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  98. <li><a class="reference internal" href="#"><code class="xref py py-mod docutils literal notranslate"><span class="pre">cgi</span></code> — Common Gateway Interface support</a><ul>

  99. <li><a class="reference internal" href="#introduction">Introduction</a></li>

  100. <li><a class="reference internal" href="#using-the-cgi-module">Using the cgi module</a></li>

  101. <li><a class="reference internal" href="#higher-level-interface">Higher Level Interface</a></li>

  102. <li><a class="reference internal" href="#functions">Functions</a></li>

  103. <li><a class="reference internal" href="#caring-about-security">Caring about security</a></li>

  104. <li><a class="reference internal" href="#installing-your-cgi-script-on-a-unix-system">Installing your CGI script on a Unix system</a></li>

  105. <li><a class="reference internal" href="#testing-your-cgi-script">Testing your CGI script</a></li>

  106. <li><a class="reference internal" href="#debugging-cgi-scripts">Debugging CGI scripts</a></li>

  107. <li><a class="reference internal" href="#common-problems-and-solutions">Common problems and solutions</a></li>

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  200.   <section id="module-cgi">

  201. <span id="cgi-common-gateway-interface-support"></span><h1><a class="reference internal" href="#module-cgi" title="cgi: Helpers for running Python scripts via the Common Gateway Interface. (deprecated)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">cgi</span></code></a> — Common Gateway Interface support<a class="headerlink" href="#module-cgi" title="Link to this heading">¶</a></h1>

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

  203. <div class="deprecated-removed" id="index-0">

  204. <p><span class="versionmodified">Deprecated since version 3.11, will be removed in version 3.13: </span>The <a class="reference internal" href="#module-cgi" title="cgi: Helpers for running Python scripts via the Common Gateway Interface. (deprecated)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">cgi</span></code></a> module is deprecated

  205. (see <span class="target" id="index-1"></span><a class="pep reference external" href="https://peps.python.org/pep-0594/#cgi"><strong>PEP 594</strong></a> for details and alternatives).</p>

  206. <p>The <code class="xref py py-class docutils literal notranslate"><span class="pre">FieldStorage</span></code> class can typically be replaced with

  207. <a class="reference internal" href="urllib.parse.html#urllib.parse.parse_qsl" title="urllib.parse.parse_qsl"><code class="xref py py-func docutils literal notranslate"><span class="pre">urllib.parse.parse_qsl()</span></code></a> for <code class="docutils literal notranslate"><span class="pre">GET</span></code> and <code class="docutils literal notranslate"><span class="pre">HEAD</span></code> requests,

  208. and the <a class="reference internal" href="email.message.html#module-email.message" title="email.message: The base class representing email messages."><code class="xref py py-mod docutils literal notranslate"><span class="pre">email.message</span></code></a> module or

  209. <a class="reference external" href="https://pypi.org/project/multipart/">multipart</a> for <code class="docutils literal notranslate"><span class="pre">POST</span></code> and <code class="docutils literal notranslate"><span class="pre">PUT</span></code>.

  210. Most <a class="reference internal" href="#functions-in-cgi-module"><span class="std std-ref">utility functions</span></a> have replacements.</p>

  211. </div>

  212. <hr class="docutils" />

  213. <p>Support module for Common Gateway Interface (CGI) scripts.</p>

  214. <p>This module defines a number of utilities for use by CGI scripts written in

  215. Python.</p>

  216. <p>The global variable <code class="docutils literal notranslate"><span class="pre">maxlen</span></code> can be set to an integer indicating the maximum

  217. size of a POST request. POST requests larger than this size will result in a

  218. <a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a> being raised during parsing. The default value of this

  219. variable is <code class="docutils literal notranslate"><span class="pre">0</span></code>, meaning the request size is unlimited.</p>

  220. <div class="availability docutils container">

  221. <p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: not Emscripten, not WASI.</p>

  222. <p>This module does not work or is not available on WebAssembly platforms

  223. <code class="docutils literal notranslate"><span class="pre">wasm32-emscripten</span></code> and <code class="docutils literal notranslate"><span class="pre">wasm32-wasi</span></code>. See

  224. <a class="reference internal" href="intro.html#wasm-availability"><span class="std std-ref">WebAssembly platforms</span></a> for more information.</p>

  225. </div>

  226. <section id="introduction">

  227. <h2>Introduction<a class="headerlink" href="#introduction" title="Link to this heading">¶</a></h2>

  228. <p id="cgi-intro">A CGI script is invoked by an HTTP server, usually to process user input

  229. submitted through an HTML <code class="docutils literal notranslate"><span class="pre">&lt;FORM&gt;</span></code> or <code class="docutils literal notranslate"><span class="pre">&lt;ISINDEX&gt;</span></code> element.</p>

  230. <p>Most often, CGI scripts live in the server’s special <code class="file docutils literal notranslate"><span class="pre">cgi-bin</span></code> directory.

  231. The HTTP server places all sorts of information about the request (such as the

  232. client’s hostname, the requested URL, the query string, and lots of other

  233. goodies) in the script’s shell environment, executes the script, and sends the

  234. script’s output back to the client.</p>

  235. <p>The script’s input is connected to the client too, and sometimes the form data

  236. is read this way; at other times the form data is passed via the “query string”

  237. part of the URL.  This module is intended to take care of the different cases

  238. and provide a simpler interface to the Python script.  It also provides a number

  239. of utilities that help in debugging scripts, and the latest addition is support

  240. for file uploads from a form (if your browser supports it).</p>

  241. <p>The output of a CGI script should consist of two sections, separated by a blank

  242. line.  The first section contains a number of headers, telling the client what

  243. kind of data is following.  Python code to generate a minimal header section

  244. looks like this:</p>

  245. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="nb">print</span><span class="p">(</span><span class="s2">&quot;Content-Type: text/html&quot;</span><span class="p">)</span>    <span class="c1"># HTML is following</span>

  246. <span class="nb">print</span><span class="p">()</span>                             <span class="c1"># blank line, end of headers</span>

