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190 lines
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HTML
<!DOCTYPE html>
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<li class="nav-item nav-item-this"><a href="">Development Server</a></li>
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<section id="development-server">
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<h1>Development Server<a class="headerlink" href="#development-server" title="Link to this heading">¶</a></h1>
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<p>Flask provides a <code class="docutils literal notranslate"><span class="pre">run</span></code> command to run the application with a development server. In
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debug mode, this server provides an interactive debugger and will reload when code is
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changed.</p>
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<div class="admonition warning">
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<p class="admonition-title">Warning</p>
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<p>Do not use the development server when deploying to production. It
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is intended for use only during local development. It is not
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designed to be particularly efficient, stable, or secure.</p>
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<p>See <a class="reference internal" href="deploying/index.html"><span class="doc">Deploying to Production</span></a> for deployment options.</p>
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</div>
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<section id="command-line">
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<h2>Command Line<a class="headerlink" href="#command-line" title="Link to this heading">¶</a></h2>
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<p>The <code class="docutils literal notranslate"><span class="pre">flask</span> <span class="pre">run</span></code> CLI command is the recommended way to run the development server. Use
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the <code class="docutils literal notranslate"><span class="pre">--app</span></code> option to point to your application, and the <code class="docutils literal notranslate"><span class="pre">--debug</span></code> option to enable
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debug mode.</p>
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<div class="highlight-text notranslate"><div class="highlight"><pre><span></span>$ flask --app hello run --debug
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</pre></div>
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</div>
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<p>This enables debug mode, including the interactive debugger and reloader, and then
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starts the server on <a class="reference external" href="http://localhost:5000/">http://localhost:5000/</a>. Use <code class="docutils literal notranslate"><span class="pre">flask</span> <span class="pre">run</span> <span class="pre">--help</span></code> to see the
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available options, and <a class="reference internal" href="cli.html"><span class="doc">Command Line Interface</span></a> for detailed instructions about configuring and using
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the CLI.</p>
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<section id="address-already-in-use">
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<span id="id1"></span><h3>Address already in use<a class="headerlink" href="#address-already-in-use" title="Link to this heading">¶</a></h3>
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<p>If another program is already using port 5000, you’ll see an <code class="docutils literal notranslate"><span class="pre">OSError</span></code>
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when the server tries to start. It may have one of the following
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messages:</p>
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<ul class="simple">
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<li><p><code class="docutils literal notranslate"><span class="pre">OSError:</span> <span class="pre">[Errno</span> <span class="pre">98]</span> <span class="pre">Address</span> <span class="pre">already</span> <span class="pre">in</span> <span class="pre">use</span></code></p></li>
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<li><p><code class="docutils literal notranslate"><span class="pre">OSError:</span> <span class="pre">[WinError</span> <span class="pre">10013]</span> <span class="pre">An</span> <span class="pre">attempt</span> <span class="pre">was</span> <span class="pre">made</span> <span class="pre">to</span> <span class="pre">access</span> <span class="pre">a</span> <span class="pre">socket</span>
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<span class="pre">in</span> <span class="pre">a</span> <span class="pre">way</span> <span class="pre">forbidden</span> <span class="pre">by</span> <span class="pre">its</span> <span class="pre">access</span> <span class="pre">permissions</span></code></p></li>
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</ul>
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<p>Either identify and stop the other program, or use
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<code class="docutils literal notranslate"><span class="pre">flask</span> <span class="pre">run</span> <span class="pre">--port</span> <span class="pre">5001</span></code> to pick a different port.</p>
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<p>You can use <code class="docutils literal notranslate"><span class="pre">netstat</span></code> or <code class="docutils literal notranslate"><span class="pre">lsof</span></code> to identify what process id is using
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a port, then use other operating system tools stop that process. The
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following example shows that process id 6847 is using port 5000.</p>
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<div class="sphinx-tabs docutils container">
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<div aria-label="Tabbed content" class="closeable" role="tablist"><button aria-controls="panel-0-0-0" aria-selected="true" class="sphinx-tabs-tab" id="tab-0-0-0" name="0-0" role="tab" tabindex="0"><code class="docutils literal notranslate"><span class="pre">netstat</span></code> (Linux)</button><button aria-controls="panel-0-0-1" aria-selected="false" class="sphinx-tabs-tab" id="tab-0-0-1" name="0-1" role="tab" tabindex="-1"><code class="docutils literal notranslate"><span class="pre">lsof</span></code> (macOS / Linux)</button><button aria-controls="panel-0-0-2" aria-selected="false" class="sphinx-tabs-tab" id="tab-0-0-2" name="0-2" role="tab" tabindex="-1"><code class="docutils literal notranslate"><span class="pre">netstat</span></code> (Windows)</button></div><div aria-labelledby="tab-0-0-0" class="sphinx-tabs-panel" id="panel-0-0-0" name="0-0" role="tabpanel" tabindex="0"><div class="highlight-text notranslate"><div class="highlight"><pre><span></span>$ netstat -nlp | grep 5000
