title: Google OAuth2 Service Accounts authorization
body: <p>I'm trying to perform Google Drive API calls using "Service Accounts" authorization.</p>
  
  <p>I'm calling the API, but <code>files.list</code> returns me empty set, meanwhile the drive is not empty.
  I guess I should specify <code>prn</code> field as well in order to perform call on behalf of the concrete user (I don't really get who is it called for when <code>prn</code> is not specified). The problem is that when I specify <code>prn</code> authorization returns me <code>access_denied</code> error.</p>
  
  <pre><code>{
    "error" : "access_denied"
  }
  </code></pre>
  
  <p>Documentation says that I should enable client API access from control panel
  <a href="http://support.google.com/a/bin/answer.py?hl=en&amp;answer=162106" rel="nofollow noreferrer">http://support.google.com/a/bin/answer.py?hl=en&amp;answer=162106</a></p>
  
  <p>Here is what I've done, but this doesn't help much, it still gives me <code>access_denied</code>.</p>
  
  <p><img src="https://i.stack.imgur.com/qv6Pz.png" alt="&#39;Authorized API clients&#39; screenshot"></p>
  
  <p>What's wrong here?</p>