228 lines
18 KiB
JavaScript
228 lines
18 KiB
JavaScript
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<section id="the-application-context">
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<h1>The Application Context<a class="headerlink" href="#the-application-context" title="Link to this heading">¶</a></h1>
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<p>The application context keeps track of the application-level data during
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a request, CLI command, or other activity. Rather than passing the
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application around to each function, the <a class="reference internal" href="api.html#flask.current_app" title="flask.current_app"><code class="xref py py-data docutils literal notranslate"><span class="pre">current_app</span></code></a> and
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<a class="reference internal" href="api.html#flask.g" title="flask.g"><code class="xref py py-data docutils literal notranslate"><span class="pre">g</span></code></a> proxies are accessed instead.</p>
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<p>This is similar to <a class="reference internal" href="reqcontext.html"><span class="doc">The Request Context</span></a>, which keeps track of
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request-level data during a request. A corresponding application context
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is pushed when a request context is pushed.</p>
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<section id="purpose-of-the-context">
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<h2>Purpose of the Context<a class="headerlink" href="#purpose-of-the-context" title="Link to this heading">¶</a></h2>
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<p>The <a class="reference internal" href="api.html#flask.Flask" title="flask.Flask"><code class="xref py py-class docutils literal notranslate"><span class="pre">Flask</span></code></a> application object has attributes, such as
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<a class="reference internal" href="api.html#flask.Flask.config" title="flask.Flask.config"><code class="xref py py-attr docutils literal notranslate"><span class="pre">config</span></code></a>, that are useful to access within views and
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<a class="reference internal" href="cli.html"><span class="doc">CLI commands</span></a>. However, importing the <code class="docutils literal notranslate"><span class="pre">app</span></code> instance
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within the modules in your project is prone to circular import issues.
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When using the <a class="reference internal" href="patterns/appfactories.html"><span class="doc">app factory pattern</span></a> or
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writing reusable <a class="reference internal" href="blueprints.html"><span class="doc">blueprints</span></a> or
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<a class="reference internal" href="extensions.html"><span class="doc">extensions</span></a> there won’t be an <code class="docutils literal notranslate"><span class="pre">app</span></code> instance to
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import at all.</p>
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<p>Flask solves this issue with the <em>application context</em>. Rather than
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referring to an <code class="docutils literal notranslate"><span class="pre">app</span></code> directly, you use the <a class="reference internal" href="api.html#flask.current_app" title="flask.current_app"><code class="xref py py-data docutils literal notranslate"><span class="pre">current_app</span></code></a>
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proxy, which points to the application handling the current activity.</p>
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<p>Flask automatically <em>pushes</em> an application context when handling a
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request. View functions, error handlers, and other functions that run
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during a request will have access to <a class="reference internal" href="api.html#flask.current_app" title="flask.current_app"><code class="xref py py-data docutils literal notranslate"><span class="pre">current_app</span></code></a>.</p>
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<p>Flask will also automatically push an app context when running CLI
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commands registered with <a class="reference internal" href="api.html#flask.Flask.cli" title="flask.Flask.cli"><code class="xref py py-attr docutils literal notranslate"><span class="pre">Flask.cli</span></code></a> using <code class="docutils literal notranslate"><span class="pre">@app.cli.command()</span></code>.</p>
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</section>
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<section id="lifetime-of-the-context">
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<h2>Lifetime of the Context<a class="headerlink" href="#lifetime-of-the-context" title="Link to this heading">¶</a></h2>
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<p>The application context is created and destroyed as necessary. When a
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Flask application begins handling a request, it pushes an application
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context and a <a class="reference internal" href="reqcontext.html"><span class="doc">request context</span></a>. When the request
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ends it pops the request context then the application context.