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tcp 0 0 127.0.0.1:5000 0.0.0.0:* LISTEN 6847/python
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</pre></div>
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</div>
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</div><div aria-labelledby="tab-0-0-1" class="sphinx-tabs-panel" hidden="true" id="panel-0-0-1" name="0-1" role="tabpanel" tabindex="0"><div class="highlight-text notranslate"><div class="highlight"><pre><span></span>$ lsof -P -i :5000
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Python 6847 IPv4 TCP localhost:5000 (LISTEN)
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</pre></div>
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</div>
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</div><div aria-labelledby="tab-0-0-2" class="sphinx-tabs-panel" hidden="true" id="panel-0-0-2" name="0-2" role="tabpanel" tabindex="0"><div class="highlight-text notranslate"><div class="highlight"><pre><span></span>> netstat -ano | findstr 5000
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TCP 127.0.0.1:5000 0.0.0.0:0 LISTENING 6847
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</pre></div>
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</div>
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</div></div>
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<p>macOS Monterey and later automatically starts a service that uses port
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5000. You can choose to disable this service instead of using a different port by
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searching for “AirPlay Receiver” in System Preferences and toggling it off.</p>
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</section>
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<section id="deferred-errors-on-reload">
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<h3>Deferred Errors on Reload<a class="headerlink" href="#deferred-errors-on-reload" title="Link to this heading">¶</a></h3>
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<p>When using the <code class="docutils literal notranslate"><span class="pre">flask</span> <span class="pre">run</span></code> command with the reloader, the server will
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continue to run even if you introduce syntax errors or other
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initialization errors into the code. Accessing the site will show the
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interactive debugger for the error, rather than crashing the server.</p>
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<p>If a syntax error is already present when calling <code class="docutils literal notranslate"><span class="pre">flask</span> <span class="pre">run</span></code>, it will
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fail immediately and show the traceback rather than waiting until the
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site is accessed. This is intended to make errors more visible initially
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while still allowing the server to handle errors on reload.</p>
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</section>
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</section>
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<section id="in-code">
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<h2>In Code<a class="headerlink" href="#in-code" title="Link to this heading">¶</a></h2>
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<p>The development server can also be started from Python with the <a class="reference internal" href="api.html#flask.Flask.run" title="flask.Flask.run"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Flask.run()</span></code></a>
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method. This method takes arguments similar to the CLI options to control the server.
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The main difference from the CLI command is that the server will crash if there are
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errors when reloading. <code class="docutils literal notranslate"><span class="pre">debug=True</span></code> can be passed to enable debug mode.</p>
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<p>Place the call in a main block, otherwise it will interfere when trying to import and
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run the application with a production server later.</p>
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<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s2">"__main__"</span><span class="p">:</span>
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<span class="n">app</span><span class="o">.</span><span class="n">run</span><span class="p">(</span><span class="n">debug</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span>
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</pre></div>
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</div>
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<div class="highlight-text notranslate"><div class="highlight"><pre><span></span>$ python hello.py
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</pre></div>
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</div>
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</section>
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</section>
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<img class="logo" src="_static/flask-vertical.png" alt="Logo of Flask"/>
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<h3>Contents</h3>
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<ul>
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<li><a class="reference internal" href="#">Development Server</a><ul>
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<li><a class="reference internal" href="#command-line">Command Line</a><ul>
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<li><a class="reference internal" href="#address-already-in-use">Address already in use</a></li>
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<li><a class="reference internal" href="#deferred-errors-on-reload">Deferred Errors on Reload</a></li>
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</ul>
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</li>
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<li><a class="reference internal" href="#in-code">In Code</a></li>
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<li><a href="index.html">Overview</a>
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<li>Previous: <a href="cli.html" title="previous chapter">Command Line Interface</a>
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