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  204.   <section id="module-struct">

  205. <span id="struct-interpret-bytes-as-packed-binary-data"></span><h1><a class="reference internal" href="#module-struct" title="struct: Interpret bytes as packed binary data."><code class="xref py py-mod docutils literal notranslate"><span class="pre">struct</span></code></a> — Interpret bytes as packed binary data<a class="headerlink" href="#module-struct" title="Link to this heading">¶</a></h1>

  206. <p><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/3.12/Lib/struct.py">Lib/struct.py</a></p>

  207. <hr class="docutils" id="index-0" />

  208. <p>This module converts between Python values and C structs represented

  209. as Python <a class="reference internal" href="stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate"><span class="pre">bytes</span></code></a> objects.  Compact <a class="reference internal" href="#struct-format-strings"><span class="std std-ref">format strings</span></a>

  210. describe the intended conversions to/from Python values.

  211. The module’s functions and objects can be used for two largely

  212. distinct applications, data exchange with external sources (files or

  213. network connections), or data transfer between the Python application

  214. and the C layer.</p>

  215. <div class="admonition note">

  216. <p class="admonition-title">Note</p>

  217. <p>When no prefix character is given, native mode is the default. It

  218. packs or unpacks data based on the platform and compiler on which

  219. the Python interpreter was built.

  220. The result of packing a given C struct includes pad bytes which

  221. maintain proper alignment for the C types involved; similarly,

  222. alignment is taken into account when unpacking.  In contrast, when

  223. communicating data between external sources, the programmer is

  224. responsible for defining byte ordering and padding between elements.

  225. See <a class="reference internal" href="#struct-alignment"><span class="std std-ref">Byte Order, Size, and Alignment</span></a> for details.</p>

  226. </div>

  227. <p>Several <a class="reference internal" href="#module-struct" title="struct: Interpret bytes as packed binary data."><code class="xref py py-mod docutils literal notranslate"><span class="pre">struct</span></code></a> functions (and methods of <a class="reference internal" href="#struct.Struct" title="struct.Struct"><code class="xref py py-class docutils literal notranslate"><span class="pre">Struct</span></code></a>) take a <em>buffer</em>

  228. argument.  This refers to objects that implement the <a class="reference internal" href="../c-api/buffer.html#bufferobjects"><span class="std std-ref">Buffer Protocol</span></a> and

  229. provide either a readable or read-writable buffer.  The most common types used

  230. for that purpose are <a class="reference internal" href="stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate"><span class="pre">bytes</span></code></a> and <a class="reference internal" href="stdtypes.html#bytearray" title="bytearray"><code class="xref py py-class docutils literal notranslate"><span class="pre">bytearray</span></code></a>, but many other types

  231. that can be viewed as an array of bytes implement the buffer protocol, so that

  232. they can be read/filled without additional copying from a <a class="reference internal" href="stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate"><span class="pre">bytes</span></code></a> object.</p>

  233. <section id="functions-and-exceptions">

  234. <h2>Functions and Exceptions<a class="headerlink" href="#functions-and-exceptions" title="Link to this heading">¶</a></h2>

  235. <p>The module defines the following exception and functions:</p>

  236. <dl class="py exception">

  237. <dt class="sig sig-object py" id="struct.error">

  238. <em class="property"><span class="pre">exception</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">struct.</span></span><span class="sig-name descname"><span class="pre">error</span></span><a class="headerlink" href="#struct.error" title="Link to this definition">¶</a></dt>

  239. <dd><p>Exception raised on various occasions; argument is a string describing what

  240. is wrong.</p>

  241. </dd></dl>

  242. 

  243. <dl class="py function">

  244. <dt class="sig sig-object py" id="struct.pack">

  245. <span class="sig-prename descclassname"><span class="pre">struct.</span></span><span class="sig-name descname"><span class="pre">pack</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">format</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">v1</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">v2</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">...</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#struct.pack" title="Link to this definition">¶</a></dt>

  246. <dd><p>Return a bytes object containing the values <em>v1</em>, <em>v2</em>, … packed according

  247. to the format string <em>format</em>.  The arguments must match the values required by

  248. the format exactly.</p>

  249. </dd></dl>

  250. 

  251. <dl class="py function">

  252. <dt class="sig sig-object py" id="struct.pack_into">

  253. <span class="sig-prename descclassname"><span class="pre">struct.</span></span><span class="sig-name descname"><span class="pre">pack_into</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">format</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">buffer</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">offset</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">v1</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">v2</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">...</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#struct.pack_into" title="Link to this definition">¶</a></dt>

  254. <dd><p>Pack the values <em>v1</em>, <em>v2</em>, … according to the format string <em>format</em> and

  255. write the packed bytes into the writable buffer <em>buffer</em> starting at

  256. position <em>offset</em>.  Note that <em>offset</em> is a required argument.</p>

  257. </dd></dl>

  258. 

  259. <dl class="py function">

  260. <dt class="sig sig-object py" id="struct.unpack">

  261. <span class="sig-prename descclassname"><span class="pre">struct.</span></span><span class="sig-name descname"><span class="pre">unpack</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">format</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">buffer</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#struct.unpack" title="Link to this definition">¶</a></dt>

  262. <dd><p>Unpack from the buffer <em>buffer</em> (presumably packed by <code class="docutils literal notranslate"><span class="pre">pack(format,</span> <span class="pre">...)</span></code>)

  263. according to the format string <em>format</em>.  The result is a tuple even if it

  264. contains exactly one item.  The buffer’s size in bytes must match the

  265. size required by the format, as reflected by <a class="reference internal" href="#struct.calcsize" title="struct.calcsize"><code class="xref py py-func docutils literal notranslate"><span class="pre">calcsize()</span></code></a>.</p>

  266. </dd></dl>

  267. 

  268. <dl class="py function">

  269. <dt class="sig sig-object py" id="struct.unpack_from">

  270. <span class="sig-prename descclassname"><span class="pre">struct.</span></span><span class="sig-name descname"><span class="pre">unpack_from</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">format</span></span></em>, <em class="sig-param"><span class="o"><span class="pre">/</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">buffer</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">offset</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">0</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#struct.unpack_from" title="Link to this definition">¶</a></dt>

  271. <dd><p>Unpack from <em>buffer</em> starting at position <em>offset</em>, according to the format

  272. string <em>format</em>.  The result is a tuple even if it contains exactly one

  273. item.  The buffer’s size in bytes, starting at position <em>offset</em>, must be at

  274. least the size required by the format, as reflected by <a class="reference internal" href="#struct.calcsize" title="struct.calcsize"><code class="xref py py-func docutils literal notranslate"><span class="pre">calcsize()</span></code></a>.</p>

  275. </dd></dl>

  276. 

