code: def start(self):
          
          c_out, c_err = (open(self.path('cmd.stdout'), 'w'),
                          open(self.path('cmd.stderr'), 'w'))
          kw = self.kw.copy()
          kw['stdout'] = c_out
          kw['stderr'] = c_err
          if not kw.get('cwd', None):
              kw['cwd'] = os.getcwd()
          pr = subprocess.Popen(self.cmd_args, **kw)
          with open(self.path('cmd.pid'), 'w') as f:
              f.write(str(pr.pid))
docs: Start the subprocess.