  247. </pre></div>

  248. </div>

  249. <p>The second section is usually HTML, which allows the client software to display

  250. nicely formatted text with header, in-line images, etc. Here’s Python code that

  251. prints a simple piece of HTML:</p>

  252. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="nb">print</span><span class="p">(</span><span class="s2">&quot;&lt;TITLE&gt;CGI script output&lt;/TITLE&gt;&quot;</span><span class="p">)</span>

  253. <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;&lt;H1&gt;This is my first CGI script&lt;/H1&gt;&quot;</span><span class="p">)</span>

  254. <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;Hello, world!&quot;</span><span class="p">)</span>

  255. </pre></div>

  256. </div>

  257. </section>

  258. <section id="using-the-cgi-module">

  259. <span id="id1"></span><h2>Using the cgi module<a class="headerlink" href="#using-the-cgi-module" title="Link to this heading">¶</a></h2>

  260. <p>Begin by writing <code class="docutils literal notranslate"><span class="pre">import</span> <span class="pre">cgi</span></code>.</p>

  261. <p>When you write a new script, consider adding these lines:</p>

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

  263. <span class="n">cgitb</span><span class="o">.</span><span class="n">enable</span><span class="p">()</span>

  264. </pre></div>

  265. </div>

  266. <p>This activates a special exception handler that will display detailed reports in

  267. the web browser if any errors occur.  If you’d rather not show the guts of your

  268. program to users of your script, you can have the reports saved to files

  269. instead, with code like this:</p>

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

  271. <span class="n">cgitb</span><span class="o">.</span><span class="n">enable</span><span class="p">(</span><span class="n">display</span><span class="o">=</span><span class="mi">0</span><span class="p">,</span> <span class="n">logdir</span><span class="o">=</span><span class="s2">&quot;/path/to/logdir&quot;</span><span class="p">)</span>

  272. </pre></div>

  273. </div>

  274. <p>It’s very helpful to use this feature during script development. The reports

  275. produced by <a class="reference internal" href="cgitb.html#module-cgitb" title="cgitb: Configurable traceback handler for CGI scripts. (deprecated)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">cgitb</span></code></a> provide information that can save you a lot of time in

  276. tracking down bugs.  You can always remove the <code class="docutils literal notranslate"><span class="pre">cgitb</span></code> line later when you

  277. have tested your script and are confident that it works correctly.</p>

  278. <p>To get at submitted form data, use the <code class="xref py py-class docutils literal notranslate"><span class="pre">FieldStorage</span></code> class. If the form

  279. contains non-ASCII characters, use the <em>encoding</em> keyword parameter set to the

  280. value of the encoding defined for the document. It is usually contained in the

  281. META tag in the HEAD section of the HTML document or by the

  282. <em class="mailheader">Content-Type</em> header.  This reads the form contents from the

  283. standard input or the environment (depending on the value of various

  284. environment variables set according to the CGI standard).  Since it may consume

  285. standard input, it should be instantiated only once.</p>

  286. <p>The <code class="xref py py-class docutils literal notranslate"><span class="pre">FieldStorage</span></code> instance can be indexed like a Python dictionary.

  287. It allows membership testing with the <a class="reference internal" href="../reference/expressions.html#in"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">in</span></code></a> operator, and also supports

  288. the standard dictionary method <a class="reference internal" href="stdtypes.html#dict.keys" title="dict.keys"><code class="xref py py-meth docutils literal notranslate"><span class="pre">keys()</span></code></a> and the built-in function

  289. <a class="reference internal" href="functions.html#len" title="len"><code class="xref py py-func docutils literal notranslate"><span class="pre">len()</span></code></a>.  Form fields containing empty strings are ignored and do not appear

  290. in the dictionary; to keep such values, provide a true value for the optional

  291. <em>keep_blank_values</em> keyword parameter when creating the <code class="xref py py-class docutils literal notranslate"><span class="pre">FieldStorage</span></code>

  292. instance.</p>

  293. <p>For instance, the following code (which assumes that the

  294. <em class="mailheader">Content-Type</em> header and blank line have already been printed)

  295. checks that the fields <code class="docutils literal notranslate"><span class="pre">name</span></code> and <code class="docutils literal notranslate"><span class="pre">addr</span></code> are both set to a non-empty

  296. string:</p>

  297. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">form</span> <span class="o">=</span> <span class="n">cgi</span><span class="o">.</span><span class="n">FieldStorage</span><span class="p">()</span>

  298. <span class="k">if</span> <span class="s2">&quot;name&quot;</span> <span class="ow">not</span> <span class="ow">in</span> <span class="n">form</span> <span class="ow">or</span> <span class="s2">&quot;addr&quot;</span> <span class="ow">not</span> <span class="ow">in</span> <span class="n">form</span><span class="p">:</span>

  299.     <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;&lt;H1&gt;Error&lt;/H1&gt;&quot;</span><span class="p">)</span>

  300.     <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;Please fill in the name and addr fields.&quot;</span><span class="p">)</span>

  301.     <span class="k">return</span>

  302. <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;&lt;p&gt;name:&quot;</span><span class="p">,</span> <span class="n">form</span><span class="p">[</span><span class="s2">&quot;name&quot;</span><span class="p">]</span><span class="o">.</span><span class="n">value</span><span class="p">)</span>

  303. <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;&lt;p&gt;addr:&quot;</span><span class="p">,</span> <span class="n">form</span><span class="p">[</span><span class="s2">&quot;addr&quot;</span><span class="p">]</span><span class="o">.</span><span class="n">value</span><span class="p">)</span>

  304. <span class="o">...</span><span class="n">further</span> <span class="n">form</span> <span class="n">processing</span> <span class="n">here</span><span class="o">...</span>

  305. </pre></div>

  306. </div>

  307. <p>Here the fields, accessed through <code class="docutils literal notranslate"><span class="pre">form[key]</span></code>, are themselves instances of

  308. <code class="xref py py-class docutils literal notranslate"><span class="pre">FieldStorage</span></code> (or <code class="xref py py-class docutils literal notranslate"><span class="pre">MiniFieldStorage</span></code>, depending on the form