  277. <dl class="py function">

  278. <dt class="sig sig-object py" id="struct.iter_unpack">

  279. <span class="sig-prename descclassname"><span class="pre">struct.</span></span><span class="sig-name descname"><span class="pre">iter_unpack</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">format</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">buffer</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#struct.iter_unpack" title="Link to this definition">¶</a></dt>

  280. <dd><p>Iteratively unpack from the buffer <em>buffer</em> according to the format

  281. string <em>format</em>.  This function returns an iterator which will read

  282. equally sized chunks from the buffer until all its contents have been

  283. consumed.  The buffer’s size in bytes must be a multiple of the size

  284. required by the format, as reflected by <a class="reference internal" href="#struct.calcsize" title="struct.calcsize"><code class="xref py py-func docutils literal notranslate"><span class="pre">calcsize()</span></code></a>.</p>

  285. <p>Each iteration yields a tuple as specified by the format string.</p>

  286. <div class="versionadded">

  287. <p><span class="versionmodified added">New in version 3.4.</span></p>

  288. </div>

  289. </dd></dl>

  290. 

  291. <dl class="py function">

  292. <dt class="sig sig-object py" id="struct.calcsize">

  293. <span class="sig-prename descclassname"><span class="pre">struct.</span></span><span class="sig-name descname"><span class="pre">calcsize</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">format</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#struct.calcsize" title="Link to this definition">¶</a></dt>

  294. <dd><p>Return the size of the struct (and hence of the bytes object produced by

  295. <code class="docutils literal notranslate"><span class="pre">pack(format,</span> <span class="pre">...)</span></code>) corresponding to the format string <em>format</em>.</p>

  296. </dd></dl>

  297. 

  298. </section>

  299. <section id="format-strings">

  300. <span id="struct-format-strings"></span><h2>Format Strings<a class="headerlink" href="#format-strings" title="Link to this heading">¶</a></h2>

  301. <p>Format strings describe the data layout when

  302. packing and unpacking data.  They are built up from <a class="reference internal" href="#format-characters"><span class="std std-ref">format characters</span></a>,

  303. which specify the type of data being packed/unpacked.  In addition,

  304. special characters control the <a class="reference internal" href="#struct-alignment"><span class="std std-ref">byte order, size and alignment</span></a>.

  305. Each format string consists of an optional prefix character which

  306. describes the overall properties of the data and one or more format

  307. characters which describe the actual data values and padding.</p>

  308. <section id="byte-order-size-and-alignment">

  309. <span id="struct-alignment"></span><h3>Byte Order, Size, and Alignment<a class="headerlink" href="#byte-order-size-and-alignment" title="Link to this heading">¶</a></h3>

  310. <p>By default, C types are represented in the machine’s native format and byte

  311. order, and properly aligned by skipping pad bytes if necessary (according to the

  312. rules used by the C compiler).

  313. This behavior is chosen so

  314. that the bytes of a packed struct correspond exactly to the memory layout

  315. of the corresponding C struct.

  316. Whether to use native byte ordering

  317. and padding or standard formats depends on the application.</p>

  318. <p id="index-1">Alternatively, the first character of the format string can be used to indicate

  319. the byte order, size and alignment of the packed data, according to the

  320. following table:</p>

  321. <table class="docutils align-default">

  322. <thead>

  323. <tr class="row-odd"><th class="head"><p>Character</p></th>

  324. <th class="head"><p>Byte order</p></th>

  325. <th class="head"><p>Size</p></th>

  326. <th class="head"><p>Alignment</p></th>

  327. </tr>

  328. </thead>

  329. <tbody>

  330. <tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">&#64;</span></code></p></td>

  331. <td><p>native</p></td>

  332. <td><p>native</p></td>

  333. <td><p>native</p></td>

  334. </tr>

  335. <tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">=</span></code></p></td>

  336. <td><p>native</p></td>

  337. <td><p>standard</p></td>

  338. <td><p>none</p></td>

  339. </tr>

  340. <tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">&lt;</span></code></p></td>

  341. <td><p>little-endian</p></td>

  342. <td><p>standard</p></td>

  343. <td><p>none</p></td>

  344. </tr>

  345. <tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">&gt;</span></code></p></td>

  346. <td><p>big-endian</p></td>

  347. <td><p>standard</p></td>

  348. <td><p>none</p></td>

  349. </tr>

  350. <tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">!</span></code></p></td>

  351. <td><p>network (= big-endian)</p></td>

  352. <td><p>standard</p></td>

  353. <td><p>none</p></td>

  354. </tr>

  355. </tbody>

  356. </table>

  357. <p>If the first character is not one of these, <code class="docutils literal notranslate"><span class="pre">'&#64;'</span></code> is assumed.</p>

  358. <div class="admonition note">

  359. <p class="admonition-title">Note</p>

  360. <p>The number 1023 (<code class="docutils literal notranslate"><span class="pre">0x3ff</span></code> in hexadecimal) has the following byte representations:</p>

  361. <ul class="simple">

  362. <li><p><code class="docutils literal notranslate"><span class="pre">03</span> <span class="pre">ff</span></code> in big-endian (<code class="docutils literal notranslate"><span class="pre">&gt;</span></code>)</p></li>

  363. <li><p><code class="docutils literal notranslate"><span class="pre">ff</span> <span class="pre">03</span></code> in little-endian (<code class="docutils literal notranslate"><span class="pre">&lt;</span></code>)</p></li>

  364. </ul>

  365. <p>Python example:</p>

  366. <div class="doctest highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="kn">import</span> <span class="nn">struct</span>

  367. <span class="gp">&gt;&gt;&gt; </span><span class="n">struct</span><span class="o">.</span><span class="n">pack</span><span class="p">(</span><span class="s1">&#39;&gt;h&#39;</span><span class="p">,</span> <span class="mi">1023</span><span class="p">)</span>

  368. <span class="go">b&#39;\x03\xff&#39;</span>

  369. <span class="gp">&gt;&gt;&gt; </span><span class="n">struct</span><span class="o">.</span><span class="n">pack</span><span class="p">(</span><span class="s1">&#39;&lt;h&#39;</span><span class="p">,</span> <span class="mi">1023</span><span class="p">)</span>

  370. <span class="go">b&#39;\xff\x03&#39;</span>

  371. </pre></div>

  372. </div>

  373. </div>

  374. <p>Native byte order is big-endian or little-endian, depending on the

  375. host system. For example, Intel x86, AMD64 (x86-64), and Apple M1 are

  376. little-endian; IBM z and many legacy architectures are big-endian.