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Typically, an application context will have the same lifetime as a
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request.</p>
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<p>See <a class="reference internal" href="reqcontext.html"><span class="doc">The Request Context</span></a> for more information about how the contexts work
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and the full life cycle of a request.</p>
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</section>
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<section id="manually-push-a-context">
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<h2>Manually Push a Context<a class="headerlink" href="#manually-push-a-context" title="Link to this heading">¶</a></h2>
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<p>If you try to access <a class="reference internal" href="api.html#flask.current_app" title="flask.current_app"><code class="xref py py-data docutils literal notranslate"><span class="pre">current_app</span></code></a>, or anything that uses it,
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outside an application context, you’ll get this error message:</p>
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<div class="highlight-pytb notranslate"><div class="highlight"><pre><span></span><span class="x">RuntimeError: Working outside of application context.</span>
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<span class="x">This typically means that you attempted to use functionality that</span>
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<span class="x">needed to interface with the current application object in some way.</span>
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<span class="x">To solve this, set up an application context with app.app_context().</span>
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</pre></div>
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</div>
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<p>If you see that error while configuring your application, such as when
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initializing an extension, you can push a context manually since you
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have direct access to the <code class="docutils literal notranslate"><span class="pre">app</span></code>. Use <a class="reference internal" href="api.html#flask.Flask.app_context" title="flask.Flask.app_context"><code class="xref py py-meth docutils literal notranslate"><span class="pre">app_context()</span></code></a> in a
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<code class="docutils literal notranslate"><span class="pre">with</span></code> block, and everything that runs in the block will have access
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to <a class="reference internal" href="api.html#flask.current_app" title="flask.current_app"><code class="xref py py-data docutils literal notranslate"><span class="pre">current_app</span></code></a>.</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span><span class="w"> </span><span class="nf">create_app</span><span class="p">():</span>
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<span class="n">app</span> <span class="o">=</span> <span class="n">Flask</span><span class="p">(</span><span class="vm">__name__</span><span class="p">)</span>
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<span class="k">with</span> <span class="n">app</span><span class="o">.</span><span class="n">app_context</span><span class="p">():</span>
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<span class="n">init_db</span><span class="p">()</span>
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<span class="k">return</span> <span class="n">app</span>
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</pre></div>
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</div>
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<p>If you see that error somewhere else in your code not related to
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configuring the application, it most likely indicates that you should
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move that code into a view function or CLI command.</p>
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</section>
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<section id="storing-data">
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<h2>Storing Data<a class="headerlink" href="#storing-data" title="Link to this heading">¶</a></h2>
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<p>The application context is a good place to store common data during a
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request or CLI command. Flask provides the <a class="reference internal" href="api.html#flask.g" title="flask.g"><code class="xref py py-data docutils literal notranslate"><span class="pre">g</span> <span class="pre">object</span></code></a> for this
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purpose. It is a simple namespace object that has the same lifetime as
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an application context.</p>
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<div class="admonition note">
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<p class="admonition-title">Note</p>
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<p>The <code class="docutils literal notranslate"><span class="pre">g</span></code> name stands for “global”, but that is referring to the
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data being global <em>within a context</em>. The data on <code class="docutils literal notranslate"><span class="pre">g</span></code> is lost
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after the context ends, and it is not an appropriate place to store
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data between requests. Use the <a class="reference internal" href="api.html#flask.session" title="flask.session"><code class="xref py py-data docutils literal notranslate"><span class="pre">session</span></code></a> or a database to
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store data across requests.</p>
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</div>
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<p>A common use for <a class="reference internal" href="api.html#flask.g" title="flask.g"><code class="xref py py-data docutils literal notranslate"><span class="pre">g</span></code></a> is to manage resources during a request.</p>
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<ol class="arabic simple">
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<li><p><code class="docutils literal notranslate"><span class="pre">get_X()</span></code> creates resource <code class="docutils literal notranslate"><span class="pre">X</span></code> if it does not exist, caching it
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as <code class="docutils literal notranslate"><span class="pre">g.X</span></code>.</p></li>
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<li><p><code class="docutils literal notranslate"><span class="pre">teardown_X()</span></code> closes or otherwise deallocates the resource if it
