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<div class="section" id="streaming-contents">
<h1>Streaming Contents<a class="headerlink" href="#streaming-contents" title="Permalink to this headline">¶</a></h1>
<p>Sometimes you want to send an enormous amount of data to the client, much
more than you want to keep in memory. When you are generating the data on
the fly though, how do you send that back to the client without the
roundtrip to the filesystem?</p>
<p>The answer is by using generators and direct responses.</p>
<div class="section" id="basic-usage">
<h2>Basic Usage<a class="headerlink" href="#basic-usage" title="Permalink to this headline">¶</a></h2>
<p>This is a basic view function that generates a lot of CSV data on the fly.
The trick is to have an inner function that uses a generator to generate
data and to then invoke that function and pass it to a response object:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">flask</span> <span class="kn">import</span> <span class="n">Response</span>
<span class="nd">@app.route</span><span class="p">(</span><span class="s">'/large.csv'</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">generate_large_csv</span><span class="p">():</span>
<span class="k">def</span> <span class="nf">generate</span><span class="p">():</span>
<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>
<span class="k">yield</span> <span class="s">','</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="o">+</span> <span class="s">'</span><span class="se">\n</span><span class="s">'</span>
<span class="k">return</span> <span class="n">Response</span><span class="p">(</span><span class="n">generate</span><span class="p">(),</span> <span class="n">mimetype</span><span class="o">=</span><span class="s">'text/csv'</span><span class="p">)</span>
</pre></div>
</div>
<p>Each <tt class="docutils literal"><span class="pre">yield</span></tt> expression is directly sent to the browser. Now though
that some WSGI middlewares might break streaming, so be careful there in
debug environments with profilers and other things you might have enabled.</p>
</div>
<div class="section" id="streaming-from-templates">
<h2>Streaming from Templates<a class="headerlink" href="#streaming-from-templates" title="Permalink to this headline">¶</a></h2>
<p>The Jinja2 template engine also supports rendering templates piece by
piece. This functionality is not directly exposed by Flask because it is
quite uncommon, but you can easily do it yourself:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">flask</span> <span class="kn">import</span> <span class="n">Response</span>
<span class="k">def</span> <span class="nf">stream_template</span><span class="p">(</span><span class="n">template_name</span><span class="p">,</span> <span class="o">**</span><span class="n">context</span><span class="p">):</span>
<span class="n">app</span><span class="o">.</span><span class="n">update_template_context</span><span class="p">(</span><span class="n">context</span><span class="p">)</span>
<span class="n">t</span> <span class="o">=</span> <span class="n">app</span><span class="o">.</span><span class="n">jinja_env</span><span class="o">.</span><span class="n">get_template</span><span class="p">(</span><span class="n">template_name</span><span class="p">)</span>
<span class="n">rv</span> <span class="o">=</span> <span class="n">t</span><span class="o">.</span><span class="n">stream</span><span class="p">(</span><span class="n">context</span><span class="p">)</span>
<span class="n">rv</span><span class="o">.</span><span class="n">enable_buffering</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span>
<span class="k">return</span> <span class="n">rv</span>
<span class="nd">@app.route</span><span class="p">(</span><span class="s">'/my-large-page.html'</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">render_large_template</span><span class="p">():</span>
<span class="n">rows</span> <span class="o">=</span> <span class="n">iter_all_rows</span><span class="p">()</span>
<span class="k">return</span> <span class="n">Response</span><span class="p">(</span><span class="n">stream_template</span><span class="p">(</span><span class="s">'the_template.html'</span><span class="p">,</span> <span class="n">rows</span><span class="o">=</span><span class="n">rows</span><span class="p">))</span>
</pre></div>
</div>
<p>The trick here is to get the template object from the Jinja2 environment
on the application and to call <tt class="xref py py-meth docutils literal"><span class="pre">stream()</span></tt> instead of
<tt class="xref py py-meth docutils literal"><span class="pre">render()</span></tt> which returns a stream object instead of a
string. Since we’re bypassing the Flask template render functions and
using the template object itself we have to make sure to update the render
context ourselves by calling <a class="reference internal" href="../api.html#flask.Flask.update_template_context" title="flask.Flask.update_template_context"><tt class="xref py py-meth docutils literal"><span class="pre">update_template_context()</span></tt></a>.
The template is then evaluated as the stream is iterated over. Since each
time you do a yield the server will flush the content to the client you
might want to buffer up a few items in the template which you can do with
<tt class="docutils literal"><span class="pre">rv.enable_buffering(size)</span></tt>. <tt class="docutils literal"><span class="pre">5</span></tt> is a sane default.</p>
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<h3><a href="../index.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">Streaming Contents</a><ul>
<li><a class="reference internal" href="#basic-usage">Basic Usage</a></li>
<li><a class="reference internal" href="#streaming-from-templates">Streaming from Templates</a></li>
</ul>
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<li><a href="../index.html">Documentation overview</a><ul>
<li><a href="index.html">Patterns for Flask</a><ul>
<li>Previous: <a href="favicon.html" title="previous chapter">Adding a favicon</a></li>
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