  377. Use <a class="reference internal" href="sys.html#sys.byteorder" title="sys.byteorder"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.byteorder</span></code></a> to check the endianness of your system.</p>

  378. <p>Native size and alignment are determined using the C compiler’s

  379. <code class="docutils literal notranslate"><span class="pre">sizeof</span></code> expression.  This is always combined with native byte order.</p>

  380. <p>Standard size depends only on the format character;  see the table in

  381. the <a class="reference internal" href="#format-characters"><span class="std std-ref">Format Characters</span></a> section.</p>

  382. <p>Note the difference between <code class="docutils literal notranslate"><span class="pre">'&#64;'</span></code> and <code class="docutils literal notranslate"><span class="pre">'='</span></code>: both use native byte order, but

  383. the size and alignment of the latter is standardized.</p>

  384. <p>The form <code class="docutils literal notranslate"><span class="pre">'!'</span></code> represents the network byte order which is always big-endian

  385. as defined in <a class="reference external" href="https://datatracker.ietf.org/doc/html/rfc1700">IETF RFC 1700</a>.</p>

  386. <p>There is no way to indicate non-native byte order (force byte-swapping); use the

  387. appropriate choice of <code class="docutils literal notranslate"><span class="pre">'&lt;'</span></code> or <code class="docutils literal notranslate"><span class="pre">'&gt;'</span></code>.</p>

  388. <p>Notes:</p>

  389. <ol class="arabic simple">

  390. <li><p>Padding is only automatically added between successive structure members.

  391. No padding is added at the beginning or the end of the encoded struct.</p></li>

  392. <li><p>No padding is added when using non-native size and alignment, e.g.

  393. with ‘&lt;’, ‘&gt;’, ‘=’, and ‘!’.</p></li>

  394. <li><p>To align the end of a structure to the alignment requirement of a

  395. particular type, end the format with the code for that type with a repeat

  396. count of zero.  See <a class="reference internal" href="#struct-examples"><span class="std std-ref">Examples</span></a>.</p></li>

  397. </ol>

  398. </section>

  399. <section id="format-characters">

  400. <span id="id1"></span><h3>Format Characters<a class="headerlink" href="#format-characters" title="Link to this heading">¶</a></h3>

  401. <p>Format characters have the following meaning; the conversion between C and

  402. Python values should be obvious given their types.  The ‘Standard size’ column

  403. refers to the size of the packed value in bytes when using standard size; that

  404. is, when the format string starts with one of <code class="docutils literal notranslate"><span class="pre">'&lt;'</span></code>, <code class="docutils literal notranslate"><span class="pre">'&gt;'</span></code>, <code class="docutils literal notranslate"><span class="pre">'!'</span></code> or

  405. <code class="docutils literal notranslate"><span class="pre">'='</span></code>.  When using native size, the size of the packed value is

  406. platform-dependent.</p>

  407. <table class="docutils align-default">

  408. <thead>

  409. <tr class="row-odd"><th class="head"><p>Format</p></th>

  410. <th class="head"><p>C Type</p></th>

  411. <th class="head"><p>Python type</p></th>

  412. <th class="head"><p>Standard size</p></th>

  413. <th class="head"><p>Notes</p></th>

  414. </tr>

  415. </thead>

  416. <tbody>

  417. <tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">x</span></code></p></td>

  418. <td><p>pad byte</p></td>

  419. <td><p>no value</p></td>

  420. <td></td>

  421. <td><p>(7)</p></td>

  422. </tr>

  423. <tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">c</span></code></p></td>

  424. <td><p><span class="c-expr sig sig-inline c"><span class="kt">char</span></span></p></td>

  425. <td><p>bytes of length 1</p></td>

  426. <td><p>1</p></td>

  427. <td></td>

  428. </tr>

  429. <tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">b</span></code></p></td>

  430. <td><p><span class="c-expr sig sig-inline c"><span class="kt">signed</span><span class="w"> </span><span class="kt">char</span></span></p></td>

  431. <td><p>integer</p></td>

  432. <td><p>1</p></td>

  433. <td><p>(1), (2)</p></td>

  434. </tr>

  435. <tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">B</span></code></p></td>

  436. <td><p><span class="c-expr sig sig-inline c"><span class="kt">unsigned</span><span class="w"> </span><span class="kt">char</span></span></p></td>

  437. <td><p>integer</p></td>

  438. <td><p>1</p></td>

  439. <td><p>(2)</p></td>

  440. </tr>

  441. <tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">?</span></code></p></td>

  442. <td><p><span class="c-expr sig sig-inline c"><span class="kt">_Bool</span></span></p></td>

  443. <td><p>bool</p></td>

  444. <td><p>1</p></td>

  445. <td><p>(1)</p></td>

  446. </tr>

  447. <tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">h</span></code></p></td>

  448. <td><p><span class="c-expr sig sig-inline c"><span class="kt">short</span></span></p></td>

  449. <td><p>integer</p></td>

  450. <td><p>2</p></td>

  451. <td><p>(2)</p></td>

  452. </tr>

  453. <tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">H</span></code></p></td>

  454. <td><p><span class="c-expr sig sig-inline c"><span class="kt">unsigned</span><span class="w"> </span><span class="kt">short</span></span></p></td>

  455. <td><p>integer</p></td>

  456. <td><p>2</p></td>

  457. <td><p>(2)</p></td>

  458. </tr>

  459. <tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">i</span></code></p></td>

  460. <td><p><span class="c-expr sig sig-inline c"><span class="kt">int</span></span></p></td>

  461. <td><p>integer</p></td>

  462. <td><p>4</p></td>

  463. <td><p>(2)</p></td>

  464. </tr>

  465. <tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">I</span></code></p></td>

  466. <td><p><span class="c-expr sig sig-inline c"><span class="kt">unsigned</span><span class="w"> </span><span class="kt">int</span></span></p></td>

  467. <td><p>integer</p></td>

  468. <td><p>4</p></td>

  469. <td><p>(2)</p></td>

  470. </tr>

  471. <tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">l</span></code></p></td>

  472. <td><p><span class="c-expr sig sig-inline c"><span class="kt">long</span></span></p></td>

  473. <td><p>integer</p></td>

  474. <td><p>4</p></td>

  475. <td><p>(2)</p></td>

  476. </tr>

  477. <tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">L</span></code></p></td>

  478. <td><p><span class="c-expr sig sig-inline c"><span class="kt">unsigned</span><span class="w"> </span><span class="kt">long</span></span></p></td>

  479. <td><p>integer</p></td>

  480. <td><p>4</p></td>

  481. <td><p>(2)</p></td>

  482. </tr>

  483. <tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">q</span></code></p></td>

  484. <td><p><span class="c-expr sig sig-inline c"><span class="kt">long</span><span class="w"> </span><span class="kt">long</span></span></p></td>

  485. <td><p>integer</p></td>

  486. <td><p>8</p></td>

  487. <td><p>(2)</p></td>

  488. </tr>

  489. <tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">Q</span></code></p></td>

  490. <td><p><span class="c-expr sig sig-inline c"><span class="kt">unsigned</span><span class="w"> </span><span class="kt">long</span><span class="w"> </span><span class="kt">long</span></span></p></td>

  491. <td><p>integer</p></td>

  492. <td><p>8</p></td>

  493. <td><p>(2)</p></td>

  494. </tr>

  495. <tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">n</span></code></p></td>

