CustomNodeGroupTemplate

import bpy


class NODE_PT_MAINPANEL(bpy.types.Panel):
    bl_label = "Custom Node Group"
    bl_idname = "NODE_PT_MAINPANEL"
    bl_space_type = 'NODE_EDITOR'
    bl_region_type = 'UI'
    bl_category = 'New Tab'

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

        row = layout.row()
        row.operator('node.test_operator')





def create_test_group(context, operator, group_name):

        #enable use nodes
    bpy.context.scene.use_nodes = True

    test_group = bpy.data.node_groups.new(group_name, 'CompositorNodeTree')

    group_in = test_group.nodes.new('NodeGroupInput')
    group_in.location = (-200,0)
    test_group.inputs.new('NodeSocketFloat','Factor Value') #0
    test_group.inputs.new('NodeSocketColor','Color Input') #1


    group_out = test_group.nodes.new('NodeGroupOutput')
    group_out.location = (400,0)
    test_group.outputs.new('NodeSocketColor','Output')


    mask_node = test_group.nodes.new(type= 'CompositorNodeBoxMask')
    mask_node.location = (0,0)
    mask_node.rotation = 1

    mix_node = test_group.nodes.new(type= 'CompositorNodeMixRGB')
    mix_node.location = (200,0)
    mix_node.use_clamp = True
    mix_node.blend_type = 'OVERLAY'


    link = test_group.links.new

    link(mask_node.outputs[0], mix_node.inputs[1])

    link(group_in.outputs[0], mix_node.inputs[0])
    link(group_in.outputs[1], mix_node.inputs[2])

    link(mix_node.outputs[0], group_out.inputs[0])

    return test_group





class NODE_OT_TEST(bpy.types.Operator):
    bl_label = "Add Custom Node Group"
    bl_idname = "node.test_operator"

    def execute(self, context):

        custom_node_name = "Test Node"
        my_group = create_test_group(self, context, custom_node_name)
        test_node = context.scene.node_tree.nodes.new('CompositorNodeGroup')
        test_node.node_tree = bpy.data.node_groups[my_group.name]
        test_node.use_custom_color = True
        test_node.color = (0.5, 0.4, 0.3)

        return {'FINISHED'}





def register():
    bpy.utils.register_class(NODE_PT_MAINPANEL)
    bpy.utils.register_class(NODE_OT_TEST)





def unregister():
    bpy.utils.unregister_class(NODE_PT_MAINPANEL)
    bpy.utils.unregister_class(NODE_OT_TEST)


if __name__ == "__main__":
    register()