  309. encoding). The <code class="xref py py-attr docutils literal notranslate"><span class="pre">value</span></code> attribute of the instance yields

  310. the string value of the field.  The <code class="xref py py-meth docutils literal notranslate"><span class="pre">getvalue()</span></code> method

  311. returns this string value directly; it also accepts an optional second argument

  312. as a default to return if the requested key is not present.</p>

  313. <p>If the submitted form data contains more than one field with the same name, the

  314. object retrieved by <code class="docutils literal notranslate"><span class="pre">form[key]</span></code> is not a <code class="xref py py-class docutils literal notranslate"><span class="pre">FieldStorage</span></code> or

  315. <code class="xref py py-class docutils literal notranslate"><span class="pre">MiniFieldStorage</span></code> instance but a list of such instances.  Similarly, in

  316. this situation, <code class="docutils literal notranslate"><span class="pre">form.getvalue(key)</span></code> would return a list of strings. If you

  317. expect this possibility (when your HTML form contains multiple fields with the

  318. same name), use the <a class="reference internal" href="#cgi.FieldStorage.getlist" title="cgi.FieldStorage.getlist"><code class="xref py py-meth docutils literal notranslate"><span class="pre">getlist()</span></code></a> method, which always returns

  319. a list of values (so that you do not need to special-case the single item

  320. case).  For example, this code concatenates any number of username fields,

  321. separated by commas:</p>

  322. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">value</span> <span class="o">=</span> <span class="n">form</span><span class="o">.</span><span class="n">getlist</span><span class="p">(</span><span class="s2">&quot;username&quot;</span><span class="p">)</span>

  323. <span class="n">usernames</span> <span class="o">=</span> <span class="s2">&quot;,&quot;</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="n">value</span><span class="p">)</span>

  324. </pre></div>

  325. </div>

  326. <p>If a field represents an uploaded file, accessing the value via the

  327. <code class="xref py py-attr docutils literal notranslate"><span class="pre">value</span></code> attribute or the <code class="xref py py-meth docutils literal notranslate"><span class="pre">getvalue()</span></code>

  328. method reads the entire file in memory as bytes.  This may not be what you

  329. want.  You can test for an uploaded file by testing either the

  330. <code class="xref py py-attr docutils literal notranslate"><span class="pre">filename</span></code> attribute or the <code class="xref py py-attr docutils literal notranslate"><span class="pre">file</span></code>

  331. attribute.  You can then read the data from the <code class="xref py py-attr docutils literal notranslate"><span class="pre">file</span></code>

  332. attribute before it is automatically closed as part of the garbage collection of

  333. the <code class="xref py py-class docutils literal notranslate"><span class="pre">FieldStorage</span></code> instance

  334. (the <a class="reference internal" href="io.html#io.RawIOBase.read" title="io.RawIOBase.read"><code class="xref py py-func docutils literal notranslate"><span class="pre">read()</span></code></a> and <a class="reference internal" href="io.html#io.IOBase.readline" title="io.IOBase.readline"><code class="xref py py-func docutils literal notranslate"><span class="pre">readline()</span></code></a> methods will

  335. return bytes):</p>

  336. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">fileitem</span> <span class="o">=</span> <span class="n">form</span><span class="p">[</span><span class="s2">&quot;userfile&quot;</span><span class="p">]</span>

  337. <span class="k">if</span> <span class="n">fileitem</span><span class="o">.</span><span class="n">file</span><span class="p">:</span>

  338.     <span class="c1"># It&#39;s an uploaded file; count lines</span>

  339.     <span class="n">linecount</span> <span class="o">=</span> <span class="mi">0</span>

  340.     <span class="k">while</span> <span class="kc">True</span><span class="p">:</span>

  341.         <span class="n">line</span> <span class="o">=</span> <span class="n">fileitem</span><span class="o">.</span><span class="n">file</span><span class="o">.</span><span class="n">readline</span><span class="p">()</span>

  342.         <span class="k">if</span> <span class="ow">not</span> <span class="n">line</span><span class="p">:</span> <span class="k">break</span>

  343.         <span class="n">linecount</span> <span class="o">=</span> <span class="n">linecount</span> <span class="o">+</span> <span class="mi">1</span>

  344. </pre></div>

  345. </div>

  346. <p><code class="xref py py-class docutils literal notranslate"><span class="pre">FieldStorage</span></code> objects also support being used in a <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a>

  347. statement, which will automatically close them when done.</p>

  348. <p>If an error is encountered when obtaining the contents of an uploaded file

  349. (for example, when the user interrupts the form submission by clicking on

  350. a Back or Cancel button) the <code class="xref py py-attr docutils literal notranslate"><span class="pre">done</span></code> attribute of the

  351. object for the field will be set to the value -1.</p>

  352. <p>The file upload draft standard entertains the possibility of uploading multiple

  353. files from one field (using a recursive <em class="mimetype">multipart/*</em> encoding).

  354. When this occurs, the item will be a dictionary-like <code class="xref py py-class docutils literal notranslate"><span class="pre">FieldStorage</span></code> item.

  355. This can be determined by testing its <code class="xref py py-attr docutils literal notranslate"><span class="pre">type</span></code> attribute, which should be

  356. <em class="mimetype">multipart/form-data</em> (or perhaps another MIME type matching

  357. <em class="mimetype">multipart/*</em>).  In this case, it can be iterated over recursively

  358. just like the top-level form object.</p>

  359. <p>When a form is submitted in the “old” format (as the query string or as a single

  360. data part of type <em class="mimetype">application/x-www-form-urlencoded</em>), the items will

  361. actually be instances of the class <code class="xref py py-class docutils literal notranslate"><span class="pre">MiniFieldStorage</span></code>.  In this case, the

  362. <code class="xref py py-attr docutils literal notranslate"><span class="pre">list</span></code>, <code class="xref py py-attr docutils literal notranslate"><span class="pre">file</span></code>, and <code class="xref py py-attr docutils literal notranslate"><span class="pre">filename</span></code> attributes are always <code class="docutils literal notranslate"><span class="pre">None</span></code>.</p>

