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<li class="nav-item nav-item-this"><a href="">Debugging Application Errors</a></li>
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<section id="debugging-application-errors">
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<h1>Debugging Application Errors<a class="headerlink" href="#debugging-application-errors" title="Link to this heading">¶</a></h1>
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<section id="in-production">
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<h2>In Production<a class="headerlink" href="#in-production" title="Link to this heading">¶</a></h2>
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<p><strong>Do not run the development server, or enable the built-in debugger, in
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a production environment.</strong> The debugger allows executing arbitrary
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Python code from the browser. It’s protected by a pin, but that should
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not be relied on for security.</p>
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<p>Use an error logging tool, such as Sentry, as described in
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<a class="reference internal" href="errorhandling.html#error-logging-tools"><span class="std std-ref">Error Logging Tools</span></a>, or enable logging and notifications as
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described in <a class="reference internal" href="logging.html"><span class="doc">Logging</span></a>.</p>
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<p>If you have access to the server, you could add some code to start an
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external debugger if <code class="docutils literal notranslate"><span class="pre">request.remote_addr</span></code> matches your IP. Some IDE
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debuggers also have a remote mode so breakpoints on the server can be
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interacted with locally. Only enable a debugger temporarily.</p>
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</section>
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<section id="the-built-in-debugger">
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<h2>The Built-In Debugger<a class="headerlink" href="#the-built-in-debugger" title="Link to this heading">¶</a></h2>
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<p>The built-in Werkzeug development server provides a debugger which shows
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an interactive traceback in the browser when an unhandled error occurs
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during a request. This debugger should only be used during development.</p>
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<img alt="screenshot of debugger in action" class="screenshot align-center" src="_images/debugger.png" />
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<div class="admonition warning">
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<p class="admonition-title">Warning</p>
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<p>The debugger allows executing arbitrary Python code from the
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browser. It is protected by a pin, but still represents a major
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security risk. Do not run the development server or debugger in a
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production environment.</p>
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</div>
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<p>The debugger is enabled by default when the development server is run in 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>When running from Python code, passing <code class="docutils literal notranslate"><span class="pre">debug=True</span></code> enables debug mode, which is
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mostly equivalent.</p>
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<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><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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<p><a class="reference internal" href="server.html"><span class="doc">Development Server</span></a> and <a class="reference internal" href="cli.html"><span class="doc">Command Line Interface</span></a> have more information about running the debugger and
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debug mode. More information about the debugger can be found in the <a class="reference external" href="https://werkzeug.palletsprojects.com/debug/">Werkzeug
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documentation</a>.</p>
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</section>
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<section id="external-debuggers">
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<h2>External Debuggers<a class="headerlink" href="#external-debuggers" title="Link to this heading">¶</a></h2>
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<p>External debuggers, such as those provided by IDEs, can offer a more
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powerful debugging experience than the built-in debugger. They can also
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be used to step through code during a request before an error is raised,
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or if no error is raised. Some even have a remote mode so you can debug
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code running on another machine.</p>
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<p>When using an external debugger, the app should still be in debug mode, otherwise Flask
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turns unhandled errors into generic 500 error pages. However, the built-in debugger and
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reloader should be disabled so they don’t interfere with the external debugger.</p>
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<div class="highlight-text notranslate"><div class="highlight"><pre><span></span>$ flask --app hello run --debug --no-debugger --no-reload
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</pre></div>
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</div>
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<p>When running from Python:</p>
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<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><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> <span class="n">use_debugger</span><span class="o">=</span><span class="kc">False</span><span class="p">,</span> <span class="n">use_reloader</span><span class="o">=</span><span class="kc">False</span><span class="p">)</span>
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</pre></div>
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</div>
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<p>Disabling these isn’t required, an external debugger will continue to work with the
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following caveats.</p>
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<ul>
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<li><p>If the built-in debugger is not disabled, it will catch unhandled exceptions before
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the external debugger can.</p></li>
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<li><p>If the reloader is not disabled, it could cause an unexpected reload if code changes
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during a breakpoint.</p></li>
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<li><p>The development server will still catch unhandled exceptions if the built-in
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debugger is disabled, otherwise it would crash on any error. If you want that (and
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usually you don’t) pass <code class="docutils literal notranslate"><span class="pre">passthrough_errors=True</span></code> to <code class="docutils literal notranslate"><span class="pre">app.run</span></code>.</p>
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<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="n">app</span><span class="o">.</span><span class="n">run</span><span class="p">(</span>
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<span class="n">debug</span><span class="o">=</span><span class="kc">True</span><span class="p">,</span> <span class="n">passthrough_errors</span><span class="o">=</span><span class="kc">True</span><span class="p">,</span>
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<span class="n">use_debugger</span><span class="o">=</span><span class="kc">False</span><span class="p">,</span> <span class="n">use_reloader</span><span class="o">=</span><span class="kc">False</span>
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<span class="p">)</span>
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</pre></div>
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<h3>Contents</h3>
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<ul>
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<li><a class="reference internal" href="#">Debugging Application Errors</a><ul>
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<li><a class="reference internal" href="#in-production">In Production</a></li>
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<li><a class="reference internal" href="#the-built-in-debugger">The Built-In Debugger</a></li>
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<li><a class="reference internal" href="#external-debuggers">External Debuggers</a></li>
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<li>Previous: <a href="errorhandling.html" title="previous chapter">Handling Application Errors</a>
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<li>Next: <a href="logging.html" title="next chapter">Logging</a>
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