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  102. <li><a class="reference internal" href="#introducing-optional-arguments">Introducing Optional arguments</a><ul>

  103. <li><a class="reference internal" href="#short-options">Short options</a></li>

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  106. <li><a class="reference internal" href="#combining-positional-and-optional-arguments">Combining Positional and Optional arguments</a></li>

  107. <li><a class="reference internal" href="#getting-a-little-more-advanced">Getting a little more advanced</a><ul>

  108. <li><a class="reference internal" href="#specifying-ambiguous-arguments">Specifying ambiguous arguments</a></li>

  109. <li><a class="reference internal" href="#conflicting-options">Conflicting options</a></li>

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  205.   <section id="argparse-tutorial">

  206. <span id="id1"></span><h1>Argparse Tutorial<a class="headerlink" href="#argparse-tutorial" title="Link to this heading">¶</a></h1>

  207. <dl class="field-list simple">

  208. <dt class="field-odd">author<span class="colon">:</span></dt>

  209. <dd class="field-odd"><p>Tshepang Mbambo</p>

  210. </dd>

  211. </dl>

  212. <p>This tutorial is intended to be a gentle introduction to <a class="reference internal" href="../library/argparse.html#module-argparse" title="argparse: Command-line option and argument parsing library."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></a>, the

  213. recommended command-line parsing module in the Python standard library.</p>

  214. <div class="admonition note">

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

  216. <p>There are two other modules that fulfill the same task, namely

  217. <a class="reference internal" href="../library/getopt.html#module-getopt" title="getopt: Portable parser for command line options; support both short and long option names."><code class="xref py py-mod docutils literal notranslate"><span class="pre">getopt</span></code></a> (an equivalent for <code class="docutils literal notranslate"><span class="pre">getopt()</span></code> from the C

  218. language) and the deprecated <a class="reference internal" href="../library/optparse.html#module-optparse" title="optparse: Command-line option parsing library. (deprecated)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">optparse</span></code></a>.

  219. Note also that <a class="reference internal" href="../library/argparse.html#module-argparse" title="argparse: Command-line option and argument parsing library."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></a> is based on <a class="reference internal" href="../library/optparse.html#module-optparse" title="optparse: Command-line option parsing library. (deprecated)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">optparse</span></code></a>,

  220. and therefore very similar in terms of usage.</p>

  221. </div>

  222. <section id="concepts">

  223. <h2>Concepts<a class="headerlink" href="#concepts" title="Link to this heading">¶</a></h2>

  224. <p>Let’s show the sort of functionality that we are going to explore in this

  225. introductory tutorial by making use of the <strong class="command">ls</strong> command:</p>

  226. <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>ls

  227. <span class="go">cpython  devguide  prog.py  pypy  rm-unused-function.patch</span>

  228. <span class="gp">$ </span>ls<span class="w"> </span>pypy

  229. <span class="go">ctypes_configure  demo  dotviewer  include  lib_pypy  lib-python ...</span>

  230. <span class="gp">$ </span>ls<span class="w"> </span>-l

  231. <span class="go">total 20</span>

  232. <span class="go">drwxr-xr-x 19 wena wena 4096 Feb 18 18:51 cpython</span>

  233. <span class="go">drwxr-xr-x  4 wena wena 4096 Feb  8 12:04 devguide</span>

  234. <span class="go">-rwxr-xr-x  1 wena wena  535 Feb 19 00:05 prog.py</span>

  235. <span class="go">drwxr-xr-x 14 wena wena 4096 Feb  7 00:59 pypy</span>

  236. <span class="go">-rw-r--r--  1 wena wena  741 Feb 18 01:01 rm-unused-function.patch</span>

  237. <span class="gp">$ </span>ls<span class="w"> </span>--help

  238. <span class="go">Usage: ls [OPTION]... [FILE]...</span>

  239. <span class="go">List information about the FILEs (the current directory by default).</span>

  240. <span class="go">Sort entries alphabetically if none of -cftuvSUX nor --sort is specified.</span>

  241. <span class="go">...</span>

  242. </pre></div>

  243. </div>

  244. <p>A few concepts we can learn from the four commands:</p>

  245. <ul class="simple">

  246. <li><p>The <strong class="command">ls</strong> command is useful when run without any options at all. It defaults

  247. to displaying the contents of the current directory.</p></li>

  248. <li><p>If we want beyond what it provides by default, we tell it a bit more. In

  249. this case, we want it to display a different directory, <code class="docutils literal notranslate"><span class="pre">pypy</span></code>.

  250. What we did is specify what is known as a positional argument. It’s named so

  251. because the program should know what to do with the value, solely based on

  252. where it appears on the command line. This concept is more relevant

  253. to a command like <strong class="command">cp</strong>, whose most basic usage is <code class="docutils literal notranslate"><span class="pre">cp</span> <span class="pre">SRC</span> <span class="pre">DEST</span></code>.

  254. The first position is <em>what you want copied,</em> and the second

  255. position is <em>where you want it copied to</em>.</p></li>

  256. <li><p>Now, say we want to change behaviour of the program. In our example,

  257. we display more info for each file instead of just showing the file names.

  258. The <code class="docutils literal notranslate"><span class="pre">-l</span></code> in that case is known as an optional argument.</p></li>

  259. <li><p>That’s a snippet of the help text. It’s very useful in that you can

  260. come across a program you have never used before, and can figure out

  261. how it works simply by reading its help text.</p></li>

  262. </ul>

  263. </section>

  264. <section id="the-basics">

  265. <h2>The basics<a class="headerlink" href="#the-basics" title="Link to this heading">¶</a></h2>

  266. <p>Let us start with a very simple example which does (almost) nothing:</p>

  267. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">argparse</span>

  268. <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span>

  269. <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span>

  270. </pre></div>

  271. </div>

  272. <p>Following is a result of running the code:</p>

  273. <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>python<span class="w"> </span>prog.py

  274. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span>--help

  275. <span class="go">usage: prog.py [-h]</span>

  276. 

  277. <span class="go">options:</span>

  278. <span class="go">  -h, --help  show this help message and exit</span>

  279. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span>--verbose

  280. <span class="go">usage: prog.py [-h]</span>

  281. <span class="go">prog.py: error: unrecognized arguments: --verbose</span>

  282. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span>foo

  283. <span class="go">usage: prog.py [-h]</span>

  284. <span class="go">prog.py: error: unrecognized arguments: foo</span>

  285. </pre></div>

  286. </div>

  287. <p>Here is what is happening:</p>

  288. <ul class="simple">

  289. <li><p>Running the script without any options results in nothing displayed to

  290. stdout. Not so useful.</p></li>

  291. <li><p>The second one starts to display the usefulness of the <a class="reference internal" href="../library/argparse.html#module-argparse" title="argparse: Command-line option and argument parsing library."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></a>

  292. module. We have done almost nothing, but already we get a nice help message.</p></li>

  293. <li><p>The <code class="docutils literal notranslate"><span class="pre">--help</span></code> option, which can also be shortened to <code class="docutils literal notranslate"><span class="pre">-h</span></code>, is the only

  294. option we get for free (i.e. no need to specify it). Specifying anything

  295. else results in an error. But even then, we do get a useful usage message,

  296. also for free.</p></li>

  297. </ul>

  298. </section>

  299. <section id="introducing-positional-arguments">

  300. <h2>Introducing Positional arguments<a class="headerlink" href="#introducing-positional-arguments" title="Link to this heading">¶</a></h2>

  301. <p>An example:</p>

  302. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">argparse</span>

  303. <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span>

  304. <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s2">&quot;echo&quot;</span><span class="p">)</span>

  305. <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span>

  306. <span class="nb">print</span><span class="p">(</span><span class="n">args</span><span class="o">.</span><span class="n">echo</span><span class="p">)</span>

  307. </pre></div>

  308. </div>

  309. <p>And running the code:</p>

  310. <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>python<span class="w"> </span>prog.py

  311. <span class="go">usage: prog.py [-h] echo</span>

  312. <span class="go">prog.py: error: the following arguments are required: echo</span>

  313. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span>--help

  314. <span class="go">usage: prog.py [-h] echo</span>

  315. 

