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<li class="nav-item nav-item-this"><a href="">Using URL Processors</a></li>
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<section id="using-url-processors">
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<h1>Using URL Processors<a class="headerlink" href="#using-url-processors" title="Link to this heading">¶</a></h1>
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<details class="changelog">
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<summary>Changelog</summary><div class="versionadded">
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<p><span class="versionmodified added">Added in version 0.7.</span></p>
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</div>
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</details><p>Flask 0.7 introduces the concept of URL processors. The idea is that you
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might have a bunch of resources with common parts in the URL that you
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don’t always explicitly want to provide. For instance you might have a
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bunch of URLs that have the language code in it but you don’t want to have
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to handle it in every single function yourself.</p>
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<p>URL processors are especially helpful when combined with blueprints. We
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will handle both application specific URL processors here as well as
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blueprint specifics.</p>
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<section id="internationalized-application-urls">
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<h2>Internationalized Application URLs<a class="headerlink" href="#internationalized-application-urls" title="Link to this heading">¶</a></h2>
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<p>Consider an application like this:</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">Flask</span><span class="p">,</span> <span class="n">g</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="nd">@app</span><span class="o">.</span><span class="n">route</span><span class="p">(</span><span class="s1">'/<lang_code>/'</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><span class="n">lang_code</span><span class="p">):</span>
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<span class="n">g</span><span class="o">.</span><span class="n">lang_code</span> <span class="o">=</span> <span class="n">lang_code</span>
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<span class="o">...</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="s1">'/<lang_code>/about'</span><span class="p">)</span>
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<span class="k">def</span><span class="w"> </span><span class="nf">about</span><span class="p">(</span><span class="n">lang_code</span><span class="p">):</span>
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<span class="n">g</span><span class="o">.</span><span class="n">lang_code</span> <span class="o">=</span> <span class="n">lang_code</span>
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<span class="o">...</span>
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</pre></div>
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</div>
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<p>This is an awful lot of repetition as you have to handle the language code
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setting on 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></code></a> object yourself in every single function.
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Sure, a decorator could be used to simplify this, but if you want to
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generate URLs from one function to another you would have to still provide
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the language code explicitly which can be annoying.</p>
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<p>For the latter, this is where <a class="reference internal" href="../api.html#flask.Flask.url_defaults" title="flask.Flask.url_defaults"><code class="xref py py-func docutils literal notranslate"><span class="pre">url_defaults()</span></code></a> functions
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come in. They can automatically inject values into a call to
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<a class="reference internal" href="../api.html#flask.url_for" title="flask.url_for"><code class="xref py py-func docutils literal notranslate"><span class="pre">url_for()</span></code></a>. The code below checks if the
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language code is not yet in the dictionary of URL values and if the
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endpoint wants a value named <code class="docutils literal notranslate"><span class="pre">'lang_code'</span></code>:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="nd">@app</span><span class="o">.</span><span class="n">url_defaults</span>
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<span class="k">def</span><span class="w"> </span><span class="nf">add_language_code</span><span class="p">(</span><span class="n">endpoint</span><span class="p">,</span> <span class="n">values</span><span class="p">):</span>
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<span class="k">if</span> <span class="s1">'lang_code'</span> <span class="ow">in</span> <span class="n">values</span> <span class="ow">or</span> <span class="ow">not</span> <span class="n">g</span><span class="o">.</span><span class="n">lang_code</span><span class="p">:</span>
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<span class="k">return</span>
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<span class="k">if</span> <span class="n">app</span><span class="o">.</span><span class="n">url_map</span><span class="o">.</span><span class="n">is_endpoint_expecting</span><span class="p">(</span><span class="n">endpoint</span><span class="p">,</span> <span class="s1">'lang_code'</span><span class="p">):</span>
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<span class="n">values</span><span class="p">[</span><span class="s1">'lang_code'</span><span class="p">]</span> <span class="o">=</span> <span class="n">g</span><span class="o">.</span><span class="n">lang_code</span>
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</pre></div>
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</div>
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<p>The method <a class="reference external" href="https://werkzeug.palletsprojects.com/en/stable/routing/#werkzeug.routing.Map.is_endpoint_expecting" title="(in Werkzeug v3.1.x)"><code class="xref py py-meth docutils literal notranslate"><span class="pre">is_endpoint_expecting()</span></code></a> of the URL
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map can be used to figure out if it would make sense to provide a language
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code for the given endpoint.</p>
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<p>The reverse of that function are
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<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>s. They are executed right
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after the request was matched and can execute code based on the URL
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values. The idea is that they pull information out of the values
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dictionary and put it somewhere else:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="nd">@app</span><span class="o">.</span><span class="n">url_value_preprocessor</span>
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<span class="k">def</span><span class="w"> </span><span class="nf">pull_lang_code</span><span class="p">(</span><span class="n">endpoint</span><span class="p">,</span> <span class="n">values</span><span class="p">):</span>
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<span class="n">g</span><span class="o">.</span><span class="n">lang_code</span> <span class="o">=</span> <span class="n">values</span><span class="o">.</span><span class="n">pop</span><span class="p">(</span><span class="s1">'lang_code'</span><span class="p">,</span> <span class="kc">None</span><span class="p">)</span>
