title: How to check if there exists a process with a given pid in Python?
body: <p>Is there a way to check to see if a pid corresponds to a valid process? I'm getting a pid from a different source other than from <code>os.getpid()</code> and I need to check to see if a process with that pid doesn't exist on the machine. </p>
  
  <p>I need it to be available in Unix and Windows. I'm also checking to see if the PID is NOT in use.</p>