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<li class="nav-item nav-item-this"><a href="">Application Structure and Lifecycle</a></li>
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<section id="application-structure-and-lifecycle">
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<h1>Application Structure and Lifecycle<a class="headerlink" href="#application-structure-and-lifecycle" title="Link to this heading">¶</a></h1>
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<p>Flask makes it pretty easy to write a web application. But there are quite a few
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different parts to an application and to each request it handles. Knowing what happens
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during application setup, serving, and handling requests will help you know what’s
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possible in Flask and how to structure your application.</p>
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<section id="application-setup">
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<h2>Application Setup<a class="headerlink" href="#application-setup" title="Link to this heading">¶</a></h2>
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<p>The first step in creating a Flask application is creating the application object. Each
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Flask application is an instance of 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> class, which collects all
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configuration, extensions, and views.</p>
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<div class="highlight-python 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">Flask</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="n">app</span><span class="o">.</span><span class="n">config</span><span class="o">.</span><span class="n">from_mapping</span><span class="p">(</span>
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<span class="n">SECRET_KEY</span><span class="o">=</span><span class="s2">"dev"</span><span class="p">,</span>
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<span class="p">)</span>
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<span class="n">app</span><span class="o">.</span><span class="n">config</span><span class="o">.</span><span class="n">from_prefixed_env</span><span class="p">()</span>
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<span class="nd">@app</span><span class="o">.</span><span class="n">route</span><span class="p">(</span><span class="s2">"/"</span><span class="p">)</span>
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<span class="k">def</span><span class="w"> </span><span class="nf">index</span><span class="p">():</span>
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<span class="k">return</span> <span class="s2">"Hello, World!"</span>
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</pre></div>
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</div>
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<p>This is known as the “application setup phase”, it’s the code you write that’s outside
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any view functions or other handlers. It can be split up between different modules and
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sub-packages, but all code that you want to be part of your application must be imported
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in order for it to be registered.</p>
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<p>All application setup must be completed before you start serving your application and
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handling requests. This is because WSGI servers divide work between multiple workers, or
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can be distributed across multiple machines. If the configuration changed in one worker,
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there’s no way for Flask to ensure consistency between other workers.</p>
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<p>Flask tries to help developers catch some of these setup ordering issues by showing an
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error if setup-related methods are called after requests are handled. In that case
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you’ll see this error:</p>
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<blockquote>
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<div><p>The setup method ‘route’ can no longer be called on the application. It has already
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handled its first request, any changes will not be applied consistently.
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Make sure all imports, decorators, functions, etc. needed to set up the application
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are done before running it.</p>
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</div></blockquote>
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<p>However, it is not possible for Flask to detect all cases of out-of-order setup. In
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general, don’t do anything to modify the <code class="docutils literal notranslate"><span class="pre">Flask</span></code> app object and <code class="docutils literal notranslate"><span class="pre">Blueprint</span></code> objects
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from within view functions that run during requests. This includes:</p>
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<ul class="simple">
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<li><p>Adding routes, view functions, and other request handlers with <code class="docutils literal notranslate"><span class="pre">@app.route</span></code>,
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<code class="docutils literal notranslate"><span class="pre">@app.errorhandler</span></code>, <code class="docutils literal notranslate"><span class="pre">@app.before_request</span></code>, etc.</p></li>
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<li><p>Registering blueprints.</p></li>
