Each district attorney in this state may appoint one or more suitable persons who shall be attorneys-at-law practicing their profession in this state and members of the bar of this state to be his assistants. Every appointment of an assistant district attorney shall be in writing under the hand of the district attorney and filed in the office of the clerk of the district court of the judicial district in which the district attorney resides, and the person so appointed shall take and file in the office of the clerk of the district court of the judicial district in which the district attorney resides an oath of office as is now prescribed by law for district attorneys before entering upon his duties as assistant district attorney. Every such appointment may be revoked by the district attorney making it, which revocation shall be in writing and filed in that clerk’s office. The assistant district attorney may attend the meetings of the boards of county commissioners, the district court, metropolitan, magistrate and probate courts in the district attorney’s district and therein discharge any duties imposed by law upon or required of the district attorney by whom he was appointed.