title: What is the type of os.environ? and Why does it not support viewkeys method
body: <p>If I print <code>os.environ</code>, the output looks like a dictionary. Some posts I read online say that it is a memory based dictionary. 
  But it does not support <code>.viewkeys()</code> method and tells me that: <code>_Environ instance does not support this method</code>. So what is the exact type of <code>os.environ</code>. If I try:</p>
  
  <pre><code>print type(os.environ)
  </code></pre>
  
  <p>I get <code>instance</code> as the answer.</p>
  
  <p>Can please clarify this behavior of <code>os.environ</code>?</p>