code: def _obtain_lock_or_raise(self):
          
          if self._has_lock():
              return
          lock_file = self._lock_file_path()
          if os.path.isfile(lock_file):
              raise IOError("Lock for file %r did already exist, delete %r in case the lock is illegal" % (self._file_path, lock_file))
  
          try:
              fd = os.open(lock_file, os.O_WRONLY | os.O_CREAT | os.O_EXCL, 0)
              os.close(fd)
          except OSError,e:
              raise IOError(str(e))
  
          self._owns_lock = True
docs: Create a lock file as flag for other instances, mark our instance as lock-holder
  
          :raise IOError: if a lock was already present or a lock file could not be written