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<li><p>Loading configuration with <code class="docutils literal notranslate"><span class="pre">app.config</span></code>.</p></li>
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<li><p>Setting up the Jinja template environment with <code class="docutils literal notranslate"><span class="pre">app.jinja_env</span></code>.</p></li>
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<li><p>Setting a session interface, instead of the default itsdangerous cookie.</p></li>
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<li><p>Setting a JSON provider with <code class="docutils literal notranslate"><span class="pre">app.json</span></code>, instead of the default provider.</p></li>
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<li><p>Creating and initializing Flask extensions.</p></li>
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</ul>
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</section>
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<section id="serving-the-application">
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<h2>Serving the Application<a class="headerlink" href="#serving-the-application" title="Link to this heading">¶</a></h2>
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<p>Flask is a WSGI application framework. The other half of WSGI is the WSGI server. During
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development, Flask, through Werkzeug, provides a development WSGI server with the
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<code class="docutils literal notranslate"><span class="pre">flask</span> <span class="pre">run</span></code> CLI command. When you are done with development, use a production server
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to serve your application, see <a class="reference internal" href="deploying/index.html"><span class="doc">Deploying to Production</span></a>.</p>
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<p>Regardless of what server you’re using, it will follow the <span class="target" id="index-0"></span><a class="pep reference external" href="https://peps.python.org/pep-3333/"><strong>PEP 3333</strong></a> WSGI spec. The
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WSGI server will be told how to access your Flask application object, which is the WSGI
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application. Then it will start listening for HTTP requests, translate the request data
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into a WSGI environ, and call the WSGI application with that data. The WSGI application
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will return data that is translated into an HTTP response.</p>
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<ol class="arabic simple">
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<li><p>Browser or other client makes HTTP request.</p></li>
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<li><p>WSGI server receives request.</p></li>
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<li><p>WSGI server converts HTTP data to WSGI <code class="docutils literal notranslate"><span class="pre">environ</span></code> dict.</p></li>
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<li><p>WSGI server calls WSGI application with the <code class="docutils literal notranslate"><span class="pre">environ</span></code>.</p></li>
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<li><p>Flask, the WSGI application, does all its internal processing to route the request
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to a view function, handle errors, etc.</p></li>
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<li><p>Flask translates View function return into WSGI response data, passes it to WSGI
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server.</p></li>
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<li><p>WSGI server creates and send an HTTP response.</p></li>
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<li><p>Client receives the HTTP response.</p></li>
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</ol>
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<section id="middleware">
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<h3>Middleware<a class="headerlink" href="#middleware" title="Link to this heading">¶</a></h3>
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<p>The WSGI application above is a callable that behaves in a certain way. Middleware
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is a WSGI application that wraps another WSGI application. It’s a similar concept to
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Python decorators. The outermost middleware will be called by the server. It can modify
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the data passed to it, then call the WSGI application (or further middleware) that it
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wraps, and so on. And it can take the return value of that call and modify it further.</p>
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<p>From the WSGI server’s perspective, there is one WSGI application, the one it calls
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directly. Typically, Flask is the “real” application at the end of the chain of
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middleware. But even Flask can call further WSGI applications, although that’s an
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advanced, uncommon use case.</p>
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<p>A common middleware you’ll see used with Flask is Werkzeug’s
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<a class="reference external" href="https://werkzeug.palletsprojects.com/en/stable/middleware/proxy_fix/#werkzeug.middleware.proxy_fix.ProxyFix" title="(in Werkzeug v3.1.x)"><code class="xref py py-class docutils literal notranslate"><span class="pre">ProxyFix</span></code></a>, which modifies the request to look
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like it came directly from a client even if it passed through HTTP proxies on the way.