  316. <span class="go">positional arguments:</span>

  317. <span class="go">  echo</span>

  318. 

  319. <span class="go">options:</span>

  320. <span class="go">  -h, --help  show this help message and exit</span>

  321. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span>foo

  322. <span class="go">foo</span>

  323. </pre></div>

  324. </div>

  325. <p>Here is what’s happening:</p>

  326. <ul class="simple">

  327. <li><p>We’ve added the <a class="reference internal" href="../library/argparse.html#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument()</span></code></a> method, which is what we use to specify

  328. which command-line options the program is willing to accept. In this case,

  329. I’ve named it <code class="docutils literal notranslate"><span class="pre">echo</span></code> so that it’s in line with its function.</p></li>

  330. <li><p>Calling our program now requires us to specify an option.</p></li>

  331. <li><p>The <a class="reference internal" href="../library/argparse.html#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a> method actually returns some data from the

  332. options specified, in this case, <code class="docutils literal notranslate"><span class="pre">echo</span></code>.</p></li>

  333. <li><p>The variable is some form of ‘magic’ that <a class="reference internal" href="../library/argparse.html#module-argparse" title="argparse: Command-line option and argument parsing library."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></a> performs for free

  334. (i.e. no need to specify which variable that value is stored in).

  335. You will also notice that its name matches the string argument given

  336. to the method, <code class="docutils literal notranslate"><span class="pre">echo</span></code>.</p></li>

  337. </ul>

  338. <p>Note however that, although the help display looks nice and all, it currently

  339. is not as helpful as it can be. For example we see that we got <code class="docutils literal notranslate"><span class="pre">echo</span></code> as a

  340. positional argument, but we don’t know what it does, other than by guessing or

  341. by reading the source code. So, let’s make it a bit more useful:</p>

  342. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">argparse</span>

  343. <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span>

  344. <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s2">&quot;echo&quot;</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s2">&quot;echo the string you use here&quot;</span><span class="p">)</span>

  345. <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span>

  346. <span class="nb">print</span><span class="p">(</span><span class="n">args</span><span class="o">.</span><span class="n">echo</span><span class="p">)</span>

  347. </pre></div>

  348. </div>

  349. <p>And we get:</p>

  350. <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span>-h

  351. <span class="go">usage: prog.py [-h] echo</span>

  352. 

  353. <span class="go">positional arguments:</span>

  354. <span class="go">  echo        echo the string you use here</span>

  355. 

  356. <span class="go">options:</span>

  357. <span class="go">  -h, --help  show this help message and exit</span>

  358. </pre></div>

  359. </div>

  360. <p>Now, how about doing something even more useful:</p>

  361. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">argparse</span>

  362. <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span>

  363. <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s2">&quot;square&quot;</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s2">&quot;display a square of a given number&quot;</span><span class="p">)</span>

  364. <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span>

  365. <span class="nb">print</span><span class="p">(</span><span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="o">**</span><span class="mi">2</span><span class="p">)</span>

  366. </pre></div>

  367. </div>

  368. <p>Following is a result of running the code:</p>

  369. <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span><span class="m">4</span>

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

  371. <span class="go">  File &quot;prog.py&quot;, line 5, in &lt;module&gt;</span>

  372. <span class="go">    print(args.square**2)</span>

  373. <span class="go">TypeError: unsupported operand type(s) for ** or pow(): &#39;str&#39; and &#39;int&#39;</span>

  374. </pre></div>

  375. </div>

  376. <p>That didn’t go so well. That’s because <a class="reference internal" href="../library/argparse.html#module-argparse" title="argparse: Command-line option and argument parsing library."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></a> treats the options we

  377. give it as strings, unless we tell it otherwise. So, let’s tell

  378. <a class="reference internal" href="../library/argparse.html#module-argparse" title="argparse: Command-line option and argument parsing library."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></a> to treat that input as an integer:</p>

  379. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">argparse</span>

  380. <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span>

  381. <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s2">&quot;square&quot;</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s2">&quot;display a square of a given number&quot;</span><span class="p">,</span>

  382.                     <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">)</span>

  383. <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span>

  384. <span class="nb">print</span><span class="p">(</span><span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="o">**</span><span class="mi">2</span><span class="p">)</span>

  385. </pre></div>

  386. </div>

  387. <p>Following is a result of running the code:</p>

  388. <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span><span class="m">4</span>

  389. <span class="go">16</span>

  390. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span>four

  391. <span class="go">usage: prog.py [-h] square</span>

  392. <span class="go">prog.py: error: argument square: invalid int value: &#39;four&#39;</span>

  393. </pre></div>

  394. </div>

  395. <p>That went well. The program now even helpfully quits on bad illegal input

  396. before proceeding.</p>

  397. </section>

  398. <section id="introducing-optional-arguments">

  399. <h2>Introducing Optional arguments<a class="headerlink" href="#introducing-optional-arguments" title="Link to this heading">¶</a></h2>

  400. <p>So far we have been playing with positional arguments. Let us

  401. have a look on how to add optional ones:</p>

  402. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">argparse</span>

  403. <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span>

  404. <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s2">&quot;--verbosity&quot;</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s2">&quot;increase output verbosity&quot;</span><span class="p">)</span>

  405. <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span>

  406. <span class="k">if</span> <span class="n">args</span><span class="o">.</span><span class="n">verbosity</span><span class="p">:</span>

  407.     <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;verbosity turned on&quot;</span><span class="p">)</span>

  408. </pre></div>

  409. </div>

  410. <p>And the output:</p>

  411. <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span>--verbosity<span class="w"> </span><span class="m">1</span>

  412. <span class="go">verbosity turned on</span>

  413. <span class="gp">$ </span>python<span class="w"> </span>prog.py

  414. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span>--help

  415. <span class="go">usage: prog.py [-h] [--verbosity VERBOSITY]</span>

  416. 

  417. <span class="go">options:</span>

  418. <span class="go">  -h, --help            show this help message and exit</span>

  419. <span class="go">  --verbosity VERBOSITY</span>

  420. <span class="go">                        increase output verbosity</span>

  421. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span>--verbosity

  422. <span class="go">usage: prog.py [-h] [--verbosity VERBOSITY]</span>

  423. <span class="go">prog.py: error: argument --verbosity: expected one argument</span>

  424. </pre></div>

  425. </div>

  426. <p>Here is what is happening:</p>

  427. <ul class="simple">

  428. <li><p>The program is written so as to display something when <code class="docutils literal notranslate"><span class="pre">--verbosity</span></code> is

  429. specified and display nothing when not.</p></li>

  430. <li><p>To show that the option is actually optional, there is no error when running

  431. the program without it. Note that by default, if an optional argument isn’t

  432. used, the relevant variable, in this case <code class="docutils literal notranslate"><span class="pre">args.verbosity</span></code>, is

  433. given <code class="docutils literal notranslate"><span class="pre">None</span></code> as a value, which is the reason it fails the truth

  434. test of the <a class="reference internal" href="../reference/compound_stmts.html#if"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">if</span></code></a> statement.</p></li>

  435. <li><p>The help message is a bit different.</p></li>

  436. <li><p>When using the <code class="docutils literal notranslate"><span class="pre">--verbosity</span></code> option, one must also specify some value,

  437. any value.</p></li>

  438. </ul>

  439. <p>The above example accepts arbitrary integer values for <code class="docutils literal notranslate"><span class="pre">--verbosity</span></code>, but for

  440. our simple program, only two values are actually useful, <code class="docutils literal notranslate"><span class="pre">True</span></code> or <code class="docutils literal notranslate"><span class="pre">False</span></code>.

