title: Having problems declaring SUMO_HOME
body: <p>I'm trying to run a test python code to use the traci library and it is returning "please declare environment SUMO_HOME".</p>
  
  <p>I'm on Ubuntu 18.4.2 and Sumo 0.32.0.I solved this problem before by running 
      <code>export SUMO_HOME=/home/gustavo/Downloads/sumo-0.32.0/tools/</code>
  ,but this time it couldn't solve the problem. So I tried implementing a line inside the python file using the os library giving the same command but from the code itself:
      <code>os.system("export SUMO_HOME=/home/gustavo/Downloads/sumo-0.32.0/tool/")</code>
  And it also didn't work, so came here to ask for help. May any of you help me, please?</p>
  
  <pre><code>import os
  import sys
  import optparse
  
  os.system("export SUMO_HOME=/home/gustavo/Downloads/sumo-0.32.0/tool/")
  # we need to import some python modules from the $SUMO_HOME/tools directory
  if 'SUMO_HOME' in os.environ:
      tools = os.path.join(os.environ['SUMO_HOME=/home/gustavo/Downloads/sumo-0.32.0/tools/'], 'tools')
      sys.path.append(tools)
  else:
      sys.exit("please declare environment variable 'SUMO_HOME'")
  
  
  from sumolib import checkBinary  # Checks for the binary in environ vars
  import traci
  
  
  def get_options():
      opt_parser = optparse.OptionParser()
      opt_parser.add_option("--nogui", action="store_true",
                           default=False, help="run the commandline version of sumo")
      options, args = opt_parser.parse_args()
      return options
  
  
  # contains TraCI control loop
  def run():
      step = 0
      while traci.simulation.getMinExpectedNumber() &gt; 0:
          traci.simulationStep()
          print(step)
          step += 1
  
      traci.close()
      sys.stdout.flush()
  
  
  # main entry point
  if __name__ == "__main__":
      options = get_options()
  
      # check binary
      if options.nogui:
          sumoBinary = checkBinary('sumo')
      else:
          sumoBinary = checkBinary('sumo-gui')
  
      # traci starts sumo as a subprocess and then this script connects and runs
      traci.start([sumoBinary, "-c", "demo.sumocfg",
                               "--tripinfo-output", "tripinfo.xml"])
      run()
  </code></pre>
  
  <p>I expected for the steps to appear on the terminal.</p>