  363. <p>A form submitted via POST that also has a query string will contain both

  364. <code class="xref py py-class docutils literal notranslate"><span class="pre">FieldStorage</span></code> and <code class="xref py py-class docutils literal notranslate"><span class="pre">MiniFieldStorage</span></code> items.</p>

  365. <div class="versionchanged">

  366. <p><span class="versionmodified changed">Changed in version 3.4: </span>The <code class="xref py py-attr docutils literal notranslate"><span class="pre">file</span></code> attribute is automatically closed upon the

  367. garbage collection of the creating <code class="xref py py-class docutils literal notranslate"><span class="pre">FieldStorage</span></code> instance.</p>

  368. </div>

  369. <div class="versionchanged">

  370. <p><span class="versionmodified changed">Changed in version 3.5: </span>Added support for the context management protocol to the

  371. <code class="xref py py-class docutils literal notranslate"><span class="pre">FieldStorage</span></code> class.</p>

  372. </div>

  373. </section>

  374. <section id="higher-level-interface">

  375. <h2>Higher Level Interface<a class="headerlink" href="#higher-level-interface" title="Link to this heading">¶</a></h2>

  376. <p>The previous section explains how to read CGI form data using the

  377. <code class="xref py py-class docutils literal notranslate"><span class="pre">FieldStorage</span></code> class.  This section describes a higher level interface

  378. which was added to this class to allow one to do it in a more readable and

  379. intuitive way.  The interface doesn’t make the techniques described in previous

  380. sections obsolete — they are still useful to process file uploads efficiently,

  381. for example.</p>

  382. <p>The interface consists of two simple methods. Using the methods you can process

  383. form data in a generic way, without the need to worry whether only one or more

  384. values were posted under one name.</p>

  385. <p>In the previous section, you learned to write following code anytime you

  386. expected a user to post more than one value under one name:</p>

  387. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">item</span> <span class="o">=</span> <span class="n">form</span><span class="o">.</span><span class="n">getvalue</span><span class="p">(</span><span class="s2">&quot;item&quot;</span><span class="p">)</span>

  388. <span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">item</span><span class="p">,</span> <span class="nb">list</span><span class="p">):</span>

  389.     <span class="c1"># The user is requesting more than one item.</span>

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

  391.     <span class="c1"># The user is requesting only one item.</span>

  392. </pre></div>

  393. </div>

  394. <p>This situation is common for example when a form contains a group of multiple

  395. checkboxes with the same name:</p>

  396. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="o">&lt;</span><span class="nb">input</span> <span class="nb">type</span><span class="o">=</span><span class="s2">&quot;checkbox&quot;</span> <span class="n">name</span><span class="o">=</span><span class="s2">&quot;item&quot;</span> <span class="n">value</span><span class="o">=</span><span class="s2">&quot;1&quot;</span> <span class="o">/&gt;</span>

  397. <span class="o">&lt;</span><span class="nb">input</span> <span class="nb">type</span><span class="o">=</span><span class="s2">&quot;checkbox&quot;</span> <span class="n">name</span><span class="o">=</span><span class="s2">&quot;item&quot;</span> <span class="n">value</span><span class="o">=</span><span class="s2">&quot;2&quot;</span> <span class="o">/&gt;</span>

  398. </pre></div>

  399. </div>

  400. <p>In most situations, however, there’s only one form control with a particular

  401. name in a form and then you expect and need only one value associated with this

  402. name.  So you write a script containing for example this code:</p>

  403. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">user</span> <span class="o">=</span> <span class="n">form</span><span class="o">.</span><span class="n">getvalue</span><span class="p">(</span><span class="s2">&quot;user&quot;</span><span class="p">)</span><span class="o">.</span><span class="n">upper</span><span class="p">()</span>

  404. </pre></div>

  405. </div>

  406. <p>The problem with the code is that you should never expect that a client will

  407. provide valid input to your scripts.  For example, if a curious user appends

  408. another <code class="docutils literal notranslate"><span class="pre">user=foo</span></code> pair to the query string, then the script would crash,

  409. because in this situation the <code class="docutils literal notranslate"><span class="pre">getvalue(&quot;user&quot;)</span></code> method call returns a list

  410. instead of a string.  Calling the <a class="reference internal" href="stdtypes.html#str.upper" title="str.upper"><code class="xref py py-meth docutils literal notranslate"><span class="pre">upper()</span></code></a> method on a list is not valid

  411. (since lists do not have a method of this name) and results in an

  412. <a class="reference internal" href="exceptions.html#AttributeError" title="AttributeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AttributeError</span></code></a> exception.</p>

  413. <p>Therefore, the appropriate way to read form data values was to always use the

  414. code which checks whether the obtained value is a single value or a list of

  415. values.  That’s annoying and leads to less readable scripts.</p>

  416. <p>A more convenient approach is to use the methods <a class="reference internal" href="#cgi.FieldStorage.getfirst" title="cgi.FieldStorage.getfirst"><code class="xref py py-meth docutils literal notranslate"><span class="pre">getfirst()</span></code></a>

  417. and <a class="reference internal" href="#cgi.FieldStorage.getlist" title="cgi.FieldStorage.getlist"><code class="xref py py-meth docutils literal notranslate"><span class="pre">getlist()</span></code></a> provided by this higher level interface.</p>

  418. <dl class="py method">

  419. <dt class="sig sig-object py" id="cgi.FieldStorage.getfirst">

  420. <span class="sig-prename descclassname"><span class="pre">FieldStorage.</span></span><span class="sig-name descname"><span class="pre">getfirst</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">name</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">default</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">None</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#cgi.FieldStorage.getfirst" title="Link to this definition">¶</a></dt>

  421. <dd><p>This method always returns only one value associated with form field <em>name</em>.