  441. Let’s modify the code accordingly:</p>

  442. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">argparse</span>

  443. <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span>

  444. <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s2">&quot;--verbose&quot;</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s2">&quot;increase output verbosity&quot;</span><span class="p">,</span>

  445.                     <span class="n">action</span><span class="o">=</span><span class="s2">&quot;store_true&quot;</span><span class="p">)</span>

  446. <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span>

  447. <span class="k">if</span> <span class="n">args</span><span class="o">.</span><span class="n">verbose</span><span class="p">:</span>

  448.     <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;verbosity turned on&quot;</span><span class="p">)</span>

  449. </pre></div>

  450. </div>

  451. <p>And the output:</p>

  452. <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span>--verbose

  453. <span class="go">verbosity turned on</span>

  454. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span>--verbose<span class="w"> </span><span class="m">1</span>

  455. <span class="go">usage: prog.py [-h] [--verbose]</span>

  456. <span class="go">prog.py: error: unrecognized arguments: 1</span>

  457. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span>--help

  458. <span class="go">usage: prog.py [-h] [--verbose]</span>

  459. 

  460. <span class="go">options:</span>

  461. <span class="go">  -h, --help  show this help message and exit</span>

  462. <span class="go">  --verbose   increase output verbosity</span>

  463. </pre></div>

  464. </div>

  465. <p>Here is what is happening:</p>

  466. <ul class="simple">

  467. <li><p>The option is now more of a flag than something that requires a value.

  468. We even changed the name of the option to match that idea.

  469. Note that we now specify a new keyword, <code class="docutils literal notranslate"><span class="pre">action</span></code>, and give it the value

  470. <code class="docutils literal notranslate"><span class="pre">&quot;store_true&quot;</span></code>. This means that, if the option is specified,

  471. assign the value <code class="docutils literal notranslate"><span class="pre">True</span></code> to <code class="docutils literal notranslate"><span class="pre">args.verbose</span></code>.

  472. Not specifying it implies <code class="docutils literal notranslate"><span class="pre">False</span></code>.</p></li>

  473. <li><p>It complains when you specify a value, in true spirit of what flags

  474. actually are.</p></li>

  475. <li><p>Notice the different help text.</p></li>

  476. </ul>

  477. <section id="short-options">

  478. <h3>Short options<a class="headerlink" href="#short-options" title="Link to this heading">¶</a></h3>

  479. <p>If you are familiar with command line usage,

  480. you will notice that I haven’t yet touched on the topic of short

  481. versions of the options. It’s quite simple:</p>

  482. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">argparse</span>

  483. <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span>

  484. <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s2">&quot;-v&quot;</span><span class="p">,</span> <span class="s2">&quot;--verbose&quot;</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s2">&quot;increase output verbosity&quot;</span><span class="p">,</span>

  485.                     <span class="n">action</span><span class="o">=</span><span class="s2">&quot;store_true&quot;</span><span class="p">)</span>

  486. <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span>

  487. <span class="k">if</span> <span class="n">args</span><span class="o">.</span><span class="n">verbose</span><span class="p">:</span>

  488.     <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;verbosity turned on&quot;</span><span class="p">)</span>

  489. </pre></div>

  490. </div>

  491. <p>And here goes:</p>

  492. <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span>-v

  493. <span class="go">verbosity turned on</span>

  494. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span>--help

  495. <span class="go">usage: prog.py [-h] [-v]</span>

  496. 

  497. <span class="go">options:</span>

  498. <span class="go">  -h, --help     show this help message and exit</span>

  499. <span class="go">  -v, --verbose  increase output verbosity</span>

  500. </pre></div>

  501. </div>

  502. <p>Note that the new ability is also reflected in the help text.</p>

  503. </section>

  504. </section>

  505. <section id="combining-positional-and-optional-arguments">

  506. <h2>Combining Positional and Optional arguments<a class="headerlink" href="#combining-positional-and-optional-arguments" title="Link to this heading">¶</a></h2>

  507. <p>Our program keeps growing in complexity:</p>

  508. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">argparse</span>

  509. <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span>

  510. <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s2">&quot;square&quot;</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span>

  511.                     <span class="n">help</span><span class="o">=</span><span class="s2">&quot;display a square of a given number&quot;</span><span class="p">)</span>

  512. <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s2">&quot;-v&quot;</span><span class="p">,</span> <span class="s2">&quot;--verbose&quot;</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s2">&quot;store_true&quot;</span><span class="p">,</span>

  513.                     <span class="n">help</span><span class="o">=</span><span class="s2">&quot;increase output verbosity&quot;</span><span class="p">)</span>

  514. <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span>

  515. <span class="n">answer</span> <span class="o">=</span> <span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="o">**</span><span class="mi">2</span>

  516. <span class="k">if</span> <span class="n">args</span><span class="o">.</span><span class="n">verbose</span><span class="p">:</span>

  517.     <span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;the square of </span><span class="si">{</span><span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="si">}</span><span class="s2"> equals </span><span class="si">{</span><span class="n">answer</span><span class="si">}</span><span class="s2">&quot;</span><span class="p">)</span>

  518. <span class="k">else</span><span class="p">:</span>

  519.     <span class="nb">print</span><span class="p">(</span><span class="n">answer</span><span class="p">)</span>

  520. </pre></div>

  521. </div>

  522. <p>And now the output:</p>

  523. <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>python<span class="w"> </span>prog.py

  524. <span class="go">usage: prog.py [-h] [-v] square</span>

  525. <span class="go">prog.py: error: the following arguments are required: square</span>

  526. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span><span class="m">4</span>

  527. <span class="go">16</span>

  528. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span><span class="m">4</span><span class="w"> </span>--verbose

  529. <span class="go">the square of 4 equals 16</span>

  530. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span>--verbose<span class="w"> </span><span class="m">4</span>

  531. <span class="go">the square of 4 equals 16</span>

  532. </pre></div>

  533. </div>

  534. <ul class="simple">

  535. <li><p>We’ve brought back a positional argument, hence the complaint.</p></li>

  536. <li><p>Note that the order does not matter.</p></li>

  537. </ul>

  538. <p>How about we give this program of ours back the ability to have

  539. multiple verbosity values, and actually get to use them:</p>

  540. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">argparse</span>

  541. <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span>

  542. <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s2">&quot;square&quot;</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span>

  543.                     <span class="n">help</span><span class="o">=</span><span class="s2">&quot;display a square of a given number&quot;</span><span class="p">)</span>

  544. <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s2">&quot;-v&quot;</span><span class="p">,</span> <span class="s2">&quot;--verbosity&quot;</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span>

  545.                     <span class="n">help</span><span class="o">=</span><span class="s2">&quot;increase output verbosity&quot;</span><span class="p">)</span>

  546. <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span>

  547. <span class="n">answer</span> <span class="o">=</span> <span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="o">**</span><span class="mi">2</span>

  548. <span class="k">if</span> <span class="n">args</span><span class="o">.</span><span class="n">verbosity</span> <span class="o">==</span> <span class="mi">2</span><span class="p">:</span>

  549.     <span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;the square of </span><span class="si">{</span><span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="si">}</span><span class="s2"> equals </span><span class="si">{</span><span class="n">answer</span><span class="si">}</span><span class="s2">&quot;</span><span class="p">)</span>

  550. <span class="k">elif</span> <span class="n">args</span><span class="o">.</span><span class="n">verbosity</span> <span class="o">==</span> <span class="mi">1</span><span class="p">:</span>

