title: Python code can't use system variable
body: <p>I have the following part of my code where I have a problem:</p>
  
  <pre><code>import os, sys
  import optparse
  import subprocess
  import random
  
  # we need to import python modules from the $SUMO_HOME/tools directory
  try:
      sys.path.append(os.path.join(os.path.dirname(__file__), '..', '..', '..', '..', "tools")) # tutorial in tests
      sys.path.append(os.path.join(os.environ.get("$SUMO_HOME", os.path.join(os.path.dirname(__file__), "..", "..", "..")), "tools")) # tutorial in docs
      from sumolib import checkBinary
  except ImportError:
      sys.exit("please declare environment variable 'SUMO_HOME' as the root directory of your sumo installation (it should contain folders 'bin', 'tools' and 'docs')")
  
  import traci
  </code></pre>
  
  <p>I declared SUMO_HOME as a system variable, but when I run this script I get the ImportError. Do you have any idea what the problem is?</p>