Build an ArcPy FieldMappings object from source and target feature classes using field map tuples.

build_field_map

#
# fm_input is a field mapping tuple
# fm_input = (("target1","source1"),("target2","source2"),("target3","source3"))


def build_field_map(target, source, fm_input):

        field_mapps = arcpy.FieldMappings()
        field_mapps.addTable(target)

        for fm_item in fm_input:
            try:
              target_field, source_field = fm_item
              print_msg("target field: " + target_field + " source field: " + source_field)
              index = field_mapps.findFieldMapIndex(target_field)
              fmap = field_mapps.getFieldMap(index)
              fmap.removeAll()
              fmap.addInputField(source,source_field)
              field_mapps.replaceFieldMap(index,fmap)
              
            except Exception as e:
                if len(e.args) > 0:
                    print_msg(e.args[0])
                else:
                    print_msg("Error, exiting")
                    exit

        
        return field_mapps