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</pre></div>
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</div>
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<p>That way you no longer have to do the <code class="code docutils literal notranslate"><span class="pre">lang_code</span></code> assignment to
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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> in every function. You can further improve that by
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writing your own decorator that prefixes URLs with the language code, but
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the more beautiful solution is using a blueprint. Once the
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<code class="docutils literal notranslate"><span class="pre">'lang_code'</span></code> is popped from the values dictionary and it will no longer
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be forwarded to the view function reducing the code to this:</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">Flask</span><span class="p">,</span> <span class="n">g</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="nd">@app</span><span class="o">.</span><span class="n">url_defaults</span>
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<span class="k">def</span><span class="w"> </span><span class="nf">add_language_code</span><span class="p">(</span><span class="n">endpoint</span><span class="p">,</span> <span class="n">values</span><span class="p">):</span>
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<span class="k">if</span> <span class="s1">'lang_code'</span> <span class="ow">in</span> <span class="n">values</span> <span class="ow">or</span> <span class="ow">not</span> <span class="n">g</span><span class="o">.</span><span class="n">lang_code</span><span class="p">:</span>
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<span class="k">return</span>
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<span class="k">if</span> <span class="n">app</span><span class="o">.</span><span class="n">url_map</span><span class="o">.</span><span class="n">is_endpoint_expecting</span><span class="p">(</span><span class="n">endpoint</span><span class="p">,</span> <span class="s1">'lang_code'</span><span class="p">):</span>
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<span class="n">values</span><span class="p">[</span><span class="s1">'lang_code'</span><span class="p">]</span> <span class="o">=</span> <span class="n">g</span><span class="o">.</span><span class="n">lang_code</span>
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<span class="nd">@app</span><span class="o">.</span><span class="n">url_value_preprocessor</span>
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<span class="k">def</span><span class="w"> </span><span class="nf">pull_lang_code</span><span class="p">(</span><span class="n">endpoint</span><span class="p">,</span> <span class="n">values</span><span class="p">):</span>
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<span class="n">g</span><span class="o">.</span><span class="n">lang_code</span> <span class="o">=</span> <span class="n">values</span><span class="o">.</span><span class="n">pop</span><span class="p">(</span><span class="s1">'lang_code'</span><span class="p">,</span> <span class="kc">None</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="s1">'/<lang_code>/'</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="o">...</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="s1">'/<lang_code>/about'</span><span class="p">)</span>
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<span class="k">def</span><span class="w"> </span><span class="nf">about</span><span class="p">():</span>
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<span class="o">...</span>
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</pre></div>
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</div>
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</section>
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<section id="internationalized-blueprint-urls">
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<h2>Internationalized Blueprint URLs<a class="headerlink" href="#internationalized-blueprint-urls" title="Link to this heading">¶</a></h2>
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<p>Because blueprints can automatically prefix all URLs with a common string
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it’s easy to automatically do that for every function. Furthermore
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blueprints can have per-blueprint URL processors which removes a whole lot
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of logic from the <a class="reference internal" href="../api.html#flask.Flask.url_defaults" title="flask.Flask.url_defaults"><code class="xref py py-meth docutils literal notranslate"><span class="pre">url_defaults()</span></code></a> function because it no
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longer has to check if the URL is really interested in a <code class="docutils literal notranslate"><span class="pre">'lang_code'</span></code>
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parameter:</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">Blueprint</span><span class="p">,</span> <span class="n">g</span>
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<span class="n">bp</span> <span class="o">=</span> <span class="n">Blueprint</span><span class="p">(</span><span class="s1">'frontend'</span><span class="p">,</span> <span class="vm">__name__</span><span class="p">,</span> <span class="n">url_prefix</span><span class="o">=</span><span class="s1">'/<lang_code>'</span><span class="p">)</span>
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<span class="nd">@bp</span><span class="o">.</span><span class="n">url_defaults</span>
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<span class="k">def</span><span class="w"> </span><span class="nf">add_language_code</span><span class="p">(</span><span class="n">endpoint</span><span class="p">,</span> <span class="n">values</span><span class="p">):</span>
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<span class="n">values</span><span class="o">.</span><span class="n">setdefault</span><span class="p">(</span><span class="s1">'lang_code'</span><span class="p">,</span> <span class="n">g</span><span class="o">.</span><span class="n">lang_code</span><span class="p">)</span>
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<span class="nd">@bp</span><span class="o">.</span><span class="n">url_value_preprocessor</span>
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<span class="k">def</span><span class="w"> </span><span class="nf">pull_lang_code</span><span class="p">(</span><span class="n">endpoint</span><span class="p">,</span> <span class="n">values</span><span class="p">):</span>
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<span class="n">g</span><span class="o">.</span><span class="n">lang_code</span> <span class="o">=</span> <span class="n">values</span><span class="o">.</span><span class="n">pop</span><span class="p">(</span><span class="s1">'lang_code'</span><span class="p">)</span>
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<span class="nd">@bp</span><span class="o">.</span><span class="n">route</span><span class="p">(</span><span class="s1">'/'</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="o">...</span>
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<span class="nd">@bp</span><span class="o">.</span><span class="n">route</span><span class="p">(</span><span class="s1">'/about'</span><span class="p">)</span>
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<span class="k">def</span><span class="w"> </span><span class="nf">about</span><span class="p">():</span>
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<span class="o">...</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="#">Using URL Processors</a><ul>
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<li><a class="reference internal" href="#internationalized-application-urls">Internationalized Application URLs</a></li>
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<li><a class="reference internal" href="#internationalized-blueprint-urls">Internationalized Blueprint URLs</a></li>
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