def _list_to_string(l, s):
  """Concatenates list items into a single string separated by `s`.

  Args:
    l: List with items to be concatenated into a single string.
    s: String or char that will be concatenated in between each item.

  Returns:
    String that has all items in list `l` concatenated with `s` separator.
  """

  return s.join(l)