  496. <td><p><code class="xref c c-type docutils literal notranslate"><span class="pre">ssize_t</span></code></p></td>

  497. <td><p>integer</p></td>

  498. <td></td>

  499. <td><p>(3)</p></td>

  500. </tr>

  501. <tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">N</span></code></p></td>

  502. <td><p><code class="xref c c-type docutils literal notranslate"><span class="pre">size_t</span></code></p></td>

  503. <td><p>integer</p></td>

  504. <td></td>

  505. <td><p>(3)</p></td>

  506. </tr>

  507. <tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">e</span></code></p></td>

  508. <td><p>(6)</p></td>

  509. <td><p>float</p></td>

  510. <td><p>2</p></td>

  511. <td><p>(4)</p></td>

  512. </tr>

  513. <tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">f</span></code></p></td>

  514. <td><p><span class="c-expr sig sig-inline c"><span class="kt">float</span></span></p></td>

  515. <td><p>float</p></td>

  516. <td><p>4</p></td>

  517. <td><p>(4)</p></td>

  518. </tr>

  519. <tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">d</span></code></p></td>

  520. <td><p><span class="c-expr sig sig-inline c"><span class="kt">double</span></span></p></td>

  521. <td><p>float</p></td>

  522. <td><p>8</p></td>

  523. <td><p>(4)</p></td>

  524. </tr>

  525. <tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">s</span></code></p></td>

  526. <td><p><span class="c-expr sig sig-inline c"><span class="kt">char</span><span class="p">[</span><span class="p">]</span></span></p></td>

  527. <td><p>bytes</p></td>

  528. <td></td>

  529. <td><p>(9)</p></td>

  530. </tr>

  531. <tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">p</span></code></p></td>

  532. <td><p><span class="c-expr sig sig-inline c"><span class="kt">char</span><span class="p">[</span><span class="p">]</span></span></p></td>

  533. <td><p>bytes</p></td>

  534. <td></td>

  535. <td><p>(8)</p></td>

  536. </tr>

  537. <tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">P</span></code></p></td>

  538. <td><p><span class="c-expr sig sig-inline c"><span class="kt">void</span><span class="p">*</span></span></p></td>

  539. <td><p>integer</p></td>

  540. <td></td>

  541. <td><p>(5)</p></td>

  542. </tr>

  543. </tbody>

  544. </table>

  545. <div class="versionchanged">

  546. <p><span class="versionmodified changed">Changed in version 3.3: </span>Added support for the <code class="docutils literal notranslate"><span class="pre">'n'</span></code> and <code class="docutils literal notranslate"><span class="pre">'N'</span></code> formats.</p>

  547. </div>

  548. <div class="versionchanged">

  549. <p><span class="versionmodified changed">Changed in version 3.6: </span>Added support for the <code class="docutils literal notranslate"><span class="pre">'e'</span></code> format.</p>

  550. </div>

  551. <p>Notes:</p>

  552. <ol class="arabic">

  553. <li><p id="index-2">The <code class="docutils literal notranslate"><span class="pre">'?'</span></code> conversion code corresponds to the <span class="c-expr sig sig-inline c"><span class="kt">_Bool</span></span> type defined by

  554. C99. If this type is not available, it is simulated using a <span class="c-expr sig sig-inline c"><span class="kt">char</span></span>. In

  555. standard mode, it is always represented by one byte.</p>

  556. </li>

  557. <li><p>When attempting to pack a non-integer using any of the integer conversion

  558. codes, if the non-integer has a <a class="reference internal" href="../reference/datamodel.html#object.__index__" title="object.__index__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__index__()</span></code></a> method then that method is

  559. called to convert the argument to an integer before packing.</p>

  560. <div class="versionchanged">

  561. <p><span class="versionmodified changed">Changed in version 3.2: </span>Added use of the <a class="reference internal" href="../reference/datamodel.html#object.__index__" title="object.__index__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__index__()</span></code></a> method for non-integers.</p>

  562. </div>

  563. </li>

  564. <li><p>The <code class="docutils literal notranslate"><span class="pre">'n'</span></code> and <code class="docutils literal notranslate"><span class="pre">'N'</span></code> conversion codes are only available for the native

  565. size (selected as the default or with the <code class="docutils literal notranslate"><span class="pre">'&#64;'</span></code> byte order character).

  566. For the standard size, you can use whichever of the other integer formats

  567. fits your application.</p></li>

  568. <li><p>For the <code class="docutils literal notranslate"><span class="pre">'f'</span></code>, <code class="docutils literal notranslate"><span class="pre">'d'</span></code> and <code class="docutils literal notranslate"><span class="pre">'e'</span></code> conversion codes, the packed

  569. representation uses the IEEE 754 binary32, binary64 or binary16 format (for

  570. <code class="docutils literal notranslate"><span class="pre">'f'</span></code>, <code class="docutils literal notranslate"><span class="pre">'d'</span></code> or <code class="docutils literal notranslate"><span class="pre">'e'</span></code> respectively), regardless of the floating-point

  571. format used by the platform.</p></li>

  572. <li><p>The <code class="docutils literal notranslate"><span class="pre">'P'</span></code> format character is only available for the native byte ordering

  573. (selected as the default or with the <code class="docutils literal notranslate"><span class="pre">'&#64;'</span></code> byte order character). The byte

  574. order character <code class="docutils literal notranslate"><span class="pre">'='</span></code> chooses to use little- or big-endian ordering based

  575. on the host system. The struct module does not interpret this as native

  576. ordering, so the <code class="docutils literal notranslate"><span class="pre">'P'</span></code> format is not available.</p></li>

  577. <li><p>The IEEE 754 binary16 “half precision” type was introduced in the 2008

  578. revision of the <a class="reference external" href="https://en.wikipedia.org/wiki/IEEE_754-2008_revision">IEEE 754 standard</a>. It has a sign

  579. bit, a 5-bit exponent and 11-bit precision (with 10 bits explicitly stored),

  580. and can represent numbers between approximately <code class="docutils literal notranslate"><span class="pre">6.1e-05</span></code> and <code class="docutils literal notranslate"><span class="pre">6.5e+04</span></code>

  581. at full precision. This type is not widely supported by C compilers: on a

  582. typical machine, an unsigned short can be used for storage, but not for math

  583. operations. See the Wikipedia page on the <a class="reference external" href="https://en.wikipedia.org/wiki/Half-precision_floating-point_format">half-precision floating-point

  584. format</a> for more information.</p></li>

  585. <li><p>When packing, <code class="docutils literal notranslate"><span class="pre">'x'</span></code> inserts one NUL byte.</p></li>

  586. <li><p>The <code class="docutils literal notranslate"><span class="pre">'p'</span></code> format character encodes a “Pascal string”, meaning a short

  587. variable-length string stored in a <em>fixed number of bytes</em>, given by the count.

