def _entity_skill_id(skill_id):
    """Helper converting a skill id to the format used in entities.

    Arguments:
        skill_id (str): skill identifier

    Returns:
        (str) skill id on the format used by skill entities
    """
    skill_id = skill_id[:-1]
    skill_id = skill_id.replace('.', '_')
    skill_id = skill_id.replace('-', '_')
    return skill_id