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There are other middleware that can handle serving static files, authentication, etc.</p>
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</section>
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</section>
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<section id="how-a-request-is-handled">
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<h2>How a Request is Handled<a class="headerlink" href="#how-a-request-is-handled" title="Link to this heading">¶</a></h2>
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<p>For us, the interesting part of the steps above is when Flask gets called by the WSGI
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server (or middleware). At that point, it will do quite a lot to handle the request and
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generate the response. At the most basic, it will match the URL to a view function, call
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the view function, and pass the return value back to the server. But there are many more
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parts that you can use to customize its behavior.</p>
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<ol class="arabic simple">
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<li><p>WSGI server calls the Flask object, which calls <a class="reference internal" href="api.html#flask.Flask.wsgi_app" title="flask.Flask.wsgi_app"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Flask.wsgi_app()</span></code></a>.</p></li>
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<li><p>A <a class="reference internal" href="api.html#flask.ctx.RequestContext" title="flask.ctx.RequestContext"><code class="xref py py-class docutils literal notranslate"><span class="pre">RequestContext</span></code></a> object is created. This converts the WSGI <code class="docutils literal notranslate"><span class="pre">environ</span></code>
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dict into a <a class="reference internal" href="api.html#flask.Request" title="flask.Request"><code class="xref py py-class docutils literal notranslate"><span class="pre">Request</span></code></a> object. It also creates an <code class="xref py py-class docutils literal notranslate"><span class="pre">AppContext</span></code> object.</p></li>
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<li><p>The <a class="reference internal" href="appcontext.html"><span class="doc">app context</span></a> is pushed, which makes <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> available.</p></li>
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<li><p>The <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> signal is sent.</p></li>
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<li><p>The <a class="reference internal" href="reqcontext.html"><span class="doc">request context</span></a> is pushed, which makes <a class="reference internal" href="api.html#flask.request" title="flask.request"><code class="xref py py-attr docutils literal notranslate"><span class="pre">request</span></code></a> and
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<a class="reference internal" href="api.html#flask.session" title="flask.session"><code class="xref py py-class docutils literal notranslate"><span class="pre">session</span></code></a> available.</p></li>
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<li><p>The session is opened, loading any existing session data using the app’s
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||
<a class="reference internal" href="api.html#flask.Flask.session_interface" title="flask.Flask.session_interface"><code class="xref py py-attr docutils literal notranslate"><span class="pre">session_interface</span></code></a>, an instance of <a class="reference internal" href="api.html#flask.sessions.SessionInterface" title="flask.sessions.SessionInterface"><code class="xref py py-class docutils literal notranslate"><span class="pre">SessionInterface</span></code></a>.</p></li>
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<li><p>The URL is matched against the URL rules registered with the <a class="reference internal" href="api.html#flask.Flask.route" title="flask.Flask.route"><code class="xref py py-meth docutils literal notranslate"><span class="pre">route()</span></code></a>
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decorator during application setup. If there is no match, the error - usually a 404,
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405, or redirect - is stored to be handled later.</p></li>
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<li><p>The <a class="reference internal" href="api.html#flask.request_started" title="flask.request_started"><code class="xref py py-data docutils literal notranslate"><span class="pre">request_started</span></code></a> signal is sent.</p></li>
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<li><p>Any <a class="reference internal" href="api.html#flask.Flask.url_value_preprocessor" title="flask.Flask.url_value_preprocessor"><code class="xref py py-meth docutils literal notranslate"><span class="pre">url_value_preprocessor()</span></code></a> decorated functions are called.</p></li>
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<li><p>Any <a class="reference internal" href="api.html#flask.Flask.before_request" title="flask.Flask.before_request"><code class="xref py py-meth docutils literal notranslate"><span class="pre">before_request()</span></code></a> decorated functions are called. If any of
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these function returns a value it is treated as the response immediately.</p></li>
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<li><p>If the URL didn’t match a route a few steps ago, that error is raised now.</p></li>
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<li><p>The <a class="reference internal" href="api.html#flask.Flask.route" title="flask.Flask.route"><code class="xref py py-meth docutils literal notranslate"><span class="pre">route()</span></code></a> decorated view function associated with the matched URL
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is called and returns a value to be used as the response.</p></li>
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<li><p>If any step so far raised an exception, and there is an <a class="reference internal" href="api.html#flask.Flask.errorhandler" title="flask.Flask.errorhandler"><code class="xref py py-meth docutils literal notranslate"><span class="pre">errorhandler()</span></code></a>
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decorated function that matches the exception class or HTTP error code, it is
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called to handle the error and return a response.</p></li>
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<li><p>Whatever returned a response value - a before request function, the view, or an
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error handler, that value is converted to a <a class="reference internal" href="api.html#flask.Response" title="flask.Response"><code class="xref py py-class docutils literal notranslate"><span class="pre">Response</span></code></a> object.</p></li>
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<li><p>Any <a class="reference internal" href="api.html#flask.after_this_request" title="flask.after_this_request"><code class="xref py py-func docutils literal notranslate"><span class="pre">after_this_request()</span></code></a> decorated functions are called, then cleared.</p></li>
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<li><p>Any <a class="reference internal" href="api.html#flask.Flask.after_request" title="flask.Flask.after_request"><code class="xref py py-meth docutils literal notranslate"><span class="pre">after_request()</span></code></a> decorated functions are called, which can modify
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the response object.</p></li>
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<li><p>The session is saved, persisting any modified session data using the app’s
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<a class="reference internal" href="api.html#flask.Flask.session_interface" title="flask.Flask.session_interface"><code class="xref py py-attr docutils literal notranslate"><span class="pre">session_interface</span></code></a>.</p></li>
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<li><p>The <a class="reference internal" href="api.html#flask.request_finished" title="flask.request_finished"><code class="xref py py-data docutils literal notranslate"><span class="pre">request_finished</span></code></a> signal is sent.</p></li>
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<li><p>If any step so far raised an exception, and it was not handled by an error handler
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function, it is handled now. HTTP exceptions are treated as responses with their
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corresponding status code, other exceptions are converted to a generic 500 response.
