
called to convert the argument to an integer before packing.
.. versionchanged:: 3.2
Use of the :meth:`__index__` method for non-integers is new in 3.2.
(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.
2007-08-15 11:28:22 -03:00
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,
2007-08-15 11:28:22 -03:00
``'10s'`` means a single 10-byte string, while ``'10c'`` means 10 characters.
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
2007-08-15 11:28:22 -03:00
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 :exc:`struct.error` is raised.
.. versionchanged:: 3.1
In 3.0, some of the integer formats wrapped out-of-range values and
raised :exc:`DeprecationWarning` instead of :exc:`struct.error`.
2007-08-15 11:28:22 -03:00
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.
2007-08-15 11:28:22 -03:00
Merged revisions 61239-61249,61252-61257,61260-61264,61269-61275,61278-61279,61285-61286,61288-61290,61298,61303-61305,61312-61314,61317,61329,61332,61344,61350-61351,61363-61376,61378-61379,61382-61383,61387-61388,61392,61395-61396,61402-61403 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r61239 | andrew.kuchling | 2008-03-05 01:44:41 +0100 (Wed, 05 Mar 2008) | 1 line Add more items; add fragmentary notes ........ r61240 | amaury.forgeotdarc | 2008-03-05 02:50:33 +0100 (Wed, 05 Mar 2008) | 13 lines Issue#2238: some syntax errors from *args or **kwargs expressions would give bogus error messages, because of untested exceptions:: >>> f(**g(1=2)) XXX undetected error Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: 'int' object is not iterable instead of the expected SyntaxError: keyword can't be an expression Will backport. ........ r61241 | neal.norwitz | 2008-03-05 06:10:48 +0100 (Wed, 05 Mar 2008) | 3 lines Remove the files/dirs after closing the DB so the tests work on Windows. Patch from Trent Nelson. Also simplified removing a file by using test_support. ........ r61242 | neal.norwitz | 2008-03-05 06:14:18 +0100 (Wed, 05 Mar 2008) | 3 lines Get this test to pass even when there is no sound card in the system. Patch from Trent Nelson. (I can't test this.) ........ r61243 | neal.norwitz | 2008-03-05 06:20:44 +0100 (Wed, 05 Mar 2008) | 3 lines Catch OSError when trying to remove a file in case removal fails. This should prevent a failure in tearDown masking any real test failure. ........ r61244 | neal.norwitz | 2008-03-05 06:38:06 +0100 (Wed, 05 Mar 2008) | 5 lines Make the timeout longer to give slow machines a chance to pass the test before timing out. This doesn't change the duration of the test under normal circumstances. This is targetted at fixing the spurious failures on the FreeBSD buildbot primarily. ........ r61245 | neal.norwitz | 2008-03-05 06:49:03 +0100 (Wed, 05 Mar 2008) | 1 line Tabs -> spaces ........ r61246 | neal.norwitz | 2008-03-05 06:50:20 +0100 (Wed, 05 Mar 2008) | 1 line Use -u urlfetch to run more tests ........ r61247 | neal.norwitz | 2008-03-05 06:51:20 +0100 (Wed, 05 Mar 2008) | 1 line test_smtplib sometimes reports leaks too, suppress it ........ r61248 | jeffrey.yasskin | 2008-03-05 07:19:56 +0100 (Wed, 05 Mar 2008) | 5 lines Fix test_socketserver on Windows after r61099 added several signal.alarm() calls (which don't exist on non-Unix platforms). Thanks to Trent Nelson for the report and patch. ........ r61249 | georg.brandl | 2008-03-05 08:10:35 +0100 (Wed, 05 Mar 2008) | 2 lines Fix some rst. ........ r61252 | thomas.heller | 2008-03-05 15:53:39 +0100 (Wed, 05 Mar 2008) | 2 lines News entry for yesterdays commit. ........ r61253 | thomas.heller | 2008-03-05 16:34:29 +0100 (Wed, 05 Mar 2008) | 3 lines Issue 1872: Changed the struct module typecode from 't' to '?', for compatibility with PEP3118. ........ r61254 | skip.montanaro | 2008-03-05 17:41:09 +0100 (Wed, 05 Mar 2008) | 4 lines Elaborate on the role of the altinstall target when installing multiple versions. ........ r61255 | georg.brandl | 2008-03-05 20:31:44 +0100 (Wed, 05 Mar 2008) | 2 lines #2239: PYTHONPATH delimiter is os.pathsep. ........ r61256 | raymond.hettinger | 2008-03-05 21:59:58 +0100 (Wed, 05 Mar 2008) | 1 line C implementation of itertools.permutations(). ........ r61257 | raymond.hettinger | 2008-03-05 22:04:32 +0100 (Wed, 05 Mar 2008) | 1 line Small code cleanup. ........ r61260 | martin.v.loewis | 2008-03-05 23:24:31 +0100 (Wed, 05 Mar 2008) | 2 lines cd PCbuild only after deleting all pyc files. ........ r61261 | raymond.hettinger | 2008-03-06 02:15:52 +0100 (Thu, 06 Mar 2008) | 1 line Add examples. ........ r61262 | andrew.kuchling | 2008-03-06 02:36:27 +0100 (Thu, 06 Mar 2008) | 1 line Add two items ........ r61263 | georg.brandl | 2008-03-06 07:47:18 +0100 (Thu, 06 Mar 2008) | 2 lines #1725737: ignore other VC directories other than CVS and SVN's too. ........ r61264 | martin.v.loewis | 2008-03-06 07:55:22 +0100 (Thu, 06 Mar 2008) | 4 lines Patch #2232: os.tmpfile might fail on Windows if the user has no permission to create files in the root directory. Will backport to 2.5. ........ r61269 | georg.brandl | 2008-03-06 08:19:15 +0100 (Thu, 06 Mar 2008) | 2 lines Expand on re.split behavior with captured expressions. ........ r61270 | georg.brandl | 2008-03-06 08:22:09 +0100 (Thu, 06 Mar 2008) | 2 lines Little clarification of assignments. ........ r61271 | georg.brandl | 2008-03-06 08:31:34 +0100 (Thu, 06 Mar 2008) | 2 lines Add isinstance/issubclass to tutorial. ........ r61272 | georg.brandl | 2008-03-06 08:34:52 +0100 (Thu, 06 Mar 2008) | 2 lines Add missing NEWS entry for r61263. ........ r61273 | georg.brandl | 2008-03-06 08:41:16 +0100 (Thu, 06 Mar 2008) | 2 lines #2225: return nonzero status code from py_compile if not all files could be compiled. ........ r61274 | georg.brandl | 2008-03-06 08:43:02 +0100 (Thu, 06 Mar 2008) | 2 lines #2220: handle matching failure more gracefully. ........ r61275 | georg.brandl | 2008-03-06 08:45:52 +0100 (Thu, 06 Mar 2008) | 2 lines Bug #2220: handle rlcompleter attribute match failure more gracefully. ........ r61278 | martin.v.loewis | 2008-03-06 14:49:47 +0100 (Thu, 06 Mar 2008) | 1 line Rely on x64 platform configuration when building _bsddb on AMD64. ........ r61279 | martin.v.loewis | 2008-03-06 14:50:28 +0100 (Thu, 06 Mar 2008) | 1 line Update db-4.4.20 build procedure. ........ r61285 | raymond.hettinger | 2008-03-06 21:52:01 +0100 (Thu, 06 Mar 2008) | 1 line More tests. ........ r61286 | raymond.hettinger | 2008-03-06 23:51:36 +0100 (Thu, 06 Mar 2008) | 1 line Issue 2246: itertools grouper object did not participate in GC (should be backported). ........ r61288 | raymond.hettinger | 2008-03-07 02:33:20 +0100 (Fri, 07 Mar 2008) | 1 line Tweak recipes and tests ........ r61289 | jeffrey.yasskin | 2008-03-07 07:22:15 +0100 (Fri, 07 Mar 2008) | 5 lines Progress on issue #1193577 by adding a polling .shutdown() method to SocketServers. The core of the patch was written by Pedro Werneck, but any bugs are mine. I've also rearranged the code for timeouts in order to avoid interfering with the shutdown poll. ........ r61290 | nick.coghlan | 2008-03-07 15:13:28 +0100 (Fri, 07 Mar 2008) | 1 line Speed up with statements by storing the __exit__ method on the stack instead of in a temp variable (bumps the magic number for pyc files) ........ r61298 | andrew.kuchling | 2008-03-07 22:09:23 +0100 (Fri, 07 Mar 2008) | 1 line Grammar fix ........ r61303 | georg.brandl | 2008-03-08 10:54:06 +0100 (Sat, 08 Mar 2008) | 2 lines #2253: fix continue vs. finally docs. ........ r61304 | marc-andre.lemburg | 2008-03-08 11:01:43 +0100 (Sat, 08 Mar 2008) | 3 lines Add new name for Mandrake: Mandriva. ........ r61305 | georg.brandl | 2008-03-08 11:05:24 +0100 (Sat, 08 Mar 2008) | 2 lines #1533486: fix types in refcount intro. ........ r61312 | facundo.batista | 2008-03-08 17:50:27 +0100 (Sat, 08 Mar 2008) | 5 lines Issue 1106316. post_mortem()'s parameter, traceback, is now optional: it defaults to the traceback of the exception that is currently being handled. ........ r61313 | jeffrey.yasskin | 2008-03-08 19:26:54 +0100 (Sat, 08 Mar 2008) | 2 lines Add tests for with and finally performance to pybench. ........ r61314 | jeffrey.yasskin | 2008-03-08 21:08:21 +0100 (Sat, 08 Mar 2008) | 2 lines Fix pybench for pythons < 2.6, tested back to 2.3. ........ r61317 | jeffrey.yasskin | 2008-03-08 22:35:15 +0100 (Sat, 08 Mar 2008) | 3 lines Well that was dumb. platform.python_implementation returns a function, not a string. ........ r61329 | georg.brandl | 2008-03-09 16:11:39 +0100 (Sun, 09 Mar 2008) | 2 lines #2249: document assertTrue and assertFalse. ........ r61332 | neal.norwitz | 2008-03-09 20:03:42 +0100 (Sun, 09 Mar 2008) | 4 lines Introduce a lock to fix a race condition which caused an exception in the test. Some buildbots were consistently failing (e.g., amd64). Also remove a couple of semi-colons. ........ r61344 | raymond.hettinger | 2008-03-11 01:19:07 +0100 (Tue, 11 Mar 2008) | 1 line Add recipe to docs. ........ r61350 | guido.van.rossum | 2008-03-11 22:18:06 +0100 (Tue, 11 Mar 2008) | 3 lines Fix the overflows in expandtabs(). "This time for sure!" (Exploit at request.) ........ r61351 | raymond.hettinger | 2008-03-11 22:37:46 +0100 (Tue, 11 Mar 2008) | 1 line Improve docs for itemgetter(). Show that it works with slices. ........ r61363 | georg.brandl | 2008-03-13 08:15:56 +0100 (Thu, 13 Mar 2008) | 2 lines #2265: fix example. ........ r61364 | georg.brandl | 2008-03-13 08:17:14 +0100 (Thu, 13 Mar 2008) | 2 lines #2270: fix typo. ........ r61365 | georg.brandl | 2008-03-13 08:21:41 +0100 (Thu, 13 Mar 2008) | 2 lines #1720705: add docs about import/threading interaction, wording by Nick. ........ r61366 | andrew.kuchling | 2008-03-13 12:07:35 +0100 (Thu, 13 Mar 2008) | 1 line Add class decorators ........ r61367 | raymond.hettinger | 2008-03-13 17:43:17 +0100 (Thu, 13 Mar 2008) | 1 line Add 2-to-3 support for the itertools moved to builtins or renamed. ........ r61368 | raymond.hettinger | 2008-03-13 17:43:59 +0100 (Thu, 13 Mar 2008) | 1 line Consistent tense. ........ r61369 | raymond.hettinger | 2008-03-13 20:03:51 +0100 (Thu, 13 Mar 2008) | 1 line Issue 2274: Add heapq.heappushpop(). ........ r61370 | raymond.hettinger | 2008-03-13 20:33:34 +0100 (Thu, 13 Mar 2008) | 1 line Simplify the nlargest() code using heappushpop(). ........ r61371 | brett.cannon | 2008-03-13 21:27:00 +0100 (Thu, 13 Mar 2008) | 4 lines Move test_thread over to unittest. Commits GHOP 237. Thanks Benjamin Peterson for the patch. ........ r61372 | brett.cannon | 2008-03-13 21:33:10 +0100 (Thu, 13 Mar 2008) | 4 lines Move test_tokenize to doctest. Done as GHOP 238 by Josip Dzolonga. ........ r61373 | brett.cannon | 2008-03-13 21:47:41 +0100 (Thu, 13 Mar 2008) | 4 lines Convert test_contains, test_crypt, and test_select to unittest. Patch from GHOP 294 by David Marek. ........ r61374 | brett.cannon | 2008-03-13 22:02:16 +0100 (Thu, 13 Mar 2008) | 4 lines Move test_gdbm to use unittest. Closes issue #1960. Thanks Giampaolo Rodola. ........ r61375 | brett.cannon | 2008-03-13 22:09:28 +0100 (Thu, 13 Mar 2008) | 4 lines Convert test_fcntl to unittest. Closes issue #2055. Thanks Giampaolo Rodola. ........ r61376 | raymond.hettinger | 2008-03-14 06:03:44 +0100 (Fri, 14 Mar 2008) | 1 line Leave heapreplace() unchanged. ........ r61378 | martin.v.loewis | 2008-03-14 14:56:09 +0100 (Fri, 14 Mar 2008) | 2 lines Patch #2284: add -x64 option to rt.bat. ........ r61379 | martin.v.loewis | 2008-03-14 14:57:59 +0100 (Fri, 14 Mar 2008) | 2 lines Use -x64 flag. ........ r61382 | brett.cannon | 2008-03-14 15:03:10 +0100 (Fri, 14 Mar 2008) | 2 lines Remove a bad test. ........ r61383 | mark.dickinson | 2008-03-14 15:23:37 +0100 (Fri, 14 Mar 2008) | 9 lines Issue 705836: Fix struct.pack(">f", 1e40) to behave consistently across platforms: it should now raise OverflowError on all platforms. (Previously it raised OverflowError only on non IEEE 754 platforms.) Also fix the (already existing) test for this behaviour so that it actually raises TestFailed instead of just referencing it. ........ r61387 | thomas.heller | 2008-03-14 22:06:21 +0100 (Fri, 14 Mar 2008) | 1 line Remove unneeded initializer. ........ r61388 | martin.v.loewis | 2008-03-14 22:19:28 +0100 (Fri, 14 Mar 2008) | 2 lines Run debug version, cd to PCbuild. ........ r61392 | georg.brandl | 2008-03-15 00:10:34 +0100 (Sat, 15 Mar 2008) | 2 lines Remove obsolete paragraph. #2288. ........ r61395 | georg.brandl | 2008-03-15 01:20:19 +0100 (Sat, 15 Mar 2008) | 2 lines Fix lots of broken links in the docs, found by Sphinx' external link checker. ........ r61396 | skip.montanaro | 2008-03-15 03:32:49 +0100 (Sat, 15 Mar 2008) | 1 line note that fork and forkpty raise OSError on failure ........ r61402 | skip.montanaro | 2008-03-15 17:04:45 +0100 (Sat, 15 Mar 2008) | 1 line add %f format to datetime - issue 1158 ........ r61403 | skip.montanaro | 2008-03-15 17:07:11 +0100 (Sat, 15 Mar 2008) | 2 lines . ........
2008-03-15 21:07:10 -03:00
For the ``'?'`` format character, the return value is either :const:`True` or
2007-08-15 11:28:22 -03:00
:const:`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.
.. _struct-examples:
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::
2007-08-15 11:28:22 -03:00
>>> 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')
2007-08-15 11:28:22 -03:00
(1, 2, 3)
>>> calcsize('hhl')
8
Merged revisions 63542-63544,63546,63553,63563-63564,63567,63569,63576 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r63542 | mark.dickinson | 2008-05-22 20:35:30 -0500 (Thu, 22 May 2008) | 5 lines Issue #2819: Add math.sum, a function that sums a sequence of floats efficiently but with no intermediate loss of precision. Based on Raymond Hettinger's ASPN recipe. Thanks Jean Brouwers for the patch. ........ r63543 | mark.dickinson | 2008-05-22 21:36:48 -0500 (Thu, 22 May 2008) | 2 lines Add tests for math.sum (Issue #2819) ........ r63544 | mark.dickinson | 2008-05-22 22:30:01 -0500 (Thu, 22 May 2008) | 2 lines Better error reporting in test_math.py ........ r63546 | raymond.hettinger | 2008-05-22 23:32:43 -0500 (Thu, 22 May 2008) | 1 line Tweak the comments and formatting. ........ r63553 | mark.dickinson | 2008-05-23 07:07:36 -0500 (Fri, 23 May 2008) | 3 lines Skip math.sum tests on non IEEE 754 platforms, and on IEEE 754 platforms that exhibit the problem described in issue #2937. ........ r63563 | martin.v.loewis | 2008-05-23 10:18:28 -0500 (Fri, 23 May 2008) | 3 lines Issue #1390: Raise ValueError in toxml when an invalid comment would otherwise be produced. ........ r63564 | raymond.hettinger | 2008-05-23 12:21:44 -0500 (Fri, 23 May 2008) | 1 line Issue 2909: show how to name unpacked fields. ........ r63567 | raymond.hettinger | 2008-05-23 12:34:34 -0500 (Fri, 23 May 2008) | 1 line Fix typo ........ r63569 | martin.v.loewis | 2008-05-23 14:33:13 -0500 (Fri, 23 May 2008) | 3 lines Mention that the leaking of variables from list comprehensions is fixed in 3.0. ........ r63576 | martin.v.loewis | 2008-05-24 04:36:45 -0500 (Sat, 24 May 2008) | 3 lines Don't try to get the window size if it was never set before. Fixes the test failure on Solaris. ........
