title: Google App Engine - headers[] and headers.add_header() for cache control
body: <p>What is the proper way to set cache control?</p>
  
  <p>Sometimes I see the use of headers[]</p>
  
  <pre><code>self.response.headers["Pragma"]="no-cache"
  self.response.headers["Cache-Control"]="no-cache, no-store, must-revalidate, pre-check=0, post-check=0"
  self.response.headers["Expires"]="Thu, 01 Dec 1994 16:00:00"
  </code></pre>
  
  <p>Other times, I see headers.add_header()</p>
  
  <pre><code>self.response.headers.add_header("Pragma","no-cache")
  self.response.headers.add_header("Cache-Control","no-cache, no-store, must-revalidate, pre-check=0, post-check=0")
  self.response.headers.add_header("Expires","Thu, 01 Dec 1994 16:00:00")
  </code></pre>
  
  <p>And even a mix of both headers[] and headers.add_header()</p>
  
  <pre><code>self.response.headers["Pragma"]="no-cache"
  self.response.headers.add_header("Cache-Control","no-cache, no-store, must-revalidate, pre-check=0, post-check=0")
  self.response.headers.add_header("Expires","Thu, 01 Dec 1994 16:00:00")
  </code></pre>