  422. The method returns only the first value in case that more values were posted

  423. under such name.  Please note that the order in which the values are received

  424. may vary from browser to browser and should not be counted on. <a class="footnote-reference brackets" href="#id3" id="id2" role="doc-noteref"><span class="fn-bracket">[</span>1<span class="fn-bracket">]</span></a>  If no such

  425. form field or value exists then the method returns the value specified by the

  426. optional parameter <em>default</em>.  This parameter defaults to <code class="docutils literal notranslate"><span class="pre">None</span></code> if not

  427. specified.</p>

  428. </dd></dl>

  429. 

  430. <dl class="py method">

  431. <dt class="sig sig-object py" id="cgi.FieldStorage.getlist">

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

  433. <dd><p>This method always returns a list of values associated with form field <em>name</em>.

  434. The method returns an empty list if no such form field or value exists for

  435. <em>name</em>.  It returns a list consisting of one item if only one such value exists.</p>

  436. </dd></dl>

  437. 

  438. <p>Using these methods you can write nice compact code:</p>

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

  440. <span class="n">form</span> <span class="o">=</span> <span class="n">cgi</span><span class="o">.</span><span class="n">FieldStorage</span><span class="p">()</span>

  441. <span class="n">user</span> <span class="o">=</span> <span class="n">form</span><span class="o">.</span><span class="n">getfirst</span><span class="p">(</span><span class="s2">&quot;user&quot;</span><span class="p">,</span> <span class="s2">&quot;&quot;</span><span class="p">)</span><span class="o">.</span><span class="n">upper</span><span class="p">()</span>    <span class="c1"># This way it&#39;s safe.</span>

  442. <span class="k">for</span> <span class="n">item</span> <span class="ow">in</span> <span class="n">form</span><span class="o">.</span><span class="n">getlist</span><span class="p">(</span><span class="s2">&quot;item&quot;</span><span class="p">):</span>

  443.     <span class="n">do_something</span><span class="p">(</span><span class="n">item</span><span class="p">)</span>

  444. </pre></div>

  445. </div>

  446. </section>

  447. <section id="functions">

  448. <span id="functions-in-cgi-module"></span><h2>Functions<a class="headerlink" href="#functions" title="Link to this heading">¶</a></h2>

  449. <p>These are useful if you want more control, or if you want to employ some of the

  450. algorithms implemented in this module in other circumstances.</p>

  451. <dl class="py function">

  452. <dt class="sig sig-object py" id="cgi.parse">

  453. <span class="sig-prename descclassname"><span class="pre">cgi.</span></span><span class="sig-name descname"><span class="pre">parse</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">fp</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">None</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">environ</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">os.environ</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">keep_blank_values</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">False</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">strict_parsing</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">False</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">separator</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">'&amp;'</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#cgi.parse" title="Link to this definition">¶</a></dt>

  454. <dd><p>Parse a query in the environment or from a file (the file defaults to

  455. <code class="docutils literal notranslate"><span class="pre">sys.stdin</span></code>).  The <em>keep_blank_values</em>, <em>strict_parsing</em> and <em>separator</em> parameters are

  456. passed to <a class="reference internal" href="urllib.parse.html#urllib.parse.parse_qs" title="urllib.parse.parse_qs"><code class="xref py py-func docutils literal notranslate"><span class="pre">urllib.parse.parse_qs()</span></code></a> unchanged.</p>

  457. <div class="deprecated-removed">

  458. <p><span class="versionmodified">Deprecated since version 3.11, will be removed in version 3.13: </span>This function, like the rest of the <a class="reference internal" href="#module-cgi" title="cgi: Helpers for running Python scripts via the Common Gateway Interface. (deprecated)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">cgi</span></code></a> module, is deprecated.

  459. It can be replaced by calling <a class="reference internal" href="urllib.parse.html#urllib.parse.parse_qs" title="urllib.parse.parse_qs"><code class="xref py py-func docutils literal notranslate"><span class="pre">urllib.parse.parse_qs()</span></code></a> directly

  460. on the desired query string (except for <code class="docutils literal notranslate"><span class="pre">multipart/form-data</span></code> input,

  461. which can be handled as described for <a class="reference internal" href="#cgi.parse_multipart" title="cgi.parse_multipart"><code class="xref py py-func docutils literal notranslate"><span class="pre">parse_multipart()</span></code></a>).</p>

  462. </div>

  463. </dd></dl>

  464. 

  465. <dl class="py function">

  466. <dt class="sig sig-object py" id="cgi.parse_multipart">

  467. <span class="sig-prename descclassname"><span class="pre">cgi.</span></span><span class="sig-name descname"><span class="pre">parse_multipart</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">fp</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">pdict</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">encoding</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">'utf-8'</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">errors</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">'replace'</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">separator</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">'&amp;'</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#cgi.parse_multipart" title="Link to this definition">¶</a></dt>

  468. <dd><p>Parse input of type <em class="mimetype">multipart/form-data</em> (for  file uploads).

  469. Arguments are <em>fp</em> for the input file, <em>pdict</em> for a dictionary containing

  470. other parameters in the <em class="mailheader">Content-Type</em> header, and <em>encoding</em>,

  471. the request encoding.</p>

  472. <p>Returns a dictionary just like <a class="reference internal" href="urllib.parse.html#urllib.parse.parse_qs" title="urllib.parse.parse_qs"><code class="xref py py-func docutils literal notranslate"><span class="pre">urllib.parse.parse_qs()</span></code></a>: keys are the

  473. field names, each value is a list of values for that field. For non-file

  474. fields, the value is a list of strings.</p>

  475. <p>This is easy to use but not much good if you are expecting megabytes to be

  476. uploaded — in that case, use the <code class="xref py py-class docutils literal notranslate"><span class="pre">FieldStorage</span></code> class instead

  477. which is much more flexible.</p>

  478. <div class="versionchanged">

  479. <p><span class="versionmodified changed">Changed in version 3.7: </span>Added the <em>encoding</em> and <em>errors</em> parameters.  For non-file fields, the

  480. value is now a list of strings, not bytes.</p>

  481. </div>

  482. <div class="versionchanged">

  483. <p><span class="versionmodified changed">Changed in version 3.10: </span>Added the <em>separator</em> parameter.</p>

  484. </div>

  485. <div class="deprecated-removed">

  486. <p><span class="versionmodified">Deprecated since version 3.11, will be removed in version 3.13: </span>This function, like the rest of the <a class="reference internal" href="#module-cgi" title="cgi: Helpers for running Python scripts via the Common Gateway Interface. (deprecated)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">cgi</span></code></a> module, is deprecated.

