Before entering upon the duties of his office, the county assessor shall take and subscribe the following oath or affirmation: 'I, ... having been elected assessor of ... county, state of Wyoming, do solemnly swear (or affirm) that I will faithfully and impartially perform the duties of assessor of the county of ..., state of Wyoming, according to law and to the best of my ability, and that I will without fear or favor assess all taxable property within the county of ..., at its fair value. So help me God'.