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176 lines
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<li class="nav-item nav-item-this"><a href="">Streaming Contents</a></li>
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<section id="streaming-contents">
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<h1>Streaming Contents<a class="headerlink" href="#streaming-contents" title="Link to this heading">¶</a></h1>
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<p>Sometimes you want to send an enormous amount of data to the client, much
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more than you want to keep in memory. When you are generating the data on
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the fly though, how do you send that back to the client without the
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roundtrip to the filesystem?</p>
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<p>The answer is by using generators and direct responses.</p>
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<section id="basic-usage">
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<h2>Basic Usage<a class="headerlink" href="#basic-usage" title="Link to this heading">¶</a></h2>
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<p>This is a basic view function that generates a lot of CSV data on the fly.
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The trick is to have an inner function that uses a generator to generate
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data and to then invoke that function and pass it to a response object:</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">route</span><span class="p">(</span><span class="s1">'/large.csv'</span><span class="p">)</span>
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<span class="k">def</span><span class="w"> </span><span class="nf">generate_large_csv</span><span class="p">():</span>
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<span class="k">def</span><span class="w"> </span><span class="nf">generate</span><span class="p">():</span>
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<span class="k">for</span> <span class="n">row</span> <span class="ow">in</span> <span class="n">iter_all_rows</span><span class="p">():</span>
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<span class="k">yield</span> <span class="sa">f</span><span class="s2">"</span><span class="si">{</span><span class="s1">','</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="n">row</span><span class="p">)</span><span class="si">}</span><span class="se">\n</span><span class="s2">"</span>
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<span class="k">return</span> <span class="n">generate</span><span class="p">(),</span> <span class="p">{</span><span class="s2">"Content-Type"</span><span class="p">:</span> <span class="s2">"text/csv"</span><span class="p">}</span>
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</pre></div>
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</div>
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<p>Each <code class="docutils literal notranslate"><span class="pre">yield</span></code> expression is directly sent to the browser. Note though
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that some WSGI middlewares might break streaming, so be careful there in
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debug environments with profilers and other things you might have enabled.</p>
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</section>
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<section id="streaming-from-templates">
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<h2>Streaming from Templates<a class="headerlink" href="#streaming-from-templates" title="Link to this heading">¶</a></h2>
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<p>The Jinja2 template engine supports rendering a template piece by
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piece, returning an iterator of strings. Flask provides the
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<a class="reference internal" href="../api.html#flask.stream_template" title="flask.stream_template"><code class="xref py py-func docutils literal notranslate"><span class="pre">stream_template()</span></code></a> and <a class="reference internal" href="../api.html#flask.stream_template_string" title="flask.stream_template_string"><code class="xref py py-func docutils literal notranslate"><span class="pre">stream_template_string()</span></code></a>
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functions to make this easier to use.</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">stream_template</span>
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<span class="nd">@app</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="s2">"/timeline"</span><span class="p">)</span>
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<span class="k">def</span><span class="w"> </span><span class="nf">timeline</span><span class="p">():</span>
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<span class="k">return</span> <span class="n">stream_template</span><span class="p">(</span><span class="s2">"timeline.html"</span><span class="p">)</span>
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</pre></div>
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</div>
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<p>The parts yielded by the render stream tend to match statement blocks in
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the template.</p>
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</section>
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<section id="streaming-with-context">
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<h2>Streaming with Context<a class="headerlink" href="#streaming-with-context" title="Link to this heading">¶</a></h2>
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<p>The <a class="reference internal" href="../api.html#flask.request" title="flask.request"><code class="xref py py-data docutils literal notranslate"><span class="pre">request</span></code></a> will not be active while the generator is
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running, because the view has already returned at that point. If you try
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to access <code class="docutils literal notranslate"><span class="pre">request</span></code>, you’ll get a <code class="docutils literal notranslate"><span class="pre">RuntimeError</span></code>.</p>
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<p>If your generator function relies on data in <code class="docutils literal notranslate"><span class="pre">request</span></code>, use the
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<a class="reference internal" href="../api.html#flask.stream_with_context" title="flask.stream_with_context"><code class="xref py py-func docutils literal notranslate"><span class="pre">stream_with_context()</span></code></a> wrapper. This will keep the request
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context active during the generator.</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">stream_with_context</span><span class="p">,</span> <span class="n">request</span>
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<span class="kn">from</span><span class="w"> </span><span class="nn">markupsafe</span><span class="w"> </span><span class="kn">import</span> <span class="n">escape</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">'/stream'</span><span class="p">)</span>
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<span class="k">def</span><span class="w"> </span><span class="nf">streamed_response</span><span class="p">():</span>
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<span class="k">def</span><span class="w"> </span><span class="nf">generate</span><span class="p">():</span>
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<span class="k">yield</span> <span class="s1">'<p>Hello '</span>
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<span class="k">yield</span> <span class="n">escape</span><span class="p">(</span><span class="n">request</span><span class="o">.</span><span class="n">args</span><span class="p">[</span><span class="s1">'name'</span><span class="p">])</span>
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<span class="k">yield</span> <span class="s1">'!</p>'</span>
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<span class="k">return</span> <span class="n">stream_with_context</span><span class="p">(</span><span class="n">generate</span><span class="p">())</span>
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</pre></div>
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</div>
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<p>It can also be used as a decorator.</p>
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<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="nd">@stream_with_context</span>
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<span class="k">def</span><span class="w"> </span><span class="nf">generate</span><span class="p">():</span>
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<span class="o">...</span>
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<span class="k">return</span> <span class="n">generate</span><span class="p">()</span>
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</pre></div>
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</div>
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<p>The <a class="reference internal" href="../api.html#flask.stream_template" title="flask.stream_template"><code class="xref py py-func docutils literal notranslate"><span class="pre">stream_template()</span></code></a> and
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<a class="reference internal" href="../api.html#flask.stream_template_string" title="flask.stream_template_string"><code class="xref py py-func docutils literal notranslate"><span class="pre">stream_template_string()</span></code></a> functions automatically
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use <a class="reference internal" href="../api.html#flask.stream_with_context" title="flask.stream_with_context"><code class="xref py py-func docutils literal notranslate"><span class="pre">stream_with_context()</span></code></a> if a request is active.</p>
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</section>
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</section>
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<h3>Contents</h3>
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<ul>
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<li><a class="reference internal" href="#">Streaming Contents</a><ul>
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<li><a class="reference internal" href="#basic-usage">Basic Usage</a></li>
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<li><a class="reference internal" href="#streaming-from-templates">Streaming from Templates</a></li>
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<li><a class="reference internal" href="#streaming-with-context">Streaming with Context</a></li>
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<li>Previous: <a href="favicon.html" title="previous chapter">Adding a favicon</a>
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<li>Next: <a href="deferredcallbacks.html" title="next chapter">Deferred Request Callbacks</a></ul>
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