title: After starting process, how to get parent's PID in the child?
body: <p>In Python, I start a new process via <code>Popen()</code>, which works fine. Now in the child process I want to find the parent's process ID.</p>
  
  <p>What is the best way to achieve this, maybe I can pass the PID via the <code>Popen</code> constructor, but how? Or is there a better way to do so?</p>
  
  <p>PS: If possible I would prefere a solution using only standard libraries.</p>