  551.     <span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;</span><span class="si">{</span><span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="si">}</span><span class="s2">^2 == </span><span class="si">{</span><span class="n">answer</span><span class="si">}</span><span class="s2">&quot;</span><span class="p">)</span>

  552. <span class="k">else</span><span class="p">:</span>

  553.     <span class="nb">print</span><span class="p">(</span><span class="n">answer</span><span class="p">)</span>

  554. </pre></div>

  555. </div>

  556. <p>And the output:</p>

  557. <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span><span class="m">4</span>

  558. <span class="go">16</span>

  559. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span><span class="m">4</span><span class="w"> </span>-v

  560. <span class="go">usage: prog.py [-h] [-v VERBOSITY] square</span>

  561. <span class="go">prog.py: error: argument -v/--verbosity: expected one argument</span>

  562. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span><span class="m">4</span><span class="w"> </span>-v<span class="w"> </span><span class="m">1</span>

  563. <span class="go">4^2 == 16</span>

  564. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span><span class="m">4</span><span class="w"> </span>-v<span class="w"> </span><span class="m">2</span>

  565. <span class="go">the square of 4 equals 16</span>

  566. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span><span class="m">4</span><span class="w"> </span>-v<span class="w"> </span><span class="m">3</span>

  567. <span class="go">16</span>

  568. </pre></div>

  569. </div>

  570. <p>These all look good except the last one, which exposes a bug in our program.

  571. Let’s fix it by restricting the values the <code class="docutils literal notranslate"><span class="pre">--verbosity</span></code> option can accept:</p>

  572. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">argparse</span>

  573. <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span>

  574. <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s2">&quot;square&quot;</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span>

  575.                     <span class="n">help</span><span class="o">=</span><span class="s2">&quot;display a square of a given number&quot;</span><span class="p">)</span>

  576. <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s2">&quot;-v&quot;</span><span class="p">,</span> <span class="s2">&quot;--verbosity&quot;</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">choices</span><span class="o">=</span><span class="p">[</span><span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">],</span>

  577.                     <span class="n">help</span><span class="o">=</span><span class="s2">&quot;increase output verbosity&quot;</span><span class="p">)</span>

  578. <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span>

  579. <span class="n">answer</span> <span class="o">=</span> <span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="o">**</span><span class="mi">2</span>

  580. <span class="k">if</span> <span class="n">args</span><span class="o">.</span><span class="n">verbosity</span> <span class="o">==</span> <span class="mi">2</span><span class="p">:</span>

  581.     <span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;the square of </span><span class="si">{</span><span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="si">}</span><span class="s2"> equals </span><span class="si">{</span><span class="n">answer</span><span class="si">}</span><span class="s2">&quot;</span><span class="p">)</span>

  582. <span class="k">elif</span> <span class="n">args</span><span class="o">.</span><span class="n">verbosity</span> <span class="o">==</span> <span class="mi">1</span><span class="p">:</span>

  583.     <span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;</span><span class="si">{</span><span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="si">}</span><span class="s2">^2 == </span><span class="si">{</span><span class="n">answer</span><span class="si">}</span><span class="s2">&quot;</span><span class="p">)</span>

  584. <span class="k">else</span><span class="p">:</span>

  585.     <span class="nb">print</span><span class="p">(</span><span class="n">answer</span><span class="p">)</span>

  586. </pre></div>

  587. </div>

  588. <p>And the output:</p>

  589. <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span><span class="m">4</span><span class="w"> </span>-v<span class="w"> </span><span class="m">3</span>

  590. <span class="go">usage: prog.py [-h] [-v {0,1,2}] square</span>

  591. <span class="go">prog.py: error: argument -v/--verbosity: invalid choice: 3 (choose from 0, 1, 2)</span>

  592. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span><span class="m">4</span><span class="w"> </span>-h

  593. <span class="go">usage: prog.py [-h] [-v {0,1,2}] square</span>

  594. 

  595. <span class="go">positional arguments:</span>

  596. <span class="go">  square                display a square of a given number</span>

  597. 

  598. <span class="go">options:</span>

  599. <span class="go">  -h, --help            show this help message and exit</span>

  600. <span class="go">  -v {0,1,2}, --verbosity {0,1,2}</span>

  601. <span class="go">                        increase output verbosity</span>

  602. </pre></div>

  603. </div>

  604. <p>Note that the change also reflects both in the error message as well as the

  605. help string.</p>

  606. <p>Now, let’s use a different approach of playing with verbosity, which is pretty

  607. common. It also matches the way the CPython executable handles its own

  608. verbosity argument (check the output of <code class="docutils literal notranslate"><span class="pre">python</span> <span class="pre">--help</span></code>):</p>

  609. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">argparse</span>

  610. <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span>

  611. <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s2">&quot;square&quot;</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span>

  612.                     <span class="n">help</span><span class="o">=</span><span class="s2">&quot;display the square of a given number&quot;</span><span class="p">)</span>

  613. <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s2">&quot;-v&quot;</span><span class="p">,</span> <span class="s2">&quot;--verbosity&quot;</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s2">&quot;count&quot;</span><span class="p">,</span>

  614.                     <span class="n">help</span><span class="o">=</span><span class="s2">&quot;increase output verbosity&quot;</span><span class="p">)</span>

  615. <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span>

  616. <span class="n">answer</span> <span class="o">=</span> <span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="o">**</span><span class="mi">2</span>

  617. <span class="k">if</span> <span class="n">args</span><span class="o">.</span><span class="n">verbosity</span> <span class="o">==</span> <span class="mi">2</span><span class="p">:</span>

  618.     <span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;the square of </span><span class="si">{</span><span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="si">}</span><span class="s2"> equals </span><span class="si">{</span><span class="n">answer</span><span class="si">}</span><span class="s2">&quot;</span><span class="p">)</span>

  619. <span class="k">elif</span> <span class="n">args</span><span class="o">.</span><span class="n">verbosity</span> <span class="o">==</span> <span class="mi">1</span><span class="p">:</span>

  620.     <span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;</span><span class="si">{</span><span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="si">}</span><span class="s2">^2 == </span><span class="si">{</span><span class="n">answer</span><span class="si">}</span><span class="s2">&quot;</span><span class="p">)</span>

  621. <span class="k">else</span><span class="p">:</span>

  622.     <span class="nb">print</span><span class="p">(</span><span class="n">answer</span><span class="p">)</span>

  623. </pre></div>

  624. </div>

  625. <p>We have introduced another action, “count”,

  626. to count the number of occurrences of specific options.</p>

  627. <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span><span class="m">4</span>

  628. <span class="go">16</span>

  629. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span><span class="m">4</span><span class="w"> </span>-v

  630. <span class="go">4^2 == 16</span>

  631. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span><span class="m">4</span><span class="w"> </span>-vv

  632. <span class="go">the square of 4 equals 16</span>

  633. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span><span class="m">4</span><span class="w"> </span>--verbosity<span class="w"> </span>--verbosity

  634. <span class="go">the square of 4 equals 16</span>

  635. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span><span class="m">4</span><span class="w"> </span>-v<span class="w"> </span><span class="m">1</span>

  636. <span class="go">usage: prog.py [-h] [-v] square</span>

  637. <span class="go">prog.py: error: unrecognized arguments: 1</span>

  638. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span><span class="m">4</span><span class="w"> </span>-h

  639. <span class="go">usage: prog.py [-h] [-v] square</span>

  640. 

  641. <span class="go">positional arguments:</span>

  642. <span class="go">  square           display a square of a given number</span>

  643. 