  588. The first byte stored is the length of the string, or 255, whichever is

  589. smaller.  The bytes of the string follow.  If the string passed in to

  590. <a class="reference internal" href="#struct.pack" title="struct.pack"><code class="xref py py-func docutils literal notranslate"><span class="pre">pack()</span></code></a> is too long (longer than the count minus 1), only the leading

  591. <code class="docutils literal notranslate"><span class="pre">count-1</span></code> bytes of the string are stored.  If the string is shorter than

  592. <code class="docutils literal notranslate"><span class="pre">count-1</span></code>, it is padded with null bytes so that exactly count bytes in all

  593. are used.  Note that for <a class="reference internal" href="#struct.unpack" title="struct.unpack"><code class="xref py py-func docutils literal notranslate"><span class="pre">unpack()</span></code></a>, the <code class="docutils literal notranslate"><span class="pre">'p'</span></code> format character consumes

  594. <code class="docutils literal notranslate"><span class="pre">count</span></code> bytes, but that the string returned can never contain more than 255

  595. bytes.</p></li>

  596. <li><p>For the <code class="docutils literal notranslate"><span class="pre">'s'</span></code> format character, the count is interpreted as the length of the

  597. bytes, not a repeat count like for the other format characters; for example,

  598. <code class="docutils literal notranslate"><span class="pre">'10s'</span></code> means a single 10-byte string mapping to or from a single

  599. Python byte string, while <code class="docutils literal notranslate"><span class="pre">'10c'</span></code> means 10

  600. separate one byte character elements (e.g., <code class="docutils literal notranslate"><span class="pre">cccccccccc</span></code>) mapping

  601. to or from ten different Python byte objects. (See <a class="reference internal" href="#struct-examples"><span class="std std-ref">Examples</span></a>

  602. for a concrete demonstration of the difference.)

  603. If a count is not given, it defaults to 1.  For packing, the string is

  604. truncated or padded with null bytes as appropriate to make it fit. For

  605. unpacking, the resulting bytes object always has exactly the specified number

  606. of bytes.  As a special case, <code class="docutils literal notranslate"><span class="pre">'0s'</span></code> means a single, empty string (while

  607. <code class="docutils literal notranslate"><span class="pre">'0c'</span></code> means 0 characters).</p></li>

  608. </ol>

  609. <p>A format character may be preceded by an integral repeat count.  For example,

  610. the format string <code class="docutils literal notranslate"><span class="pre">'4h'</span></code> means exactly the same as <code class="docutils literal notranslate"><span class="pre">'hhhh'</span></code>.</p>

  611. <p>Whitespace characters between formats are ignored; a count and its format must

  612. not contain whitespace though.</p>

  613. <p>When packing a value <code class="docutils literal notranslate"><span class="pre">x</span></code> using one of the integer formats (<code class="docutils literal notranslate"><span class="pre">'b'</span></code>,

  614. <code class="docutils literal notranslate"><span class="pre">'B'</span></code>, <code class="docutils literal notranslate"><span class="pre">'h'</span></code>, <code class="docutils literal notranslate"><span class="pre">'H'</span></code>, <code class="docutils literal notranslate"><span class="pre">'i'</span></code>, <code class="docutils literal notranslate"><span class="pre">'I'</span></code>, <code class="docutils literal notranslate"><span class="pre">'l'</span></code>, <code class="docutils literal notranslate"><span class="pre">'L'</span></code>,

  615. <code class="docutils literal notranslate"><span class="pre">'q'</span></code>, <code class="docutils literal notranslate"><span class="pre">'Q'</span></code>), if <code class="docutils literal notranslate"><span class="pre">x</span></code> is outside the valid range for that format

  616. then <a class="reference internal" href="#struct.error" title="struct.error"><code class="xref py py-exc docutils literal notranslate"><span class="pre">struct.error</span></code></a> is raised.</p>

  617. <div class="versionchanged">

  618. <p><span class="versionmodified changed">Changed in version 3.1: </span>Previously, some of the integer formats wrapped out-of-range values and

  619. raised <a class="reference internal" href="exceptions.html#DeprecationWarning" title="DeprecationWarning"><code class="xref py py-exc docutils literal notranslate"><span class="pre">DeprecationWarning</span></code></a> instead of <a class="reference internal" href="#struct.error" title="struct.error"><code class="xref py py-exc docutils literal notranslate"><span class="pre">struct.error</span></code></a>.</p>

  620. </div>

  621. <p id="index-3">For the <code class="docutils literal notranslate"><span class="pre">'?'</span></code> format character, the return value is either <a class="reference internal" href="constants.html#True" title="True"><code class="xref py py-const docutils literal notranslate"><span class="pre">True</span></code></a> or

  622. <a class="reference internal" href="constants.html#False" title="False"><code class="xref py py-const docutils literal notranslate"><span class="pre">False</span></code></a>. When packing, the truth value of the argument object is used.

  623. Either 0 or 1 in the native or standard bool representation will be packed, and

  624. any non-zero value will be <code class="docutils literal notranslate"><span class="pre">True</span></code> when unpacking.</p>

  625. </section>

  626. <section id="examples">

  627. <span id="struct-examples"></span><h3>Examples<a class="headerlink" href="#examples" title="Link to this heading">¶</a></h3>

  628. <div class="admonition note">

  629. <p class="admonition-title">Note</p>

  630. <p>Native byte order examples (designated by the <code class="docutils literal notranslate"><span class="pre">'&#64;'</span></code> format prefix or

  631. lack of any prefix character) may not match what the reader’s

  632. machine produces as

  633. that depends on the platform and compiler.</p>

  634. </div>

  635. <p>Pack and unpack integers of three different sizes, using big endian

  636. ordering:</p>

  637. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="kn">from</span> <span class="nn">struct</span> <span class="kn">import</span> <span class="o">*</span>

  638. <span class="gp">&gt;&gt;&gt; </span><span class="n">pack</span><span class="p">(</span><span class="s2">&quot;&gt;bhl&quot;</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">)</span>

  639. <span class="go">b&#39;\x01\x00\x02\x00\x00\x00\x03&#39;</span>

  640. <span class="gp">&gt;&gt;&gt; </span><span class="n">unpack</span><span class="p">(</span><span class="s1">&#39;&gt;bhl&#39;</span><span class="p">,</span> <span class="sa">b</span><span class="s1">&#39;</span><span class="se">\x01\x00\x02\x00\x00\x00\x03</span><span class="s1">&#39;</span><span class="p">)</span>

  641. <span class="go">(1, 2, 3)</span>

  642. <span class="gp">&gt;&gt;&gt; </span><span class="n">calcsize</span><span class="p">(</span><span class="s1">&#39;&gt;bhl&#39;</span><span class="p">)</span>

  643. <span class="go">7</span>

  644. </pre></div>

  645. </div>

  646. <p>Attempt to pack an integer which is too large for the defined field:</p>

  647. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">pack</span><span class="p">(</span><span class="s2">&quot;&gt;h&quot;</span><span class="p">,</span> <span class="mi">99999</span><span class="p">)</span>

