title: Python: subprocess with different working directory
body: <p>I have a python script that is under this directory:</p>
  
  <pre><code>work/project/test/a.py
  </code></pre>
  
  <p>Inside <code>a.py</code>, I use <code>subprocess.POPEN</code> to launch the process from another directory,</p>
  
  <pre><code>work/to_launch/file1.pl, file2.py, file3.py, ...
  </code></pre>
  
  <p>Python Code:</p>
  
  <pre><code>subprocess.POPEN("usr/bin/perl ../to_launch/file1.pl") 
  </code></pre>
  
  <p>and under work/project/, I type the following</p>
  
  <pre><code>[user@machine project]python test/a.py,
  </code></pre>
  
  <p>error "file2.py, 'No such file or directory'"</p>
  
  <p>How can I add <code>work/to_launch/</code>, so that these dependent files <code>file2.py</code> can be found? </p>