  644. <span class="go">options:</span>

  645. <span class="go">  -h, --help       show this help message and exit</span>

  646. <span class="go">  -v, --verbosity  increase output verbosity</span>

  647. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span><span class="m">4</span><span class="w"> </span>-vvv

  648. <span class="go">16</span>

  649. </pre></div>

  650. </div>

  651. <ul class="simple">

  652. <li><p>Yes, it’s now more of a flag (similar to <code class="docutils literal notranslate"><span class="pre">action=&quot;store_true&quot;</span></code>) in the

  653. previous version of our script. That should explain the complaint.</p></li>

  654. <li><p>It also behaves similar to “store_true” action.</p></li>

  655. <li><p>Now here’s a demonstration of what the “count” action gives. You’ve probably

  656. seen this sort of usage before.</p></li>

  657. <li><p>And if you don’t specify the <code class="docutils literal notranslate"><span class="pre">-v</span></code> flag, that flag is considered to have

  658. <code class="docutils literal notranslate"><span class="pre">None</span></code> value.</p></li>

  659. <li><p>As should be expected, specifying the long form of the flag, we should get

  660. the same output.</p></li>

  661. <li><p>Sadly, our help output isn’t very informative on the new ability our script

  662. has acquired, but that can always be fixed by improving the documentation for

  663. our script (e.g. via the <code class="docutils literal notranslate"><span class="pre">help</span></code> keyword argument).</p></li>

  664. <li><p>That last output exposes a bug in our program.</p></li>

  665. </ul>

  666. <p>Let’s fix:</p>

  667. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">argparse</span>

  668. <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span>

  669. <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s2">&quot;square&quot;</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span>

  670.                     <span class="n">help</span><span class="o">=</span><span class="s2">&quot;display a square of a given number&quot;</span><span class="p">)</span>

  671. <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s2">&quot;-v&quot;</span><span class="p">,</span> <span class="s2">&quot;--verbosity&quot;</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s2">&quot;count&quot;</span><span class="p">,</span>

  672.                     <span class="n">help</span><span class="o">=</span><span class="s2">&quot;increase output verbosity&quot;</span><span class="p">)</span>

  673. <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span>

  674. <span class="n">answer</span> <span class="o">=</span> <span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="o">**</span><span class="mi">2</span>

  675. 

  676. <span class="c1"># bugfix: replace == with &gt;=</span>

  677. <span class="k">if</span> <span class="n">args</span><span class="o">.</span><span class="n">verbosity</span> <span class="o">&gt;=</span> <span class="mi">2</span><span class="p">:</span>

  678.     <span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;the square of </span><span class="si">{</span><span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="si">}</span><span class="s2"> equals </span><span class="si">{</span><span class="n">answer</span><span class="si">}</span><span class="s2">&quot;</span><span class="p">)</span>

  679. <span class="k">elif</span> <span class="n">args</span><span class="o">.</span><span class="n">verbosity</span> <span class="o">&gt;=</span> <span class="mi">1</span><span class="p">:</span>

  680.     <span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;</span><span class="si">{</span><span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="si">}</span><span class="s2">^2 == </span><span class="si">{</span><span class="n">answer</span><span class="si">}</span><span class="s2">&quot;</span><span class="p">)</span>

  681. <span class="k">else</span><span class="p">:</span>

  682.     <span class="nb">print</span><span class="p">(</span><span class="n">answer</span><span class="p">)</span>

  683. </pre></div>

  684. </div>

  685. <p>And this is what it gives:</p>

  686. <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span><span class="m">4</span><span class="w"> </span>-vvv

  687. <span class="go">the square of 4 equals 16</span>

  688. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span><span class="m">4</span><span class="w"> </span>-vvvv

  689. <span class="go">the square of 4 equals 16</span>

  690. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span><span class="m">4</span>

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

  692. <span class="go">  File &quot;prog.py&quot;, line 11, in &lt;module&gt;</span>

  693. <span class="go">    if args.verbosity &gt;= 2:</span>

  694. <span class="go">TypeError: &#39;&gt;=&#39; not supported between instances of &#39;NoneType&#39; and &#39;int&#39;</span>

  695. </pre></div>

  696. </div>

  697. <ul class="simple">

  698. <li><p>First output went well, and fixes the bug we had before.

  699. That is, we want any value &gt;= 2 to be as verbose as possible.</p></li>

  700. <li><p>Third output not so good.</p></li>

  701. </ul>

  702. <p>Let’s fix that bug:</p>

  703. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">argparse</span>

  704. <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span>

  705. <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s2">&quot;square&quot;</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span>

  706.                     <span class="n">help</span><span class="o">=</span><span class="s2">&quot;display a square of a given number&quot;</span><span class="p">)</span>

  707. <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s2">&quot;-v&quot;</span><span class="p">,</span> <span class="s2">&quot;--verbosity&quot;</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s2">&quot;count&quot;</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="mi">0</span><span class="p">,</span>

  708.                     <span class="n">help</span><span class="o">=</span><span class="s2">&quot;increase output verbosity&quot;</span><span class="p">)</span>

  709. <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span>

  710. <span class="n">answer</span> <span class="o">=</span> <span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="o">**</span><span class="mi">2</span>

  711. <span class="k">if</span> <span class="n">args</span><span class="o">.</span><span class="n">verbosity</span> <span class="o">&gt;=</span> <span class="mi">2</span><span class="p">:</span>

  712.     <span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;the square of </span><span class="si">{</span><span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="si">}</span><span class="s2"> equals </span><span class="si">{</span><span class="n">answer</span><span class="si">}</span><span class="s2">&quot;</span><span class="p">)</span>

  713. <span class="k">elif</span> <span class="n">args</span><span class="o">.</span><span class="n">verbosity</span> <span class="o">&gt;=</span> <span class="mi">1</span><span class="p">:</span>

  714.     <span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;</span><span class="si">{</span><span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="si">}</span><span class="s2">^2 == </span><span class="si">{</span><span class="n">answer</span><span class="si">}</span><span class="s2">&quot;</span><span class="p">)</span>

  715. <span class="k">else</span><span class="p">:</span>

  716.     <span class="nb">print</span><span class="p">(</span><span class="n">answer</span><span class="p">)</span>

  717. </pre></div>

  718. </div>

  719. <p>We’ve just introduced yet another keyword, <code class="docutils literal notranslate"><span class="pre">default</span></code>.

  720. We’ve set it to <code class="docutils literal notranslate"><span class="pre">0</span></code> in order to make it comparable to the other int values.

  721. Remember that by default,

  722. if an optional argument isn’t specified,

  723. it gets the <code class="docutils literal notranslate"><span class="pre">None</span></code> value, and that cannot be compared to an int value

  724. (hence the <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> exception).</p>

  725. <p>And:</p>

  726. <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span><span class="m">4</span>

  727. <span class="go">16</span>

  728. </pre></div>

  729. </div>

  730. <p>You can go quite far just with what we’ve learned so far,

  731. and we have only scratched the surface.