  487. It can be replaced with the functionality in the <a class="reference internal" href="email.html#module-email" title="email: Package supporting the parsing, manipulating, and generating email messages."><code class="xref py py-mod docutils literal notranslate"><span class="pre">email</span></code></a> package

  488. (e.g. <a class="reference internal" href="email.message.html#email.message.EmailMessage" title="email.message.EmailMessage"><code class="xref py py-class docutils literal notranslate"><span class="pre">email.message.EmailMessage</span></code></a>/<a class="reference internal" href="email.compat32-message.html#email.message.Message" title="email.message.Message"><code class="xref py py-class docutils literal notranslate"><span class="pre">email.message.Message</span></code></a>)

  489. which implements the same MIME RFCs, or with the

  490. <a class="reference external" href="https://pypi.org/project/multipart/">multipart</a> PyPI project.</p>

  491. </div>

  492. </dd></dl>

  493. 

  494. <dl class="py function">

  495. <dt class="sig sig-object py" id="cgi.parse_header">

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

  497. <dd><p>Parse a MIME header (such as <em class="mailheader">Content-Type</em>) into a main value and a

  498. dictionary of parameters.</p>

  499. <div class="deprecated-removed">

  500. <p><span class="versionmodified">Deprecated since version 3.11, will be removed in version 3.13: </span>This function, like the rest of the <a class="reference internal" href="#module-cgi" title="cgi: Helpers for running Python scripts via the Common Gateway Interface. (deprecated)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">cgi</span></code></a> module, is deprecated.

  501. It can be replaced with the functionality in the <a class="reference internal" href="email.html#module-email" title="email: Package supporting the parsing, manipulating, and generating email messages."><code class="xref py py-mod docutils literal notranslate"><span class="pre">email</span></code></a> package,

  502. which implements the same MIME RFCs.</p>

  503. <p>For example, with <a class="reference internal" href="email.message.html#email.message.EmailMessage" title="email.message.EmailMessage"><code class="xref py py-class docutils literal notranslate"><span class="pre">email.message.EmailMessage</span></code></a>:</p>

  504. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">email.message</span> <span class="kn">import</span> <span class="n">EmailMessage</span>

  505. <span class="n">msg</span> <span class="o">=</span> <span class="n">EmailMessage</span><span class="p">()</span>

  506. <span class="n">msg</span><span class="p">[</span><span class="s1">&#39;content-type&#39;</span><span class="p">]</span> <span class="o">=</span> <span class="s1">&#39;application/json; charset=&quot;utf8&quot;&#39;</span>

  507. <span class="n">main</span><span class="p">,</span> <span class="n">params</span> <span class="o">=</span> <span class="n">msg</span><span class="o">.</span><span class="n">get_content_type</span><span class="p">(),</span> <span class="n">msg</span><span class="p">[</span><span class="s1">&#39;content-type&#39;</span><span class="p">]</span><span class="o">.</span><span class="n">params</span>

  508. </pre></div>

  509. </div>

  510. </div>

  511. </dd></dl>

  512. 

  513. <dl class="py function">

  514. <dt class="sig sig-object py" id="cgi.test">

  515. <span class="sig-prename descclassname"><span class="pre">cgi.</span></span><span class="sig-name descname"><span class="pre">test</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#cgi.test" title="Link to this definition">¶</a></dt>

  516. <dd><p>Robust test CGI script, usable as main program. Writes minimal HTTP headers and

  517. formats all information provided to the script in HTML format.</p>

  518. </dd></dl>

  519. 

  520. <dl class="py function">

  521. <dt class="sig sig-object py" id="cgi.print_environ">

  522. <span class="sig-prename descclassname"><span class="pre">cgi.</span></span><span class="sig-name descname"><span class="pre">print_environ</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#cgi.print_environ" title="Link to this definition">¶</a></dt>

  523. <dd><p>Format the shell environment in HTML.</p>

  524. </dd></dl>

  525. 

  526. <dl class="py function">

  527. <dt class="sig sig-object py" id="cgi.print_form">

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

  529. <dd><p>Format a form in HTML.</p>

  530. </dd></dl>

  531. 

  532. <dl class="py function">

  533. <dt class="sig sig-object py" id="cgi.print_directory">

  534. <span class="sig-prename descclassname"><span class="pre">cgi.</span></span><span class="sig-name descname"><span class="pre">print_directory</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#cgi.print_directory" title="Link to this definition">¶</a></dt>

  535. <dd><p>Format the current directory in HTML.</p>

  536. </dd></dl>

  537. 

  538. <dl class="py function">

  539. <dt class="sig sig-object py" id="cgi.print_environ_usage">

  540. <span class="sig-prename descclassname"><span class="pre">cgi.</span></span><span class="sig-name descname"><span class="pre">print_environ_usage</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#cgi.print_environ_usage" title="Link to this definition">¶</a></dt>

  541. <dd><p>Print a list of useful (used by CGI) environment variables in HTML.</p>

  542. </dd></dl>

  543. 

