operator_simple.py

import bpy


class TEXT_Cycle_TextBlocks(bpy.types.Operator):

    bl_idname = "text.cycle_textblocks"
    bl_label = "switch text content of current viewer"
    bl_options = {'REGISTER', 'UNDO'}

    direction = bpy.props.IntProperty(default=-1)

    def execute(self, context):
        edit_text = bpy.context.edit_text

        texts = bpy.data.texts
        num_texts = len(texts)

        def get_index_of_text(text_name):
            for idx, text in enumerate(texts):
                if text.name == text_name:
                    return idx
        
        current_idx = get_index_of_text(edit_text.name)
        new_idx = (current_idx + self.direction) % num_texts
        context.space_data.text = texts[new_idx]

        return {'FINISHED'}


def register():
    bpy.utils.register_class(TEXT_Cycle_TextBlocks)


def unregister():
    bpy.utils.unregister_class(TEXT_Cycle_TextBlocks)


if __name__ == "__main__":
    register()