  732. The <a class="reference internal" href="../library/argparse.html#module-argparse" title="argparse: Command-line option and argument parsing library."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></a> module is very powerful,

  733. and we’ll explore a bit more of it before we end this tutorial.</p>

  734. </section>

  735. <section id="getting-a-little-more-advanced">

  736. <h2>Getting a little more advanced<a class="headerlink" href="#getting-a-little-more-advanced" title="Link to this heading">¶</a></h2>

  737. <p>What if we wanted to expand our tiny program to perform other powers,

  738. not just squares:</p>

  739. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">argparse</span>

  740. <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span>

  741. <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s2">&quot;x&quot;</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s2">&quot;the base&quot;</span><span class="p">)</span>

  742. <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s2">&quot;y&quot;</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s2">&quot;the exponent&quot;</span><span class="p">)</span>

  743. <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s2">&quot;-v&quot;</span><span class="p">,</span> <span class="s2">&quot;--verbosity&quot;</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s2">&quot;count&quot;</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="mi">0</span><span class="p">)</span>

  744. <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span>

  745. <span class="n">answer</span> <span class="o">=</span> <span class="n">args</span><span class="o">.</span><span class="n">x</span><span class="o">**</span><span class="n">args</span><span class="o">.</span><span class="n">y</span>

  746. <span class="k">if</span> <span class="n">args</span><span class="o">.</span><span class="n">verbosity</span> <span class="o">&gt;=</span> <span class="mi">2</span><span class="p">:</span>

  747.     <span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;</span><span class="si">{</span><span class="n">args</span><span class="o">.</span><span class="n">x</span><span class="si">}</span><span class="s2"> to the power </span><span class="si">{</span><span class="n">args</span><span class="o">.</span><span class="n">y</span><span class="si">}</span><span class="s2"> equals </span><span class="si">{</span><span class="n">answer</span><span class="si">}</span><span class="s2">&quot;</span><span class="p">)</span>

  748. <span class="k">elif</span> <span class="n">args</span><span class="o">.</span><span class="n">verbosity</span> <span class="o">&gt;=</span> <span class="mi">1</span><span class="p">:</span>

  749.     <span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;</span><span class="si">{</span><span class="n">args</span><span class="o">.</span><span class="n">x</span><span class="si">}</span><span class="s2">^</span><span class="si">{</span><span class="n">args</span><span class="o">.</span><span class="n">y</span><span class="si">}</span><span class="s2"> == </span><span class="si">{</span><span class="n">answer</span><span class="si">}</span><span class="s2">&quot;</span><span class="p">)</span>

  750. <span class="k">else</span><span class="p">:</span>

  751.     <span class="nb">print</span><span class="p">(</span><span class="n">answer</span><span class="p">)</span>

  752. </pre></div>

  753. </div>

  754. <p>Output:</p>

  755. <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>python<span class="w"> </span>prog.py

  756. <span class="go">usage: prog.py [-h] [-v] x y</span>

  757. <span class="go">prog.py: error: the following arguments are required: x, y</span>

  758. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span>-h

  759. <span class="go">usage: prog.py [-h] [-v] x y</span>

  760. 

  761. <span class="go">positional arguments:</span>

  762. <span class="go">  x                the base</span>

  763. <span class="go">  y                the exponent</span>

  764. 

  765. <span class="go">options:</span>

  766. <span class="go">  -h, --help       show this help message and exit</span>

  767. <span class="go">  -v, --verbosity</span>

  768. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span><span class="m">4</span><span class="w"> </span><span class="m">2</span><span class="w"> </span>-v

  769. <span class="go">4^2 == 16</span>

  770. </pre></div>

  771. </div>

  772. <p>Notice that so far we’ve been using verbosity level to <em>change</em> the text

  773. that gets displayed. The following example instead uses verbosity level

  774. to display <em>more</em> text instead:</p>

  775. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">argparse</span>

  776. <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span>

  777. <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s2">&quot;x&quot;</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s2">&quot;the base&quot;</span><span class="p">)</span>

  778. <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s2">&quot;y&quot;</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s2">&quot;the exponent&quot;</span><span class="p">)</span>

  779. <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s2">&quot;-v&quot;</span><span class="p">,</span> <span class="s2">&quot;--verbosity&quot;</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s2">&quot;count&quot;</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="mi">0</span><span class="p">)</span>

  780. <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span>

  781. <span class="n">answer</span> <span class="o">=</span> <span class="n">args</span><span class="o">.</span><span class="n">x</span><span class="o">**</span><span class="n">args</span><span class="o">.</span><span class="n">y</span>

  782. <span class="k">if</span> <span class="n">args</span><span class="o">.</span><span class="n">verbosity</span> <span class="o">&gt;=</span> <span class="mi">2</span><span class="p">:</span>

  783.     <span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;Running &#39;</span><span class="si">{</span><span class="vm">__file__</span><span class="si">}</span><span class="s2">&#39;&quot;</span><span class="p">)</span>

  784. <span class="k">if</span> <span class="n">args</span><span class="o">.</span><span class="n">verbosity</span> <span class="o">&gt;=</span> <span class="mi">1</span><span class="p">:</span>

  785.     <span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;</span><span class="si">{</span><span class="n">args</span><span class="o">.</span><span class="n">x</span><span class="si">}</span><span class="s2">^</span><span class="si">{</span><span class="n">args</span><span class="o">.</span><span class="n">y</span><span class="si">}</span><span class="s2"> == &quot;</span><span class="p">,</span> <span class="n">end</span><span class="o">=</span><span class="s2">&quot;&quot;</span><span class="p">)</span>

  786. <span class="nb">print</span><span class="p">(</span><span class="n">answer</span><span class="p">)</span>

  787. </pre></div>

  788. </div>

  789. <p>Output:</p>

  790. <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span><span class="m">4</span><span class="w"> </span><span class="m">2</span>

  791. <span class="go">16</span>

  792. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span><span class="m">4</span><span class="w"> </span><span class="m">2</span><span class="w"> </span>-v

  793. <span class="go">4^2 == 16</span>

  794. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span><span class="m">4</span><span class="w"> </span><span class="m">2</span><span class="w"> </span>-vv

  795. <span class="go">Running &#39;prog.py&#39;</span>

  796. <span class="go">4^2 == 16</span>

  797. </pre></div>

  798. </div>

  799. <section id="specifying-ambiguous-arguments">

  800. <span id="id2"></span><h3>Specifying ambiguous arguments<a class="headerlink" href="#specifying-ambiguous-arguments" title="Link to this heading">¶</a></h3>

  801. <p>When there is ambiguity in deciding whether an argument is positional or for an

  802. argument, <code class="docutils literal notranslate"><span class="pre">--</span></code> can be used to tell <a class="reference internal" href="../library/argparse.html#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a> that

  803. everything after that is a positional argument:</p>

  804. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s1">&#39;PROG&#39;</span><span class="p">)</span>

  805. <span class="gp">&gt;&gt;&gt; </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">&#39;-n&#39;</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">&#39;+&#39;</span><span class="p">)</span>

  806. <span class="gp">&gt;&gt;&gt; </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">&#39;args&#39;</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">&#39;*&#39;</span><span class="p">)</span>

  807. 

  808. <span class="gp">&gt;&gt;&gt; </span><span class="c1"># ambiguous, so parse_args assumes it&#39;s an option</span>

  809. <span class="gp">&gt;&gt;&gt; </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">&#39;-f&#39;</span><span class="p">])</span>

  810. <span class="go">usage: PROG [-h] [-n N [N ...]] [args ...]</span>

  811. <span class="go">PROG: error: unrecognized arguments: -f</span>

  812. 

  813. <span class="gp">&gt;&gt;&gt; </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">&#39;--&#39;</span><span class="p">,</span> <span class="s1">&#39;-f&#39;</span><span class="p">])</span>

  814. <span class="go">Namespace(args=[&#39;-f&#39;], n=None)</span>

  815. 

  816. <span class="gp">&gt;&gt;&gt; </span><span class="c1"># ambiguous, so the -n option greedily accepts arguments</span>

  817. <span class="gp">&gt;&gt;&gt; </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">&#39;-n&#39;</span><span class="p">,</span> <span class="s1">&#39;1&#39;</span><span class="p">,</span> <span class="s1">&#39;2&#39;</span><span class="p">,</span> <span class="s1">&#39;3&#39;</span><span class="p">])</span>

  818. <span class="go">Namespace(args=[], n=[&#39;1&#39;, &#39;2&#39;, &#39;3&#39;])</span>

  819. 