  544. </section>

  545. <section id="caring-about-security">

  546. <span id="cgi-security"></span><h2>Caring about security<a class="headerlink" href="#caring-about-security" title="Link to this heading">¶</a></h2>

  547. <p id="index-2">There’s one important rule: if you invoke an external program (via

  548. <a class="reference internal" href="os.html#os.system" title="os.system"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.system()</span></code></a>, <a class="reference internal" href="os.html#os.popen" title="os.popen"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.popen()</span></code></a> or other functions with similar

  549. functionality), make very sure you don’t pass arbitrary strings received from

  550. the client to the shell.  This is a well-known security hole whereby clever

  551. hackers anywhere on the web can exploit a gullible CGI script to invoke

  552. arbitrary shell commands.  Even parts of the URL or field names cannot be

  553. trusted, since the request doesn’t have to come from your form!</p>

  554. <p>To be on the safe side, if you must pass a string gotten from a form to a shell

  555. command, you should make sure the string contains only alphanumeric characters,

  556. dashes, underscores, and periods.</p>

  557. </section>

  558. <section id="installing-your-cgi-script-on-a-unix-system">

  559. <h2>Installing your CGI script on a Unix system<a class="headerlink" href="#installing-your-cgi-script-on-a-unix-system" title="Link to this heading">¶</a></h2>

  560. <p>Read the documentation for your HTTP server and check with your local system

  561. administrator to find the directory where CGI scripts should be installed;

  562. usually this is in a directory <code class="file docutils literal notranslate"><span class="pre">cgi-bin</span></code> in the server tree.</p>

  563. <p>Make sure that your script is readable and executable by “others”; the Unix file

  564. mode should be <code class="docutils literal notranslate"><span class="pre">0o755</span></code> octal (use <code class="docutils literal notranslate"><span class="pre">chmod</span> <span class="pre">0755</span> <span class="pre">filename</span></code>).  Make sure that the

  565. first line of the script contains <code class="docutils literal notranslate"><span class="pre">#!</span></code> starting in column 1 followed by the

  566. pathname of the Python interpreter, for instance:</p>

  567. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="ch">#!/usr/local/bin/python</span>

  568. </pre></div>

  569. </div>

  570. <p>Make sure the Python interpreter exists and is executable by “others”.</p>

  571. <p>Make sure that any files your script needs to read or write are readable or

  572. writable, respectively, by “others” — their mode should be <code class="docutils literal notranslate"><span class="pre">0o644</span></code> for

  573. readable and <code class="docutils literal notranslate"><span class="pre">0o666</span></code> for writable.  This is because, for security reasons, the

  574. HTTP server executes your script as user “nobody”, without any special

  575. privileges.  It can only read (write, execute) files that everybody can read

  576. (write, execute).  The current directory at execution time is also different (it

  577. is usually the server’s cgi-bin directory) and the set of environment variables

  578. is also different from what you get when you log in.  In particular, don’t count

  579. on the shell’s search path for executables (<span class="target" id="index-3"></span><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PATH</span></code>) or the Python module

  580. search path (<span class="target" id="index-4"></span><a class="reference internal" href="../using/cmdline.html#envvar-PYTHONPATH"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONPATH</span></code></a>) to be set to anything interesting.</p>

  581. <p>If you need to load modules from a directory which is not on Python’s default

  582. module search path, you can change the path in your script, before importing

  583. other modules.  For example:</p>

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

  585. <span class="n">sys</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">insert</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="s2">&quot;/usr/home/joe/lib/python&quot;</span><span class="p">)</span>

  586. <span class="n">sys</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">insert</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="s2">&quot;/usr/local/lib/python&quot;</span><span class="p">)</span>

  587. </pre></div>

  588. </div>

  589. <p>(This way, the directory inserted last will be searched first!)</p>

  590. <p>Instructions for non-Unix systems will vary; check your HTTP server’s

  591. documentation (it will usually have a section on CGI scripts).</p>

  592. </section>

  593. <section id="testing-your-cgi-script">

  594. <h2>Testing your CGI script<a class="headerlink" href="#testing-your-cgi-script" title="Link to this heading">¶</a></h2>

  595. <p>Unfortunately, a CGI script will generally not run when you try it from the

  596. command line, and a script that works perfectly from the command line may fail

  597. mysteriously when run from the server.  There’s one reason why you should still

  598. test your script from the command line: if it contains a syntax error, the

  599. Python interpreter won’t execute it at all, and the HTTP server will most likely

  600. send a cryptic error to the client.</p>

  601. <p>Assuming your script has no syntax errors, yet it does not work, you have no

  602. choice but to read the next section.</p>

  603. </section>

  604. <section id="debugging-cgi-scripts">

  605. <h2>Debugging CGI scripts<a class="headerlink" href="#debugging-cgi-scripts" title="Link to this heading">¶</a></h2>

  606. <p id="index-5">First of all, check for trivial installation errors — reading the section

  607. above on installing your CGI script carefully can save you a lot of time.  If

  608. you wonder whether you have understood the installation procedure correctly, try

  609. installing a copy of this module file (<code class="file docutils literal notranslate"><span class="pre">cgi.py</span></code>) as a CGI script.  When

  610. invoked as a script, the file will dump its environment and the contents of the

  611. form in HTML format. Give it the right mode etc., and send it a request.  If it’s

  612. installed in the standard <code class="file docutils literal notranslate"><span class="pre">cgi-bin</span></code> directory, it should be possible to

  613. send it a request by entering a URL into your browser of the form:</p>

  614. <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>http://yourhostname/cgi-bin/cgi.py?name=Joe+Blow&amp;addr=At+Home

  615. </pre></div>

  616. </div>

  617. <p>If this gives an error of type 404, the server cannot find the script – perhaps

  618. you need to install it in a different directory.  If it gives another error,

  619. there’s an installation problem that you should fix before trying to go any

  620. further.  If you get a nicely formatted listing of the environment and form

  621. content (in this example, the fields should be listed as “addr” with value “At

  622. Home” and “name” with value “Joe Blow”), the <code class="file docutils literal notranslate"><span class="pre">cgi.py</span></code> script has been

  623. installed correctly.  If you follow the same procedure for your own script, you

  624. should now be able to debug it.</p>

  625. <p>The next step could be to call the <a class="reference internal" href="#module-cgi" title="cgi: Helpers for running Python scripts via the Common Gateway Interface. (deprecated)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">cgi</span></code></a> module’s <a class="reference internal" href="test.html#module-test" title="test: Regression tests package containing the testing suite for Python."><code class="xref py py-func docutils literal notranslate"><span class="pre">test()</span></code></a> function

  626. from your script: replace its main code with the single statement</p>

  627. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">cgi</span><span class="o">.</span><span class="n">test</span><span class="p">()</span>

  628. </pre></div>

  629. </div>

  630. <p>This should produce the same results as those gotten from installing the

  631. <code class="file docutils literal notranslate"><span class="pre">cgi.py</span></code> file itself.</p>

  632. <p>When an ordinary Python script raises an unhandled exception (for whatever

  633. reason: of a typo in a module name, a file that can’t be opened, etc.), the

  634. Python interpreter prints a nice traceback and exits.  While the Python

  635. interpreter will still do this when your CGI script raises an exception, most

  636. likely the traceback will end up in one of the HTTP server’s log files, or be

  637. discarded altogether.</p>

  638. <p>Fortunately, once you have managed to get your script to execute <em>some</em> code,

  639. you can easily send tracebacks to the web browser using the <a class="reference internal" href="cgitb.html#module-cgitb" title="cgitb: Configurable traceback handler for CGI scripts. (deprecated)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">cgitb</span></code></a> module.

