def format_filters(variant):
  """Gets a human-readable string showing the filters applied to variant.

  Returns a string with the filter field values of variant separated by commas.
  If the filter field isn't set, returns '.'.

  Args:
    variant: third_party.nucleus.protos.Variant.

  Returns:
    A string.
  """
  return ','.join(variant.filter) if variant.filter else '.'