City functions relating to the assessment of property for taxation, the collection of taxes levied for municipal purposes, the collection of assessments, and the sale of property for the nonpayment of taxes or assessments, may be transferred pursuant to this article. Where a city transfers the assessment function to a county it may request the county to prepare only a secured roll in the first year of transfer and thereafter both the secured and unsecured rolls. Where a city which has been collecting taxes upon a calendar-year basis transfers such function to a county under this article, said collection by the county shall be continued upon such calendar-year basis.