The style of ordinances of a city of the first class shall be: "Be it ordained by the mayor and city council of the city of ................," and all ordinances of a general nature shall, within fifteen days after they are passed, be published in a legal newspaper in or of general circulation within the city, or in book, pamphlet, or electronic form, to be distributed or sold, as may be provided by ordinance. Every ordinance fixing a penalty or forfeiture for its violation shall, before the ordinance takes effect, be published for at least one week in the manner prescribed in this section. In cases of riots, infectious diseases, or other impending danger, or any other emergency requiring its immediate operation, such ordinance shall take effect upon the proclamation of the mayor immediately upon its first publication as provided in this section.