  820. <span class="gp">&gt;&gt;&gt; </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">&#39;-n&#39;</span><span class="p">,</span> <span class="s1">&#39;1&#39;</span><span class="p">,</span> <span class="s1">&#39;--&#39;</span><span class="p">,</span> <span class="s1">&#39;2&#39;</span><span class="p">,</span> <span class="s1">&#39;3&#39;</span><span class="p">])</span>

  821. <span class="go">Namespace(args=[&#39;2&#39;, &#39;3&#39;], n=[&#39;1&#39;])</span>

  822. </pre></div>

  823. </div>

  824. </section>

  825. <section id="conflicting-options">

  826. <h3>Conflicting options<a class="headerlink" href="#conflicting-options" title="Link to this heading">¶</a></h3>

  827. <p>So far, we have been working with two methods of an

  828. <a class="reference internal" href="../library/argparse.html#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">argparse.ArgumentParser</span></code></a> instance. Let’s introduce a third one,

  829. <a class="reference internal" href="../library/argparse.html#argparse.ArgumentParser.add_mutually_exclusive_group" title="argparse.ArgumentParser.add_mutually_exclusive_group"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_mutually_exclusive_group()</span></code></a>. It allows for us to specify options that

  830. conflict with each other. Let’s also change the rest of the program so that

  831. the new functionality makes more sense:

  832. we’ll introduce the <code class="docutils literal notranslate"><span class="pre">--quiet</span></code> option,

  833. which will be the opposite of the <code class="docutils literal notranslate"><span class="pre">--verbose</span></code> one:</p>

  834. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">argparse</span>

  835. 

  836. <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span>

  837. <span class="n">group</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_mutually_exclusive_group</span><span class="p">()</span>

  838. <span class="n">group</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s2">&quot;-v&quot;</span><span class="p">,</span> <span class="s2">&quot;--verbose&quot;</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s2">&quot;store_true&quot;</span><span class="p">)</span>

  839. <span class="n">group</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s2">&quot;-q&quot;</span><span class="p">,</span> <span class="s2">&quot;--quiet&quot;</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s2">&quot;store_true&quot;</span><span class="p">)</span>

  840. <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s2">&quot;x&quot;</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s2">&quot;the base&quot;</span><span class="p">)</span>

  841. <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s2">&quot;y&quot;</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s2">&quot;the exponent&quot;</span><span class="p">)</span>

  842. <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span>

  843. <span class="n">answer</span> <span class="o">=</span> <span class="n">args</span><span class="o">.</span><span class="n">x</span><span class="o">**</span><span class="n">args</span><span class="o">.</span><span class="n">y</span>

  844. 

  845. <span class="k">if</span> <span class="n">args</span><span class="o">.</span><span class="n">quiet</span><span class="p">:</span>

  846.     <span class="nb">print</span><span class="p">(</span><span class="n">answer</span><span class="p">)</span>

  847. <span class="k">elif</span> <span class="n">args</span><span class="o">.</span><span class="n">verbose</span><span class="p">:</span>

  848.     <span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;</span><span class="si">{</span><span class="n">args</span><span class="o">.</span><span class="n">x</span><span class="si">}</span><span class="s2"> to the power </span><span class="si">{</span><span class="n">args</span><span class="o">.</span><span class="n">y</span><span class="si">}</span><span class="s2"> equals </span><span class="si">{</span><span class="n">answer</span><span class="si">}</span><span class="s2">&quot;</span><span class="p">)</span>

  849. <span class="k">else</span><span class="p">:</span>

  850.     <span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;</span><span class="si">{</span><span class="n">args</span><span class="o">.</span><span class="n">x</span><span class="si">}</span><span class="s2">^</span><span class="si">{</span><span class="n">args</span><span class="o">.</span><span class="n">y</span><span class="si">}</span><span class="s2"> == </span><span class="si">{</span><span class="n">answer</span><span class="si">}</span><span class="s2">&quot;</span><span class="p">)</span>

  851. </pre></div>

  852. </div>

  853. <p>Our program is now simpler, and we’ve lost some functionality for the sake of

  854. demonstration. Anyways, here’s the output:</p>

  855. <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span><span class="m">4</span><span class="w"> </span><span class="m">2</span>

  856. <span class="go">4^2 == 16</span>

  857. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span><span class="m">4</span><span class="w"> </span><span class="m">2</span><span class="w"> </span>-q

  858. <span class="go">16</span>

  859. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span><span class="m">4</span><span class="w"> </span><span class="m">2</span><span class="w"> </span>-v

  860. <span class="go">4 to the power 2 equals 16</span>

  861. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span><span class="m">4</span><span class="w"> </span><span class="m">2</span><span class="w"> </span>-vq

  862. <span class="go">usage: prog.py [-h] [-v | -q] x y</span>

  863. <span class="go">prog.py: error: argument -q/--quiet: not allowed with argument -v/--verbose</span>

  864. <span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span><span class="m">4</span><span class="w"> </span><span class="m">2</span><span class="w"> </span>-v<span class="w"> </span>--quiet

  865. <span class="go">usage: prog.py [-h] [-v | -q] x y</span>

  866. <span class="go">prog.py: error: argument -q/--quiet: not allowed with argument -v/--verbose</span>

  867. </pre></div>

  868. </div>

  869. <p>That should be easy to follow. I’ve added that last output so you can see the

  870. sort of flexibility you get, i.e. mixing long form options with short form

  871. ones.</p>

  872. <p>Before we conclude, you probably want to tell your users the main purpose of

  873. your program, just in case they don’t know:</p>

  874. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">argparse</span>

  875. 

  876. <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">description</span><span class="o">=</span><span class="s2">&quot;calculate X to the power of Y&quot;</span><span class="p">)</span>

  877. <span class="n">group</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_mutually_exclusive_group</span><span class="p">()</span>

  878. <span class="n">group</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s2">&quot;-v&quot;</span><span class="p">,</span> <span class="s2">&quot;--verbose&quot;</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s2">&quot;store_true&quot;</span><span class="p">)</span>

  879. <span class="n">group</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s2">&quot;-q&quot;</span><span class="p">,</span> <span class="s2">&quot;--quiet&quot;</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s2">&quot;store_true&quot;</span><span class="p">)</span>

  880. <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s2">&quot;x&quot;</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s2">&quot;the base&quot;</span><span class="p">)</span>

  881. <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s2">&quot;y&quot;</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s2">&quot;the exponent&quot;</span><span class="p">)</span>

  882. <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span>

  883. <span class="n">answer</span> <span class="o">=</span> <span class="n">args</span><span class="o">.</span><span class="n">x</span><span class="o">**</span><span class="n">args</span><span class="o">.</span><span class="n">y</span>

  884. 

  885. <span class="k">if</span> <span class="n">args</span><span class="o">.</span><span class="n">quiet</span><span class="p">:</span>

  886.     <span class="nb">print</span><span class="p">(</span><span class="n">answer</span><span class="p">)</span>

  887. <span class="k">elif</span> <span class="n">args</span><span class="o">.</span><span class="n">verbose</span><span class="p">:</span>

  888.     <span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;</span><span class="si">{</span><span class="n">args</span><span class="o">.</span><span class="n">x</span><span class="si">}</span><span class="s2"> to the power </span><span class="si">{</span><span class="n">args</span><span class="o">.</span><span class="n">y</span><span class="si">}</span><span class="s2"> equals </span><span class="si">{</span><span class="n">answer</span><span class="si">}</span><span class="s2">&quot;</span><span class="p">)</span>

  889. <span class="k">else</span><span class="p">:</span>

  890.     <span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;</span><span class="si">{</span><span class="n">args</span><span class="o">.</span><span class="n">x</span><span class="si">}</span><span class="s2">^</span><span class="si">{</span><span class="n">args</span><span class="o">.</span><span class="n">y</span><span class="si">}</span><span class="s2"> == </span><span class="si">{</span><span class="n">answer</span><span class="si">}</span><span class="s2">&quot;</span><span class="p">)</span>

  891. </pre></div>

  892. </div>

  893. <p>Note that slight difference in the usage text. Note the <code class="docutils literal notranslate"><span class="pre">[-v</span> <span class="pre">|</span> <span class="pre">-q]</span></code>,

  894. which tells us that we can either use <code class="docutils literal notranslate"><span class="pre">-v</span></code> or <code class="docutils literal notranslate"><span class="pre">-q</span></code>,

  895. but not both at the same time:</p>

  896. <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span>--help

  897. <span class="go">usage: prog.py [-h] [-v | -q] x y</span>

  898. 

