3dview_panel.py

import bpy


class LayoutDemoPanel(bpy.types.Panel):
    """Creates a Panel in the scene context of the properties editor"""
    bl_label = "Layout Demo"
    bl_idname = "SCENE_PT_layout"
    bl_space_type = 'VIEW_3D'
    bl_region_type = 'UI'
    bl_context = "scene"

    def draw(self, context):
        layout = self.layout

        scene = context.scene

        # Big render button
        row = layout.row()
        row.scale_y = 3.0
        row.operator("render.render", text='start')

        # Big render button
        row = layout.row()
        row.scale_y = 3.0
        row.operator("render.render", text='stop')


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


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


if __name__ == "__main__":
    register()