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exists. It is registered as a <a class="reference internal" href="api.html#flask.Flask.teardown_appcontext" title="flask.Flask.teardown_appcontext"><code class="xref py py-meth docutils literal notranslate"><span class="pre">teardown_appcontext()</span></code></a>
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handler.</p></li>
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</ol>
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<p>For example, you can manage a database connection using this pattern:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span><span class="w"> </span><span class="nn">flask</span><span class="w"> </span><span class="kn">import</span> <span class="n">g</span>
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<span class="k">def</span><span class="w"> </span><span class="nf">get_db</span><span class="p">():</span>
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<span class="k">if</span> <span class="s1">'db'</span> <span class="ow">not</span> <span class="ow">in</span> <span class="n">g</span><span class="p">:</span>
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<span class="n">g</span><span class="o">.</span><span class="n">db</span> <span class="o">=</span> <span class="n">connect_to_database</span><span class="p">()</span>
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<span class="k">return</span> <span class="n">g</span><span class="o">.</span><span class="n">db</span>
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<span class="nd">@app</span><span class="o">.</span><span class="n">teardown_appcontext</span>
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<span class="k">def</span><span class="w"> </span><span class="nf">teardown_db</span><span class="p">(</span><span class="n">exception</span><span class="p">):</span>
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<span class="n">db</span> <span class="o">=</span> <span class="n">g</span><span class="o">.</span><span class="n">pop</span><span class="p">(</span><span class="s1">'db'</span><span class="p">,</span> <span class="kc">None</span><span class="p">)</span>
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<span class="k">if</span> <span class="n">db</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span><span class="p">:</span>
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<span class="n">db</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
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</pre></div>
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</div>
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<p>During a request, every call to <code class="docutils literal notranslate"><span class="pre">get_db()</span></code> will return the same
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connection, and it will be closed automatically at the end of the
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request.</p>
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<p>You can use <a class="reference external" href="https://werkzeug.palletsprojects.com/en/stable/local/#werkzeug.local.LocalProxy" title="(in Werkzeug v3.1.x)"><code class="xref py py-class docutils literal notranslate"><span class="pre">LocalProxy</span></code></a> to make a new context
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local from <code class="docutils literal notranslate"><span class="pre">get_db()</span></code>:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span><span class="w"> </span><span class="nn">werkzeug.local</span><span class="w"> </span><span class="kn">import</span> <span class="n">LocalProxy</span>
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<span class="n">db</span> <span class="o">=</span> <span class="n">LocalProxy</span><span class="p">(</span><span class="n">get_db</span><span class="p">)</span>
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</pre></div>
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</div>
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<p>Accessing <code class="docutils literal notranslate"><span class="pre">db</span></code> will call <code class="docutils literal notranslate"><span class="pre">get_db</span></code> internally, in the same way that
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<a class="reference internal" href="api.html#flask.current_app" title="flask.current_app"><code class="xref py py-data docutils literal notranslate"><span class="pre">current_app</span></code></a> works.</p>
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</section>
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<section id="events-and-signals">
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<h2>Events and Signals<a class="headerlink" href="#events-and-signals" title="Link to this heading">¶</a></h2>
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<p>The application will call functions registered with <a class="reference internal" href="api.html#flask.Flask.teardown_appcontext" title="flask.Flask.teardown_appcontext"><code class="xref py py-meth docutils literal notranslate"><span class="pre">teardown_appcontext()</span></code></a>
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when the application context is popped.</p>
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<p>The following signals are sent: <a class="reference internal" href="api.html#flask.appcontext_pushed" title="flask.appcontext_pushed"><code class="xref py py-data docutils literal notranslate"><span class="pre">appcontext_pushed</span></code></a>,
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<a class="reference internal" href="api.html#flask.appcontext_tearing_down" title="flask.appcontext_tearing_down"><code class="xref py py-data docutils literal notranslate"><span class="pre">appcontext_tearing_down</span></code></a>, and <a class="reference internal" href="api.html#flask.appcontext_popped" title="flask.appcontext_popped"><code class="xref py py-data docutils literal notranslate"><span class="pre">appcontext_popped</span></code></a>.</p>
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<h3>Contents</h3>
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<ul>
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<li><a class="reference internal" href="#">The Application Context</a><ul>
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<li><a class="reference internal" href="#purpose-of-the-context">Purpose of the Context</a></li>
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<li><a class="reference internal" href="#lifetime-of-the-context">Lifetime of the Context</a></li>
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<li><a class="reference internal" href="#manually-push-a-context">Manually Push a Context</a></li>
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<li><a class="reference internal" href="#storing-data">Storing Data</a></li>
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<li><a class="reference internal" href="#events-and-signals">Events and Signals</a></li>
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<li><a href="index.html">Overview</a>
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<li>Previous: <a href="lifecycle.html" title="previous chapter">Application Structure and Lifecycle</a>
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<li>Next: <a href="reqcontext.html" title="next chapter">The Request Context</a>
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