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The <a class="reference internal" href="api.html#flask.got_request_exception" title="flask.got_request_exception"><code class="xref py py-data docutils literal notranslate"><span class="pre">got_request_exception</span></code></a> signal is sent.</p></li>
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<li><p>The response object’s status, headers, and body are returned to the WSGI server.</p></li>
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<li><p>Any <a class="reference internal" href="api.html#flask.Flask.teardown_request" title="flask.Flask.teardown_request"><code class="xref py py-meth docutils literal notranslate"><span class="pre">teardown_request()</span></code></a> decorated functions are called.</p></li>
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<li><p>The <a class="reference internal" href="api.html#flask.request_tearing_down" title="flask.request_tearing_down"><code class="xref py py-data docutils literal notranslate"><span class="pre">request_tearing_down</span></code></a> signal is sent.</p></li>
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<li><p>The request context is popped, <a class="reference internal" href="api.html#flask.request" title="flask.request"><code class="xref py py-attr docutils literal notranslate"><span class="pre">request</span></code></a> and <a class="reference internal" href="api.html#flask.session" title="flask.session"><code class="xref py py-class docutils literal notranslate"><span class="pre">session</span></code></a> are no longer
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available.</p></li>
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<li><p>Any <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> decorated functions are called.</p></li>
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<li><p>The <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> signal is sent.</p></li>
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<li><p>The app context is popped, <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 <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> are no longer
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available.</p></li>
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<li><p>The <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> signal is sent.</p></li>
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</ol>
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<p>There are even more decorators and customization points than this, but that aren’t part
|
||
of every request lifecycle. They’re more specific to certain things you might use during
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||
a request, such as templates, building URLs, or handling JSON data. See the rest of this
|
||
documentation, as well as the <a class="reference internal" href="api.html"><span class="doc">API</span></a> to explore further.</p>
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</section>
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</section>
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|
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|
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|
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<h3>Contents</h3>
|
||
<ul>
|
||
<li><a class="reference internal" href="#">Application Structure and Lifecycle</a><ul>
|
||
<li><a class="reference internal" href="#application-setup">Application Setup</a></li>
|
||
<li><a class="reference internal" href="#serving-the-application">Serving the Application</a><ul>
|
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<li><a class="reference internal" href="#middleware">Middleware</a></li>
|
||
</ul>
|
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</li>
|
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<li><a class="reference internal" href="#how-a-request-is-handled">How a Request is Handled</a></li>
|
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</ul>
|
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<h3>Navigation</h3>
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<ul>
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<li><a href="index.html">Overview</a>
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<ul>
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<li>Previous: <a href="views.html" title="previous chapter">Class-based Views</a>
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<li>Next: <a href="appcontext.html" title="next chapter">The Application Context</a>
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<input type="text" name="q" aria-labelledby="searchlabel" autocomplete="off" autocorrect="off" autocapitalize="off" spellcheck="false"/>
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<input type="submit" value="Go" />
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</form>
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</div>
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</search>
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<script>document.getElementById('searchbox').style.display = "block"</script><div id="ethical-ad-placement"></div>
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</div>
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</div>
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<div class="clearer"></div>
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</div>
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<div class="footer" role="contentinfo">
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© Copyright 2010 Pallets.
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</body>
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