Contracts entered into by a municipality shall be in writing, signed and executed in the name of the city or town by the officers authorized to make the same and by the party contracting. In cases not otherwise directed by law or ordinance, such contracts shall be entered into and executed by the mayor in the name of the city or town and all obligations for the payment of money by the municipality, except for bonds and interest coupons, shall be attested by the clerk. This section shall not be construed to cover purchases for the ordinary needs of the municipality.