export.py

import bpy


def write_some_data(context, filepath, use_some_setting):
    print("running write_some_data...")
    with open(filepath, 'w', encoding='utf-8') as f:
        current_object = bpy.context.scene.objects.active
        mesh = current_object.data
        f.write("local v = {\n")
        for i,vertex in mesh.vertices.items():
            x,y,z = vertex.co
            f.write("{{{:5f}, {:5f}, {:5f}}},\n".format(x,y,z))
    
        f.write("}\n")
        f.write("local f = {\n")        
    
        for i,face in mesh.polygons.items():
            a,b,c = face.vertices
            color_index = current_object.material_slots[face.material_index].material.get('pico8_index', 7)
            f.write("{{{}, {}, {}, {}}},\n".format(a+1, b+1, c+1, color_index))
    
        f.write("}\n")
        

    return {'FINISHED'}


# ExportHelper is a helper class, defines filename and
# invoke() function which calls the file selector.
from bpy_extras.io_utils import ExportHelper
from bpy.props import StringProperty, BoolProperty, EnumProperty
from bpy.types import Operator


class ExportSomeData(Operator, ExportHelper):
    """This appears in the tooltip of the operator and in the generated docs"""
    bl_idname = "export.pico8" 
    bl_label = "Export to Pico-8"

    # ExportHelper mixin class uses this
    filename_ext = ".txt"

    filter_glob = StringProperty(
            default="*.txt",
            options={'HIDDEN'},
            maxlen=255,  # Max internal buffer length, longer would be clamped.
            )
    )

    def execute(self, context):
        return write_some_data(context, self.filepath)


# Only needed if you want to add into a dynamic menu
def menu_func_export(self, context):
    self.layout.operator(ExportSomeData.bl_idname, text="Text Export Operator")


def register():
    bpy.utils.register_class(ExportSomeData)
    bpy.types.INFO_MT_file_export.append(menu_func_export)


def unregister():
    bpy.utils.unregister_class(ExportSomeData)
    bpy.types.INFO_MT_file_export.remove(menu_func_export)


if __name__ == "__main__":
    register()

    # test call
    bpy.ops.export.pico8('INVOKE_DEFAULT')