  640. If you haven’t done so already, just add the lines:</p>

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

  642. <span class="n">cgitb</span><span class="o">.</span><span class="n">enable</span><span class="p">()</span>

  643. </pre></div>

  644. </div>

  645. <p>to the top of your script.  Then try running it again; when a problem occurs,

  646. you should see a detailed report that will likely make apparent the cause of the

  647. crash.</p>

  648. <p>If you suspect that there may be a problem in importing the <a class="reference internal" href="cgitb.html#module-cgitb" title="cgitb: Configurable traceback handler for CGI scripts. (deprecated)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">cgitb</span></code></a> module,

  649. you can use an even more robust approach (which only uses built-in modules):</p>

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

  651. <span class="n">sys</span><span class="o">.</span><span class="n">stderr</span> <span class="o">=</span> <span class="n">sys</span><span class="o">.</span><span class="n">stdout</span>

  652. <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;Content-Type: text/plain&quot;</span><span class="p">)</span>

  653. <span class="nb">print</span><span class="p">()</span>

  654. <span class="o">...</span><span class="n">your</span> <span class="n">code</span> <span class="n">here</span><span class="o">...</span>

  655. </pre></div>

  656. </div>

  657. <p>This relies on the Python interpreter to print the traceback.  The content type

  658. of the output is set to plain text, which disables all HTML processing.  If your

  659. script works, the raw HTML will be displayed by your client.  If it raises an

  660. exception, most likely after the first two lines have been printed, a traceback

  661. will be displayed. Because no HTML interpretation is going on, the traceback

  662. will be readable.</p>

  663. </section>

  664. <section id="common-problems-and-solutions">

  665. <h2>Common problems and solutions<a class="headerlink" href="#common-problems-and-solutions" title="Link to this heading">¶</a></h2>

  666. <ul class="simple">

  667. <li><p>Most HTTP servers buffer the output from CGI scripts until the script is

  668. completed.  This means that it is not possible to display a progress report on

  669. the client’s display while the script is running.</p></li>

  670. <li><p>Check the installation instructions above.</p></li>

  671. <li><p>Check the HTTP server’s log files.  (<code class="docutils literal notranslate"><span class="pre">tail</span> <span class="pre">-f</span> <span class="pre">logfile</span></code> in a separate window

  672. may be useful!)</p></li>

  673. <li><p>Always check a script for syntax errors first, by doing something like

  674. <code class="docutils literal notranslate"><span class="pre">python</span> <span class="pre">script.py</span></code>.</p></li>

  675. <li><p>If your script does not have any syntax errors, try adding <code class="docutils literal notranslate"><span class="pre">import</span> <span class="pre">cgitb;</span>

  676. <span class="pre">cgitb.enable()</span></code> to the top of the script.</p></li>

  677. <li><p>When invoking external programs, make sure they can be found. Usually, this

  678. means using absolute path names — <span class="target" id="index-6"></span><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PATH</span></code> is usually not set to a very

  679. useful value in a CGI script.</p></li>

  680. <li><p>When reading or writing external files, make sure they can be read or written

  681. by the userid under which your CGI script will be running: this is typically the

  682. userid under which the web server is running, or some explicitly specified

  683. userid for a web server’s <code class="docutils literal notranslate"><span class="pre">suexec</span></code> feature.</p></li>

  684. <li><p>Don’t try to give a CGI script a set-uid mode.  This doesn’t work on most

  685. systems, and is a security liability as well.</p></li>

  686. </ul>

  687. <p class="rubric">Footnotes</p>

  688. <aside class="footnote-list brackets">

  689. <aside class="footnote brackets" id="id3" role="doc-footnote">

  690. <span class="label"><span class="fn-bracket">[</span><a role="doc-backlink" href="#id2">1</a><span class="fn-bracket">]</span></span>

  691. <p>Note that some recent versions of the HTML specification do state what

  692. order the field values should be supplied in, but knowing whether a request

  693. was received from a conforming browser, or even from a browser at all, is

  694. tedious and error-prone.</p>

  695. </aside>

  696. </aside>

  697. </section>

  698. </section>

  699. 

  700. 

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  707.   <div>

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

  709.     <ul>

  710. <li><a class="reference internal" href="#"><code class="xref py py-mod docutils literal notranslate"><span class="pre">cgi</span></code> — Common Gateway Interface support</a><ul>

  711. <li><a class="reference internal" href="#introduction">Introduction</a></li>

  712. <li><a class="reference internal" href="#using-the-cgi-module">Using the cgi module</a></li>

  713. <li><a class="reference internal" href="#higher-level-interface">Higher Level Interface</a></li>

  714. <li><a class="reference internal" href="#functions">Functions</a></li>

  715. <li><a class="reference internal" href="#caring-about-security">Caring about security</a></li>

  716. <li><a class="reference internal" href="#installing-your-cgi-script-on-a-unix-system">Installing your CGI script on a Unix system</a></li>

  717. <li><a class="reference internal" href="#testing-your-cgi-script">Testing your CGI script</a></li>

  718. <li><a class="reference internal" href="#debugging-cgi-scripts">Debugging CGI scripts</a></li>

  719. <li><a class="reference internal" href="#common-problems-and-solutions">Common problems and solutions</a></li>

  720. </ul>

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