  648. <span class="gt">Traceback (most recent call last):</span>

  649.   File <span class="nb">&quot;&lt;stdin&gt;&quot;</span>, line <span class="m">1</span>, in <span class="n">&lt;module&gt;</span>

  650. <span class="gr">struct.error</span>: <span class="n">&#39;h&#39; format requires -32768 &lt;= number &lt;= 32767</span>

  651. </pre></div>

  652. </div>

  653. <p>Demonstrate the difference between <code class="docutils literal notranslate"><span class="pre">'s'</span></code> and <code class="docutils literal notranslate"><span class="pre">'c'</span></code> format

  654. characters:</p>

  655. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">pack</span><span class="p">(</span><span class="s2">&quot;@ccc&quot;</span><span class="p">,</span> <span class="sa">b</span><span class="s1">&#39;1&#39;</span><span class="p">,</span> <span class="sa">b</span><span class="s1">&#39;2&#39;</span><span class="p">,</span> <span class="sa">b</span><span class="s1">&#39;3&#39;</span><span class="p">)</span>

  656. <span class="go">b&#39;123&#39;</span>

  657. <span class="gp">&gt;&gt;&gt; </span><span class="n">pack</span><span class="p">(</span><span class="s2">&quot;@3s&quot;</span><span class="p">,</span> <span class="sa">b</span><span class="s1">&#39;123&#39;</span><span class="p">)</span>

  658. <span class="go">b&#39;123&#39;</span>

  659. </pre></div>

  660. </div>

  661. <p>Unpacked fields can be named by assigning them to variables or by wrapping

  662. the result in a named tuple:</p>

  663. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">record</span> <span class="o">=</span> <span class="sa">b</span><span class="s1">&#39;raymond   </span><span class="se">\x32\x12\x08\x01\x08</span><span class="s1">&#39;</span>

  664. <span class="gp">&gt;&gt;&gt; </span><span class="n">name</span><span class="p">,</span> <span class="n">serialnum</span><span class="p">,</span> <span class="n">school</span><span class="p">,</span> <span class="n">gradelevel</span> <span class="o">=</span> <span class="n">unpack</span><span class="p">(</span><span class="s1">&#39;&lt;10sHHb&#39;</span><span class="p">,</span> <span class="n">record</span><span class="p">)</span>

  665. 

  666. <span class="gp">&gt;&gt;&gt; </span><span class="kn">from</span> <span class="nn">collections</span> <span class="kn">import</span> <span class="n">namedtuple</span>

  667. <span class="gp">&gt;&gt;&gt; </span><span class="n">Student</span> <span class="o">=</span> <span class="n">namedtuple</span><span class="p">(</span><span class="s1">&#39;Student&#39;</span><span class="p">,</span> <span class="s1">&#39;name serialnum school gradelevel&#39;</span><span class="p">)</span>

  668. <span class="gp">&gt;&gt;&gt; </span><span class="n">Student</span><span class="o">.</span><span class="n">_make</span><span class="p">(</span><span class="n">unpack</span><span class="p">(</span><span class="s1">&#39;&lt;10sHHb&#39;</span><span class="p">,</span> <span class="n">record</span><span class="p">))</span>

  669. <span class="go">Student(name=b&#39;raymond   &#39;, serialnum=4658, school=264, gradelevel=8)</span>

  670. </pre></div>

  671. </div>

  672. <p>The ordering of format characters may have an impact on size in native

  673. mode since padding is implicit. In standard mode, the user is

  674. responsible for inserting any desired padding.

  675. Note in

  676. the first <code class="docutils literal notranslate"><span class="pre">pack</span></code> call below that three NUL bytes were added after the

  677. packed <code class="docutils literal notranslate"><span class="pre">'#'</span></code> to align the following integer on a four-byte boundary.

  678. In this example, the output was produced on a little endian machine:</p>

  679. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">pack</span><span class="p">(</span><span class="s1">&#39;@ci&#39;</span><span class="p">,</span> <span class="sa">b</span><span class="s1">&#39;#&#39;</span><span class="p">,</span> <span class="mh">0x12131415</span><span class="p">)</span>

  680. <span class="go">b&#39;#\x00\x00\x00\x15\x14\x13\x12&#39;</span>

  681. <span class="gp">&gt;&gt;&gt; </span><span class="n">pack</span><span class="p">(</span><span class="s1">&#39;@ic&#39;</span><span class="p">,</span> <span class="mh">0x12131415</span><span class="p">,</span> <span class="sa">b</span><span class="s1">&#39;#&#39;</span><span class="p">)</span>

  682. <span class="go">b&#39;\x15\x14\x13\x12#&#39;</span>

  683. <span class="gp">&gt;&gt;&gt; </span><span class="n">calcsize</span><span class="p">(</span><span class="s1">&#39;@ci&#39;</span><span class="p">)</span>

  684. <span class="go">8</span>

  685. <span class="gp">&gt;&gt;&gt; </span><span class="n">calcsize</span><span class="p">(</span><span class="s1">&#39;@ic&#39;</span><span class="p">)</span>

  686. <span class="go">5</span>

  687. </pre></div>

  688. </div>

  689. <p>The following format <code class="docutils literal notranslate"><span class="pre">'llh0l'</span></code> results in two pad bytes being added

  690. at the end, assuming the platform’s longs are aligned on 4-byte boundaries:</p>

  691. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">pack</span><span class="p">(</span><span class="s1">&#39;@llh0l&#39;</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">)</span>

  692. <span class="go">b&#39;\x00\x00\x00\x01\x00\x00\x00\x02\x00\x03\x00\x00&#39;</span>

  693. </pre></div>

  694. </div>

  695. <div class="admonition seealso">

  696. <p class="admonition-title">See also</p>

  697. <dl class="simple">

  698. <dt>Module <a class="reference internal" href="array.html#module-array" title="array: Space efficient arrays of uniformly typed numeric values."><code class="xref py py-mod docutils literal notranslate"><span class="pre">array</span></code></a></dt><dd><p>Packed binary storage of homogeneous data.</p>

  699. </dd>

  700. <dt>Module <a class="reference internal" href="json.html#module-json" title="json: Encode and decode the JSON format."><code class="xref py py-mod docutils literal notranslate"><span class="pre">json</span></code></a></dt><dd><p>JSON encoder and decoder.</p>

  701. </dd>

  702. <dt>Module <a class="reference internal" href="pickle.html#module-pickle" title="pickle: Convert Python objects to streams of bytes and back."><code class="xref py py-mod docutils literal notranslate"><span class="pre">pickle</span></code></a></dt><dd><p>Python object serialization.</p>

  703. </dd>

  704. </dl>

  705. </div>

  706. </section>

  707. </section>

  708. <section id="applications">

  709. <span id="id2"></span><h2>Applications<a class="headerlink" href="#applications" title="Link to this heading">¶</a></h2>

  710. <p>Two main applications for the <a class="reference internal" href="#module-struct" title="struct: Interpret bytes as packed binary data."><code class="xref py py-mod docutils literal notranslate"><span class="pre">struct</span></code></a> module exist, data

  711. interchange between Python and C code within an application or another

  712. application compiled using the same compiler (<a class="reference internal" href="#struct-native-formats"><span class="std std-ref">native formats</span></a>), and

  713. data interchange between applications using agreed upon data layout

  714. (<a class="reference internal" href="#struct-standard-formats"><span class="std std-ref">standard formats</span></a>).  Generally speaking, the format strings

  715. constructed for these two domains are distinct.</p>

  716. <section id="native-formats">

  717. <span id="struct-native-formats"></span><h3>Native Formats<a class="headerlink" href="#native-formats" title="Link to this heading">¶</a></h3>

  718. <p>When constructing format strings which mimic native layouts, the

  719. compiler and machine architecture determine byte ordering and padding.