  899. <span class="go">calculate X to the power of Y</span>

  900. 

  901. <span class="go">positional arguments:</span>

  902. <span class="go">  x              the base</span>

  903. <span class="go">  y              the exponent</span>

  904. 

  905. <span class="go">options:</span>

  906. <span class="go">  -h, --help     show this help message and exit</span>

  907. <span class="go">  -v, --verbose</span>

  908. <span class="go">  -q, --quiet</span>

  909. </pre></div>

  910. </div>

  911. </section>

  912. </section>

  913. <section id="how-to-translate-the-argparse-output">

  914. <h2>How to translate the argparse output<a class="headerlink" href="#how-to-translate-the-argparse-output" title="Link to this heading">¶</a></h2>

  915. <p>The output of the <a class="reference internal" href="../library/argparse.html#module-argparse" title="argparse: Command-line option and argument parsing library."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></a> module such as its help text and error

  916. messages are all made translatable using the <a class="reference internal" href="../library/gettext.html#module-gettext" title="gettext: Multilingual internationalization services."><code class="xref py py-mod docutils literal notranslate"><span class="pre">gettext</span></code></a> module. This

  917. allows applications to easily localize messages produced by

  918. <a class="reference internal" href="../library/argparse.html#module-argparse" title="argparse: Command-line option and argument parsing library."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></a>. See also <a class="reference internal" href="../library/gettext.html#i18n-howto"><span class="std std-ref">Internationalizing your programs and modules</span></a>.</p>

  919. <p>For instance, in this <a class="reference internal" href="../library/argparse.html#module-argparse" title="argparse: Command-line option and argument parsing library."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></a> output:</p>

  920. <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>python<span class="w"> </span>prog.py<span class="w"> </span>--help

  921. <span class="go">usage: prog.py [-h] [-v | -q] x y</span>

  922. 

  923. <span class="go">calculate X to the power of Y</span>

  924. 

  925. <span class="go">positional arguments:</span>

  926. <span class="go">  x              the base</span>

  927. <span class="go">  y              the exponent</span>

  928. 

  929. <span class="go">options:</span>

  930. <span class="go">  -h, --help     show this help message and exit</span>

  931. <span class="go">  -v, --verbose</span>

  932. <span class="go">  -q, --quiet</span>

  933. </pre></div>

  934. </div>

  935. <p>The strings <code class="docutils literal notranslate"><span class="pre">usage:</span></code>, <code class="docutils literal notranslate"><span class="pre">positional</span> <span class="pre">arguments:</span></code>, <code class="docutils literal notranslate"><span class="pre">options:</span></code> and

  936. <code class="docutils literal notranslate"><span class="pre">show</span> <span class="pre">this</span> <span class="pre">help</span> <span class="pre">message</span> <span class="pre">and</span> <span class="pre">exit</span></code> are all translatable.</p>

  937. <p>In order to translate these strings, they must first be extracted

  938. into a <code class="docutils literal notranslate"><span class="pre">.po</span></code> file. For example, using <a class="reference external" href="https://babel.pocoo.org/">Babel</a>,

  939. run this command:</p>

  940. <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>pybabel<span class="w"> </span>extract<span class="w"> </span>-o<span class="w"> </span>messages.po<span class="w"> </span>/usr/lib/python3.12/argparse.py

  941. </pre></div>

  942. </div>

  943. <p>This command will extract all translatable strings from the <a class="reference internal" href="../library/argparse.html#module-argparse" title="argparse: Command-line option and argument parsing library."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></a>

  944. module and output them into a file named <code class="docutils literal notranslate"><span class="pre">messages.po</span></code>. This command assumes

  945. that your Python installation is in <code class="docutils literal notranslate"><span class="pre">/usr/lib</span></code>.</p>

  946. <p>You can find out the location of the <a class="reference internal" href="../library/argparse.html#module-argparse" title="argparse: Command-line option and argument parsing library."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></a> module on your system

  947. using this script:</p>

  948. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">argparse</span>

  949. <span class="nb">print</span><span class="p">(</span><span class="n">argparse</span><span class="o">.</span><span class="vm">__file__</span><span class="p">)</span>

  950. </pre></div>

  951. </div>

  952. <p>Once the messages in the <code class="docutils literal notranslate"><span class="pre">.po</span></code> file are translated and the translations are

  953. installed using <a class="reference internal" href="../library/gettext.html#module-gettext" title="gettext: Multilingual internationalization services."><code class="xref py py-mod docutils literal notranslate"><span class="pre">gettext</span></code></a>, <a class="reference internal" href="../library/argparse.html#module-argparse" title="argparse: Command-line option and argument parsing library."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></a> will be able to display the

  954. translated messages.</p>

  955. <p>To translate your own strings in the <a class="reference internal" href="../library/argparse.html#module-argparse" title="argparse: Command-line option and argument parsing library."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></a> output, use <a class="reference internal" href="../library/gettext.html#module-gettext" title="gettext: Multilingual internationalization services."><code class="xref py py-mod docutils literal notranslate"><span class="pre">gettext</span></code></a>.</p>

  956. </section>

  957. <section id="conclusion">

  958. <h2>Conclusion<a class="headerlink" href="#conclusion" title="Link to this heading">¶</a></h2>

  959. <p>The <a class="reference internal" href="../library/argparse.html#module-argparse" title="argparse: Command-line option and argument parsing library."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></a> module offers a lot more than shown here.

  960. Its docs are quite detailed and thorough, and full of examples.

  961. Having gone through this tutorial, you should easily digest them

  962. without feeling overwhelmed.</p>

  963. </section>

  964. </section>

  965. 

  966. 

  967.             <div class="clearer"></div>

  968.           </div>

  969.         </div>

  970.       </div>

  971.       <div class="sphinxsidebar" role="navigation" aria-label="main navigation">

  972.         <div class="sphinxsidebarwrapper">

  973.   <div>

  974.     <h3><a href="../contents.html">Table of Contents</a></h3>

  975.     <ul>

  976. <li><a class="reference internal" href="#">Argparse Tutorial</a><ul>

  977. <li><a class="reference internal" href="#concepts">Concepts</a></li>

  978. <li><a class="reference internal" href="#the-basics">The basics</a></li>

  979. <li><a class="reference internal" href="#introducing-positional-arguments">Introducing Positional arguments</a></li>

  980. <li><a class="reference internal" href="#introducing-optional-arguments">Introducing Optional arguments</a><ul>

  981. <li><a class="reference internal" href="#short-options">Short options</a></li>

  982. </ul>

  983. </li>

  984. <li><a class="reference internal" href="#combining-positional-and-optional-arguments">Combining Positional and Optional arguments</a></li>

  985. <li><a class="reference internal" href="#getting-a-little-more-advanced">Getting a little more advanced</a><ul>

  986. <li><a class="reference internal" href="#specifying-ambiguous-arguments">Specifying ambiguous arguments</a></li>

  987. <li><a class="reference internal" href="#conflicting-options">Conflicting options</a></li>

  988. </ul>

  989. </li>

  990. <li><a class="reference internal" href="#how-to-translate-the-argparse-output">How to translate the argparse output</a></li>

  991. <li><a class="reference internal" href="#conclusion">Conclusion</a></li>

  992. </ul>

  993. </li>

  994. </ul>

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