2008-05-26 14:36:47 -03:00
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'
Merged revisions 63542-63544,63546,63553,63563-63564,63567,63569,63576 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r63542 | mark.dickinson | 2008-05-22 20:35:30 -0500 (Thu, 22 May 2008) | 5 lines Issue #2819: Add math.sum, a function that sums a sequence of floats efficiently but with no intermediate loss of precision. Based on Raymond Hettinger's ASPN recipe. Thanks Jean Brouwers for the patch. ........ r63543 | mark.dickinson | 2008-05-22 21:36:48 -0500 (Thu, 22 May 2008) | 2 lines Add tests for math.sum (Issue #2819) ........ r63544 | mark.dickinson | 2008-05-22 22:30:01 -0500 (Thu, 22 May 2008) | 2 lines Better error reporting in test_math.py ........ r63546 | raymond.hettinger | 2008-05-22 23:32:43 -0500 (Thu, 22 May 2008) | 1 line Tweak the comments and formatting. ........ r63553 | mark.dickinson | 2008-05-23 07:07:36 -0500 (Fri, 23 May 2008) | 3 lines Skip math.sum tests on non IEEE 754 platforms, and on IEEE 754 platforms that exhibit the problem described in issue #2937. ........ r63563 | martin.v.loewis | 2008-05-23 10:18:28 -0500 (Fri, 23 May 2008) | 3 lines Issue #1390: Raise ValueError in toxml when an invalid comment would otherwise be produced. ........ r63564 | raymond.hettinger | 2008-05-23 12:21:44 -0500 (Fri, 23 May 2008) | 1 line Issue 2909: show how to name unpacked fields. ........ r63567 | raymond.hettinger | 2008-05-23 12:34:34 -0500 (Fri, 23 May 2008) | 1 line Fix typo ........ r63569 | martin.v.loewis | 2008-05-23 14:33:13 -0500 (Fri, 23 May 2008) | 3 lines Mention that the leaking of variables from list comprehensions is fixed in 3.0. ........ r63576 | martin.v.loewis | 2008-05-24 04:36:45 -0500 (Sat, 24 May 2008) | 3 lines Don't try to get the window size if it was never set before. Fixes the test failure on Solaris. ........
2008-05-26 14:36:47 -03:00
>>> 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)
2007-08-15 11:28:22 -03:00
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.
2007-08-15 11:28:22 -03:00
.. seealso::
Module :mod:`array`
Packed binary storage of homogeneous data.
Module :mod:`xdrlib`
Packing and unpacking of XDR data.
.. _struct-objects:
Classes
-------
2007-08-15 11:28:22 -03:00
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.
2007-08-15 11:28:22 -03:00
Compiled Struct objects support the following methods and attributes:
2007-08-15 11:28:22 -03:00
.. method:: pack(v1, v2, ...)
2007-08-15 11:28:22 -03:00
Identical to the :func:`pack` function, using the compiled format.
(``len(result)`` will equal :attr:`self.size`.)
2007-08-15 11:28:22 -03:00
.. method:: pack_into(buffer, offset, v1, v2, ...)
2007-08-15 11:28:22 -03:00
Identical to the :func:`pack_into` function, using the compiled format.
2007-08-15 11:28:22 -03:00
.. method:: unpack(buffer)
2007-08-15 11:28:22 -03:00
Identical to the :func:`unpack` function, using the compiled format.
(``len(buffer)`` must equal :attr:`self.size`).
2007-08-15 11:28:22 -03:00
.. method:: unpack_from(buffer, offset=0)
2007-08-15 11:28:22 -03:00
Identical to the :func:`unpack_from` function, using the compiled format.
(``len(buffer[offset:])`` must be at least :attr:`self.size`).
2007-08-15 11:28:22 -03:00
.. attribute:: format
2007-08-15 11:28:22 -03:00
The format string used to construct this Struct object.
