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<section id="request-content-checksums">
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<h1>Request Content Checksums<a class="headerlink" href="#request-content-checksums" title="Link to this heading">¶</a></h1>
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<p>Various pieces of code can consume the request data and preprocess it.
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For instance JSON data ends up on the request object already read and
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processed, form data ends up there as well but goes through a different
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code path. This seems inconvenient when you want to calculate the
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checksum of the incoming request data. This is necessary sometimes for
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some APIs.</p>
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<p>Fortunately this is however very simple to change by wrapping the input
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stream.</p>
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<p>The following example calculates the SHA1 checksum of the incoming data as
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it gets read and stores it in the WSGI environment:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span><span class="w"> </span><span class="nn">hashlib</span>
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<span class="k">class</span><span class="w"> </span><span class="nc">ChecksumCalcStream</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
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<span class="k">def</span><span class="w"> </span><span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">stream</span><span class="p">):</span>
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<span class="bp">self</span><span class="o">.</span><span class="n">_stream</span> <span class="o">=</span> <span class="n">stream</span>
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<span class="bp">self</span><span class="o">.</span><span class="n">_hash</span> <span class="o">=</span> <span class="n">hashlib</span><span class="o">.</span><span class="n">sha1</span><span class="p">()</span>
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<span class="k">def</span><span class="w"> </span><span class="nf">read</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="nb">bytes</span><span class="p">):</span>
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<span class="n">rv</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">_stream</span><span class="o">.</span><span class="n">read</span><span class="p">(</span><span class="nb">bytes</span><span class="p">)</span>
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<span class="bp">self</span><span class="o">.</span><span class="n">_hash</span><span class="o">.</span><span class="n">update</span><span class="p">(</span><span class="n">rv</span><span class="p">)</span>
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<span class="k">return</span> <span class="n">rv</span>
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<span class="k">def</span><span class="w"> </span><span class="nf">readline</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">size_hint</span><span class="p">):</span>
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<span class="n">rv</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">_stream</span><span class="o">.</span><span class="n">readline</span><span class="p">(</span><span class="n">size_hint</span><span class="p">)</span>
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<span class="bp">self</span><span class="o">.</span><span class="n">_hash</span><span class="o">.</span><span class="n">update</span><span class="p">(</span><span class="n">rv</span><span class="p">)</span>
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<span class="k">return</span> <span class="n">rv</span>
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<span class="k">def</span><span class="w"> </span><span class="nf">generate_checksum</span><span class="p">(</span><span class="n">request</span><span class="p">):</span>
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<span class="n">env</span> <span class="o">=</span> <span class="n">request</span><span class="o">.</span><span class="n">environ</span>
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<span class="n">stream</span> <span class="o">=</span> <span class="n">ChecksumCalcStream</span><span class="p">(</span><span class="n">env</span><span class="p">[</span><span class="s1">'wsgi.input'</span><span class="p">])</span>
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<span class="n">env</span><span class="p">[</span><span class="s1">'wsgi.input'</span><span class="p">]</span> <span class="o">=</span> <span class="n">stream</span>
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<span class="k">return</span> <span class="n">stream</span><span class="o">.</span><span class="n">_hash</span>
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</pre></div>
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</div>
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<p>To use this, all you need to do is to hook the calculating stream in
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before the request starts consuming data. (Eg: be careful accessing
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<code class="docutils literal notranslate"><span class="pre">request.form</span></code> or anything of that nature. <code class="docutils literal notranslate"><span class="pre">before_request_handlers</span></code>
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for instance should be careful not to access it).</p>
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<p>Example usage:</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">'/special-api'</span><span class="p">,</span> <span class="n">methods</span><span class="o">=</span><span class="p">[</span><span class="s1">'POST'</span><span class="p">])</span>
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<span class="k">def</span><span class="w"> </span><span class="nf">special_api</span><span class="p">():</span>
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<span class="nb">hash</span> <span class="o">=</span> <span class="n">generate_checksum</span><span class="p">(</span><span class="n">request</span><span class="p">)</span>
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<span class="c1"># Accessing this parses the input stream</span>
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<span class="n">files</span> <span class="o">=</span> <span class="n">request</span><span class="o">.</span><span class="n">files</span>
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<span class="c1"># At this point the hash is fully constructed.</span>
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<span class="n">checksum</span> <span class="o">=</span> <span class="nb">hash</span><span class="o">.</span><span class="n">hexdigest</span><span class="p">()</span>
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<span class="k">return</span> <span class="sa">f</span><span class="s2">"Hash was: </span><span class="si">{</span><span class="n">checksum</span><span class="si">}</span><span class="s2">"</span>
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</pre></div>
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