  720. In such cases, the <code class="docutils literal notranslate"><span class="pre">&#64;</span></code> format character should be used to specify

  721. native byte ordering and data sizes.  Internal pad bytes are normally inserted

  722. automatically.  It is possible that a zero-repeat format code will be

  723. needed at the end of a format string to round up to the correct

  724. byte boundary for proper alignment of consecutive chunks of data.</p>

  725. <p>Consider these two simple examples (on a 64-bit, little-endian

  726. machine):</p>

  727. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">calcsize</span><span class="p">(</span><span class="s1">&#39;@lhl&#39;</span><span class="p">)</span>

  728. <span class="go">24</span>

  729. <span class="gp">&gt;&gt;&gt; </span><span class="n">calcsize</span><span class="p">(</span><span class="s1">&#39;@llh&#39;</span><span class="p">)</span>

  730. <span class="go">18</span>

  731. </pre></div>

  732. </div>

  733. <p>Data is not padded to an 8-byte boundary at the end of the second

  734. format string without the use of extra padding.  A zero-repeat format

  735. code solves that problem:</p>

  736. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">calcsize</span><span class="p">(</span><span class="s1">&#39;@llh0l&#39;</span><span class="p">)</span>

  737. <span class="go">24</span>

  738. </pre></div>

  739. </div>

  740. <p>The <code class="docutils literal notranslate"><span class="pre">'x'</span></code> format code can be used to specify the repeat, but for

  741. native formats it is better to use a zero-repeat format like <code class="docutils literal notranslate"><span class="pre">'0l'</span></code>.</p>

  742. <p>By default, native byte ordering and alignment is used, but it is

  743. better to be explicit and use the <code class="docutils literal notranslate"><span class="pre">'&#64;'</span></code> prefix character.</p>

  744. </section>

  745. <section id="standard-formats">

  746. <span id="struct-standard-formats"></span><h3>Standard Formats<a class="headerlink" href="#standard-formats" title="Link to this heading">¶</a></h3>

  747. <p>When exchanging data beyond your process such as networking or storage,

  748. be precise.  Specify the exact byte order, size, and alignment.  Do

  749. not assume they match the native order of a particular machine.

  750. For example, network byte order is big-endian, while many popular CPUs

  751. are little-endian.  By defining this explicitly, the user need not

  752. care about the specifics of the platform their code is running on.

  753. The first character should typically be <code class="docutils literal notranslate"><span class="pre">&lt;</span></code> or <code class="docutils literal notranslate"><span class="pre">&gt;</span></code>

  754. (or <code class="docutils literal notranslate"><span class="pre">!</span></code>).  Padding is the responsibility of the programmer.  The

  755. zero-repeat format character won’t work.  Instead, the user must

  756. explicitly add <code class="docutils literal notranslate"><span class="pre">'x'</span></code> pad bytes where needed.  Revisiting the

  757. examples from the previous section, we have:</p>

  758. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">calcsize</span><span class="p">(</span><span class="s1">&#39;&lt;qh6xq&#39;</span><span class="p">)</span>

  759. <span class="go">24</span>

  760. <span class="gp">&gt;&gt;&gt; </span><span class="n">pack</span><span class="p">(</span><span class="s1">&#39;&lt;qh6xq&#39;</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">)</span> <span class="o">==</span> <span class="n">pack</span><span class="p">(</span><span class="s1">&#39;@lhl&#39;</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">)</span>

  761. <span class="go">True</span>

  762. <span class="gp">&gt;&gt;&gt; </span><span class="n">calcsize</span><span class="p">(</span><span class="s1">&#39;@llh&#39;</span><span class="p">)</span>

  763. <span class="go">18</span>

  764. <span class="gp">&gt;&gt;&gt; </span><span class="n">pack</span><span class="p">(</span><span class="s1">&#39;@llh&#39;</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">)</span> <span class="o">==</span> <span class="n">pack</span><span class="p">(</span><span class="s1">&#39;&lt;qqh&#39;</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">)</span>

  765. <span class="go">True</span>

  766. <span class="gp">&gt;&gt;&gt; </span><span class="n">calcsize</span><span class="p">(</span><span class="s1">&#39;&lt;qqh6x&#39;</span><span class="p">)</span>

  767. <span class="go">24</span>

  768. <span class="gp">&gt;&gt;&gt; </span><span class="n">calcsize</span><span class="p">(</span><span class="s1">&#39;@llh0l&#39;</span><span class="p">)</span>

  769. <span class="go">24</span>

  770. <span class="gp">&gt;&gt;&gt; </span><span class="n">pack</span><span class="p">(</span><span class="s1">&#39;@llh0l&#39;</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">)</span> <span class="o">==</span> <span class="n">pack</span><span class="p">(</span><span class="s1">&#39;&lt;qqh6x&#39;</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">)</span>

  771. <span class="go">True</span>

  772. </pre></div>

  773. </div>

  774. <p>The above results (executed on a 64-bit machine) aren’t guaranteed to

  775. match when executed on different machines.  For example, the examples

  776. below were executed on a 32-bit machine:</p>

  777. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">calcsize</span><span class="p">(</span><span class="s1">&#39;&lt;qqh6x&#39;</span><span class="p">)</span>

  778. <span class="go">24</span>

  779. <span class="gp">&gt;&gt;&gt; </span><span class="n">calcsize</span><span class="p">(</span><span class="s1">&#39;@llh0l&#39;</span><span class="p">)</span>

  780. <span class="go">12</span>

  781. <span class="gp">&gt;&gt;&gt; </span><span class="n">pack</span><span class="p">(</span><span class="s1">&#39;@llh0l&#39;</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">)</span> <span class="o">==</span> <span class="n">pack</span><span class="p">(</span><span class="s1">&#39;&lt;qqh6x&#39;</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">)</span>

