title: Setting file on S3 as read-only with Boto
body: <p>How do you set a key on S3 using Boto so you won't accidentally overwrite it later? Is there a way to make it read-only or at least give you some sort of error if you eventually try to write to the same key again.</p>
  
  <p>I've tried changing ACLs, both the key's and the bucket's, and nothing seemed to make any difference.</p>
  
  <p>When I remove the <code>write</code> permission using DragonDisk (so it becomes <code>&lt;Policy: roddds (owner) = READ, roddds (owner) = READ_ACP&gt;</code>), but if I run <code>keyfile.set_contents_from_string()</code> it changes it back to full control.</p>
  
  <p>I am accessing the bucket as the owner.</p>