def left_zfill(word, n_digits):
    """
    DESCRIPTION:
    Given a string a number, the function introduces zeros by the left side 
    until the string the size specified in the number.
    :param word: [string] the string that is to be extended. 
    :param n_digits: [int] the number of digits of the final word. 
    :return: [string] extended word. 
    """
    return '0' * (n_digits - len(word)) + word