  782. <span class="go">False</span>

  783. </pre></div>

  784. </div>

  785. </section>

  786. </section>

  787. <section id="classes">

  788. <span id="struct-objects"></span><h2>Classes<a class="headerlink" href="#classes" title="Link to this heading">¶</a></h2>

  789. <p>The <a class="reference internal" href="#module-struct" title="struct: Interpret bytes as packed binary data."><code class="xref py py-mod docutils literal notranslate"><span class="pre">struct</span></code></a> module also defines the following type:</p>

  790. <dl class="py class">

  791. <dt class="sig sig-object py" id="struct.Struct">

  792. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">struct.</span></span><span class="sig-name descname"><span class="pre">Struct</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">format</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#struct.Struct" title="Link to this definition">¶</a></dt>

  793. <dd><p>Return a new Struct object which writes and reads binary data according to

  794. the format string <em>format</em>.  Creating a <code class="docutils literal notranslate"><span class="pre">Struct</span></code> object once and calling its

  795. methods is more efficient than calling module-level functions with the

  796. same format since the format string is only compiled once.</p>

  797. <div class="admonition note">

  798. <p class="admonition-title">Note</p>

  799. <p>The compiled versions of the most recent format strings passed to

  800. the module-level functions are cached, so programs that use only a few

  801. format strings needn’t worry about reusing a single <a class="reference internal" href="#struct.Struct" title="struct.Struct"><code class="xref py py-class docutils literal notranslate"><span class="pre">Struct</span></code></a>

  802. instance.</p>

  803. </div>

  804. <p>Compiled Struct objects support the following methods and attributes:</p>

  805. <dl class="py method">

  806. <dt class="sig sig-object py" id="struct.Struct.pack">

  807. <span class="sig-name descname"><span class="pre">pack</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">v1</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">v2</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">...</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#struct.Struct.pack" title="Link to this definition">¶</a></dt>

  808. <dd><p>Identical to the <a class="reference internal" href="#struct.pack" title="struct.pack"><code class="xref py py-func docutils literal notranslate"><span class="pre">pack()</span></code></a> function, using the compiled format.

  809. (<code class="docutils literal notranslate"><span class="pre">len(result)</span></code> will equal <a class="reference internal" href="#struct.Struct.size" title="struct.Struct.size"><code class="xref py py-attr docutils literal notranslate"><span class="pre">size</span></code></a>.)</p>

  810. </dd></dl>

  811. 

  812. <dl class="py method">

  813. <dt class="sig sig-object py" id="struct.Struct.pack_into">

  814. <span class="sig-name descname"><span class="pre">pack_into</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">buffer</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">offset</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">v1</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">v2</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">...</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#struct.Struct.pack_into" title="Link to this definition">¶</a></dt>

  815. <dd><p>Identical to the <a class="reference internal" href="#struct.pack_into" title="struct.pack_into"><code class="xref py py-func docutils literal notranslate"><span class="pre">pack_into()</span></code></a> function, using the compiled format.</p>

  816. </dd></dl>

  817. 

  818. <dl class="py method">

  819. <dt class="sig sig-object py" id="struct.Struct.unpack">

  820. <span class="sig-name descname"><span class="pre">unpack</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">buffer</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#struct.Struct.unpack" title="Link to this definition">¶</a></dt>

  821. <dd><p>Identical to the <a class="reference internal" href="#struct.unpack" title="struct.unpack"><code class="xref py py-func docutils literal notranslate"><span class="pre">unpack()</span></code></a> function, using the compiled format.

  822. The buffer’s size in bytes must equal <a class="reference internal" href="#struct.Struct.size" title="struct.Struct.size"><code class="xref py py-attr docutils literal notranslate"><span class="pre">size</span></code></a>.</p>

  823. </dd></dl>

  824. 

  825. <dl class="py method">

  826. <dt class="sig sig-object py" id="struct.Struct.unpack_from">

  827. <span class="sig-name descname"><span class="pre">unpack_from</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">buffer</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">offset</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">0</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#struct.Struct.unpack_from" title="Link to this definition">¶</a></dt>

  828. <dd><p>Identical to the <a class="reference internal" href="#struct.unpack_from" title="struct.unpack_from"><code class="xref py py-func docutils literal notranslate"><span class="pre">unpack_from()</span></code></a> function, using the compiled format.

  829. The buffer’s size in bytes, starting at position <em>offset</em>, must be at least

  830. <a class="reference internal" href="#struct.Struct.size" title="struct.Struct.size"><code class="xref py py-attr docutils literal notranslate"><span class="pre">size</span></code></a>.</p>

  831. </dd></dl>

  832. 

  833. <dl class="py method">

  834. <dt class="sig sig-object py" id="struct.Struct.iter_unpack">

  835. <span class="sig-name descname"><span class="pre">iter_unpack</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">buffer</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#struct.Struct.iter_unpack" title="Link to this definition">¶</a></dt>

  836. <dd><p>Identical to the <a class="reference internal" href="#struct.iter_unpack" title="struct.iter_unpack"><code class="xref py py-func docutils literal notranslate"><span class="pre">iter_unpack()</span></code></a> function, using the compiled format.

  837. The buffer’s size in bytes must be a multiple of <a class="reference internal" href="#struct.Struct.size" title="struct.Struct.size"><code class="xref py py-attr docutils literal notranslate"><span class="pre">size</span></code></a>.</p>

  838. <div class="versionadded">

  839. <p><span class="versionmodified added">New in version 3.4.</span></p>

  840. </div>

  841. </dd></dl>

  842. 

  843. <dl class="py attribute">

  844. <dt class="sig sig-object py" id="struct.Struct.format">

  845. <span class="sig-name descname"><span class="pre">format</span></span><a class="headerlink" href="#struct.Struct.format" title="Link to this definition">¶</a></dt>

  846. <dd><p>The format string used to construct this Struct object.</p>

  847. <div class="versionchanged">

  848. <p><span class="versionmodified changed">Changed in version 3.7: </span>The format string type is now <a class="reference internal" href="stdtypes.html#str" title="str"><code class="xref py py-class docutils literal notranslate"><span class="pre">str</span></code></a> instead of <a class="reference internal" href="stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate"><span class="pre">bytes</span></code></a>.</p>

  849. </div>

  850. </dd></dl>

  851. 

  852. <dl class="py attribute">

  853. <dt class="sig sig-object py" id="struct.Struct.size">

  854. <span class="sig-name descname"><span class="pre">size</span></span><a class="headerlink" href="#struct.Struct.size" title="Link to this definition">¶</a></dt>

  855. <dd><p>The calculated size of the struct (and hence of the bytes object produced

  856. by the <a class="reference internal" href="#struct.pack" title="struct.pack"><code class="xref py py-meth docutils literal notranslate"><span class="pre">pack()</span></code></a> method) corresponding to <a class="reference internal" href="functions.html#format" title="format"><code class="xref py py-attr docutils literal notranslate"><span class="pre">format</span></code></a>.</p>

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