title: ConfigParser and String interpolation with env variable
body: <p>it's a little bit I'm out of python syntax and I have a problem in reading a <code>.ini</code> file with interpolated values.</p>
  
  <p>this is my ini file:</p>
  
  <pre><code>[DEFAULT]
  home=$HOME
  test_home=$home
  
  [test]
  test_1=$test_home/foo.csv
  test_2=$test_home/bar.csv
  </code></pre>
  
  <p>Those lines</p>
  
  <pre><code>from ConfigParser import SafeConfigParser
  
  parser = SafeConfigParser()
  parser.read('config.ini')
  
  print parser.get('test', 'test_1')
  </code></pre>
  
  <p>does output </p>
  
  <pre><code>$test_home/foo.csv
  </code></pre>
  
  <p>while I'm expecting </p>
  
  <pre><code>/Users/nkint/foo.csv
  </code></pre>
  
  <h3>EDIT:</h3>
  
  <p>I supposed that the <code>$</code> syntax was implicitly included in the so called <strong>string interpolation</strong> (referring to the <a href="https://docs.python.org/2/library/configparser.html" rel="noreferrer">manual</a>):</p>
  
  <blockquote>
    <p>On top of the core functionality, SafeConfigParser supports
    interpolation. This means values can contain format strings which
    refer to other values in the same section, or values in a special
    DEFAULT section.</p>
  </blockquote>
  
  <p>But I'm wrong. How to handle this case?</p>