2007-08-15 11:28:22 -03:00
.. attribute:: size
Merged revisions 57221-57391 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r57227 | facundo.batista | 2007-08-20 17:16:21 -0700 (Mon, 20 Aug 2007) | 5 lines Catch ProtocolError exceptions and include the header information in test output (to make it easier to debug test failures caused by problems in the server). [GSoC - Alan McIntyre] ........ r57229 | mark.hammond | 2007-08-20 18:04:47 -0700 (Mon, 20 Aug 2007) | 5 lines [ 1761786 ] distutils.util.get_platform() return value on 64bit Windows As discussed on distutils-sig: Allows the generated installer name on 64bit Windows platforms to be different than the name generated for 32bit Windows platforms. ........ r57230 | mark.hammond | 2007-08-20 18:05:16 -0700 (Mon, 20 Aug 2007) | 5 lines [ 1761786 ] distutils.util.get_platform() return value on 64bit Windows As discussed on distutils-sig: Allows the generated installer name on 64bit Windows platforms to be different than the name generated for 32bit Windows platforms. ........ r57253 | georg.brandl | 2007-08-20 23:01:18 -0700 (Mon, 20 Aug 2007) | 2 lines Demand version 2.5.1 since 2.5 has a bug with codecs.open context managers. ........ r57254 | georg.brandl | 2007-08-20 23:03:43 -0700 (Mon, 20 Aug 2007) | 2 lines Revert accidental checkins from last commit. ........ r57255 | georg.brandl | 2007-08-20 23:07:08 -0700 (Mon, 20 Aug 2007) | 2 lines Bug #1777160: mention explicitly that e.g. -1**2 is -1. ........ r57256 | georg.brandl | 2007-08-20 23:12:19 -0700 (Mon, 20 Aug 2007) | 3 lines Bug #1777168: replace operator names "opa"... with "op1"... and mark everything up as literal, to enhance readability. ........ r57259 | facundo.batista | 2007-08-21 09:57:18 -0700 (Tue, 21 Aug 2007) | 8 lines Added test for behavior of operations on an unconnected SMTP object, and tests for NOOP, RSET, and VRFY. Corrected typo in a comment for testNonnumericPort. Added a check for constructing SMTP objects when non-numeric ports are included in the host name. Derived a server from SMTPServer to test various ESMTP/SMTP capabilities. Check that a second HELO to DebuggingServer returns an error. [GSoC - Alan McIntyre] ........ r57279 | skip.montanaro | 2007-08-22 12:02:16 -0700 (Wed, 22 Aug 2007) | 2 lines Note that BeOS is unsupported as of Python 2.6. ........ r57280 | skip.montanaro | 2007-08-22 12:05:21 -0700 (Wed, 22 Aug 2007) | 1 line whoops - need to check in configure as well ........ r57284 | alex.martelli | 2007-08-22 14:14:17 -0700 (Wed, 22 Aug 2007) | 5 lines Fix compile.c so that it records 0.0 and -0.0 as separate constants in a code object's co_consts tuple; add a test to show that the previous behavior (where these two constants were "collapsed" into one) causes serious malfunctioning. ........ r57286 | gregory.p.smith | 2007-08-22 14:32:34 -0700 (Wed, 22 Aug 2007) | 3 lines stop leaving log.0000001 __db.00* and xxx.db turds in developer sandboxes when bsddb3 tests are run. ........ r57301 | jeffrey.yasskin | 2007-08-22 16:14:27 -0700 (Wed, 22 Aug 2007) | 3 lines When setup.py fails to find the necessary bits to build some modules, have it print a slightly more informative message. ........ r57320 | brett.cannon | 2007-08-23 07:53:17 -0700 (Thu, 23 Aug 2007) | 2 lines Make test_runpy re-entrant. ........ r57324 | georg.brandl | 2007-08-23 10:54:11 -0700 (Thu, 23 Aug 2007) | 2 lines Bug #1768121: fix wrong/missing opcode docs. ........ r57326 | georg.brandl | 2007-08-23 10:57:05 -0700 (Thu, 23 Aug 2007) | 2 lines Bug #1766421: "return code" vs. "status code". ........ r57328 | georg.brandl | 2007-08-23 11:08:06 -0700 (Thu, 23 Aug 2007) | 2 lines Second half of #1752175: #ifdef out references to PyImport_DynLoadFiletab if HAVE_DYNAMIC_LOADING is not defined. ........ r57331 | georg.brandl | 2007-08-23 11:11:33 -0700 (Thu, 23 Aug 2007) | 2 lines Use try-except-finally in contextlib. ........ r57343 | georg.brandl | 2007-08-23 13:35:00 -0700 (Thu, 23 Aug 2007) | 2 lines Bug #1697820: document that the old slice protocol is still used by builtin types. ........ r57345 | georg.brandl | 2007-08-23 13:40:01 -0700 (Thu, 23 Aug 2007) | 2 lines Bug #1573854: fix docs for sqlite3 cursor rowcount attr. ........ r57347 | georg.brandl | 2007-08-23 13:50:23 -0700 (Thu, 23 Aug 2007) | 2 lines Bug #1694833: fix imp.find_module() docs wrt. packages. ........ r57348 | georg.brandl | 2007-08-23 13:53:28 -0700 (Thu, 23 Aug 2007) | 2 lines Bug #1594966: fix misleading usage example ........ r57349 | georg.brandl | 2007-08-23 13:55:44 -0700 (Thu, 23 Aug 2007) | 2 lines Clarify wording a bit. ........ r57351 | georg.brandl | 2007-08-23 14:18:44 -0700 (Thu, 23 Aug 2007) | 2 lines Bug #1752332: httplib no longer uses socket.getaddrinfo(). ........ r57352 | georg.brandl | 2007-08-23 14:21:36 -0700 (Thu, 23 Aug 2007) | 2 lines Bug #1734111: document struct.Struct.size. ........ r57353 | georg.brandl | 2007-08-23 14:27:57 -0700 (Thu, 23 Aug 2007) | 2 lines Bug #1688564: document os.path.join's absolute path behavior in the docstring. ........ r57354 | georg.brandl | 2007-08-23 14:36:05 -0700 (Thu, 23 Aug 2007) | 2 lines Bug #1625381: clarify match vs search introduction. ........ r57355 | georg.brandl | 2007-08-23 14:42:54 -0700 (Thu, 23 Aug 2007) | 2 lines Bug #1758696: more info about descriptors. ........ r57357 | georg.brandl | 2007-08-23 14:55:57 -0700 (Thu, 23 Aug 2007) | 2 lines Patch #1779550: remove redundant code in logging. ........ r57378 | gregory.p.smith | 2007-08-23 22:11:38 -0700 (Thu, 23 Aug 2007) | 2 lines Fix bug 1725856. ........ r57382 | georg.brandl | 2007-08-23 23:10:01 -0700 (Thu, 23 Aug 2007) | 2 lines uuid creation is now threadsafe, backport from py3k rev. 57375. ........ r57389 | georg.brandl | 2007-08-24 04:47:37 -0700 (Fri, 24 Aug 2007) | 2 lines Bug #1765375: fix stripping of unwanted LDFLAGS. ........ r57391 | guido.van.rossum | 2007-08-24 07:53:14 -0700 (Fri, 24 Aug 2007) | 2 lines Fix silly typo in test name. ........
2007-08-24 13:32:05 -03:00
The calculated size of the struct (and hence of the bytes object produced
by the :meth:`pack` method) corresponding to :attr:`format`.
Merged revisions 57221-57391 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r57227 | facundo.batista | 2007-08-20 17:16:21 -0700 (Mon, 20 Aug 2007) | 5 lines Catch ProtocolError exceptions and include the header information in test output (to make it easier to debug test failures caused by problems in the server). [GSoC - Alan McIntyre] ........ r57229 | mark.hammond | 2007-08-20 18:04:47 -0700 (Mon, 20 Aug 2007) | 5 lines [ 1761786 ] distutils.util.get_platform() return value on 64bit Windows As discussed on distutils-sig: Allows the generated installer name on 64bit Windows platforms to be different than the name generated for 32bit Windows platforms. ........ r57230 | mark.hammond | 2007-08-20 18:05:16 -0700 (Mon, 20 Aug 2007) | 5 lines [ 1761786 ] distutils.util.get_platform() return value on 64bit Windows As discussed on distutils-sig: Allows the generated installer name on 64bit Windows platforms to be different than the name generated for 32bit Windows platforms. ........ r57253 | georg.brandl | 2007-08-20 23:01:18 -0700 (Mon, 20 Aug 2007) | 2 lines Demand version 2.5.1 since 2.5 has a bug with codecs.open context managers. ........ r57254 | georg.brandl | 2007-08-20 23:03:43 -0700 (Mon, 20 Aug 2007) | 2 lines Revert accidental checkins from last commit. ........ r57255 | georg.brandl | 2007-08-20 23:07:08 -0700 (Mon, 20 Aug 2007) | 2 lines Bug #1777160: mention explicitly that e.g. -1**2 is -1. ........ r57256 | georg.brandl | 2007-08-20 23:12:19 -0700 (Mon, 20 Aug 2007) | 3 lines Bug #1777168: replace operator names "opa"... with "op1"... and mark everything up as literal, to enhance readability. ........ r57259 | facundo.batista | 2007-08-21 09:57:18 -0700 (Tue, 21 Aug 2007) | 8 lines Added test for behavior of operations on an unconnected SMTP object, and tests for NOOP, RSET, and VRFY. Corrected typo in a comment for testNonnumericPort. Added a check for constructing SMTP objects when non-numeric ports are included in the host name. Derived a server from SMTPServer to test various ESMTP/SMTP capabilities. Check that a second HELO to DebuggingServer returns an error. [GSoC - Alan McIntyre] ........ r57279 | skip.montanaro | 2007-08-22 12:02:16 -0700 (Wed, 22 Aug 2007) | 2 lines Note that BeOS is unsupported as of Python 2.6. ........ r57280 | skip.montanaro | 2007-08-22 12:05:21 -0700 (Wed, 22 Aug 2007) | 1 line whoops - need to check in configure as well ........ r57284 | alex.martelli | 2007-08-22 14:14:17 -0700 (Wed, 22 Aug 2007) | 5 lines Fix compile.c so that it records 0.0 and -0.0 as separate constants in a code object's co_consts tuple; add a test to show that the previous behavior (where these two constants were "collapsed" into one) causes serious malfunctioning. ........ r57286 | gregory.p.smith | 2007-08-22 14:32:34 -0700 (Wed, 22 Aug 2007) | 3 lines stop leaving log.0000001 __db.00* and xxx.db turds in developer sandboxes when bsddb3 tests are run. ........ r57301 | jeffrey.yasskin | 2007-08-22 16:14:27 -0700 (Wed, 22 Aug 2007) | 3 lines When setup.py fails to find the necessary bits to build some modules, have it print a slightly more informative message. ........ r57320 | brett.cannon | 2007-08-23 07:53:17 -0700 (Thu, 23 Aug 2007) | 2 lines Make test_runpy re-entrant. ........ r57324 | georg.brandl | 2007-08-23 10:54:11 -0700 (Thu, 23 Aug 2007) | 2 lines Bug #1768121: fix wrong/missing opcode docs. ........ r57326 | georg.brandl | 2007-08-23 10:57:05 -0700 (Thu, 23 Aug 2007) | 2 lines Bug #1766421: "return code" vs. "status code". ........ r57328 | georg.brandl | 2007-08-23 11:08:06 -0700 (Thu, 23 Aug 2007) | 2 lines Second half of #1752175: #ifdef out references to PyImport_DynLoadFiletab if HAVE_DYNAMIC_LOADING is not defined. ........ r57331 | georg.brandl | 2007-08-23 11:11:33 -0700 (Thu, 23 Aug 2007) | 2 lines Use try-except-finally in contextlib. ........ r57343 | georg.brandl | 2007-08-23 13:35:00 -0700 (Thu, 23 Aug 2007) | 2 lines Bug #1697820: document that the old slice protocol is still used by builtin types. ........ r57345 | georg.brandl | 2007-08-23 13:40:01 -0700 (Thu, 23 Aug 2007) | 2 lines Bug #1573854: fix docs for sqlite3 cursor rowcount attr. ........ r57347 | georg.brandl | 2007-08-23 13:50:23 -0700 (Thu, 23 Aug 2007) | 2 lines Bug #1694833: fix imp.find_module() docs wrt. packages. ........ r57348 | georg.brandl | 2007-08-23 13:53:28 -0700 (Thu, 23 Aug 2007) | 2 lines Bug #1594966: fix misleading usage example ........ r57349 | georg.brandl | 2007-08-23 13:55:44 -0700 (Thu, 23 Aug 2007) | 2 lines Clarify wording a bit. ........ r57351 | georg.brandl | 2007-08-23 14:18:44 -0700 (Thu, 23 Aug 2007) | 2 lines Bug #1752332: httplib no longer uses socket.getaddrinfo(). ........ r57352 | georg.brandl | 2007-08-23 14:21:36 -0700 (Thu, 23 Aug 2007) | 2 lines Bug #1734111: document struct.Struct.size. ........ r57353 | georg.brandl | 2007-08-23 14:27:57 -0700 (Thu, 23 Aug 2007) | 2 lines Bug #1688564: document os.path.join's absolute path behavior in the docstring. ........ r57354 | georg.brandl | 2007-08-23 14:36:05 -0700 (Thu, 23 Aug 2007) | 2 lines Bug #1625381: clarify match vs search introduction. ........ r57355 | georg.brandl | 2007-08-23 14:42:54 -0700 (Thu, 23 Aug 2007) | 2 lines Bug #1758696: more info about descriptors. ........ r57357 | georg.brandl | 2007-08-23 14:55:57 -0700 (Thu, 23 Aug 2007) | 2 lines Patch #1779550: remove redundant code in logging. ........ r57378 | gregory.p.smith | 2007-08-23 22:11:38 -0700 (Thu, 23 Aug 2007) | 2 lines Fix bug 1725856. ........ r57382 | georg.brandl | 2007-08-23 23:10:01 -0700 (Thu, 23 Aug 2007) | 2 lines uuid creation is now threadsafe, backport from py3k rev. 57375. ........ r57389 | georg.brandl | 2007-08-24 04:47:37 -0700 (Fri, 24 Aug 2007) | 2 lines Bug #1765375: fix stripping of unwanted LDFLAGS. ........ r57391 | guido.van.rossum | 2007-08-24 07:53:14 -0700 (Fri, 24 Aug 2007) | 2 lines Fix silly typo in test name. ........
2007-08-24 13:32:05 -03:00