def to_gtin14(value):
    """ Convert various product codes to  to GTIN-14.

    Will convert UPC-A (GTIN-12), GTIN-13 (EAN / UCC-13), GTIN-8 (EAN / UCC-8)
    to GTIN-14 by left zero padding.

    See http://www.gtin.info/

    Returns:
      (str): The GTIN-14 code.
    """
    return str(value).zfill(14)
