Python launch file to start a simulated world and include sdf models (does not spawn robot)

CMakeLists.txt

cmake_minimum_required(VERSION 3.8)
project(my_package)

if(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
  add_compile_options(-Wall -Wextra -Wpedantic)
endif()

# find dependencies
find_package(ament_cmake REQUIRED)
# uncomment the following section in order to fill in
# further dependencies manually.
# find_package(<dependency> REQUIRED)

install(DIRECTORY 
  launch
  worlds
  models
  DESTINATION share/${PROJECT_NAME})

ament_package()

my_custom_scene.world

<?xml version="1.0" ?>

<sdf version="1.6">
  <world name="my_demo">

   <!-- shadows, ambient, grid, origin -->
    <scene>
      <shadows>false</shadows>
      <ambient>0.3 0.3 0.3 1</ambient>
      <background>1 1 1 1</background>
      <grid>false</grid>
      <origin_visual>false</origin_visual>
    </scene>

    <gui fullscreen='0'>
      <camera name='user_camera'>
        <pose>-2.0736 -2.82419 3.5577 -0 0.629796 0.656</pose>
        <view_controller>orbit</view_controller>
        <projection_type>perspective</projection_type>
      </camera>
    </gui>

    <include>
      <uri>model://sun</uri>
    </include>
    
    <include>
      <uri>model://ground_plane</uri>
    </include>

        <model name="cafe_table">
            <static>true</static>
            <include>
                <uri>model://cafe_table</uri>
                <pose>1.5 -0.5 0 0 0 0</pose>
            </include>
        </model>

        <model name="cardboard_box_1">
            <static>true</static>
            <include>
                <uri>model://cardboard_box</uri>
                <pose>1.5 -0.5 0.924471 0 0 0</pose>
            </include>
        </model>

        <model name="cafe_table2">
            <static>true</static>
            <include>
                <uri>model://cafe_table</uri>
                <pose>0.5 1.5 0 0 0 0</pose>
            </include>
        </model>

        <model name="cardboard_box_2">
            <static>true</static>
            <include>
                <uri>model://cardboard_box</uri>
                <pose>0.5 1.5 0.924471 0 0 0</pose>
            </include>
        </model>

  </world>
</sdf>

start_simulation.launch.py

#!/usr/bin/python3
# -*- coding: utf-8 -*-
import os

from ament_index_python.packages import get_package_share_directory
from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument
from launch.actions import IncludeLaunchDescription
from launch.launch_description_sources import PythonLaunchDescriptionSource
from ament_index_python.packages import get_package_prefix


def generate_launch_description():

    pkg_gazebo_ros = get_package_share_directory('gazebo_ros')
    package_name = "my_package"
    pkg_share_directory = get_package_share_directory('my_package')

    # We get the whole install dir
    # We do this to avoid having to copy or softlink manually the packages so that gazebo can find them
    install_dir = get_package_prefix(package_name)

    # Set the path to the WORLD model files. Is to find the models inside the models folder in my_package package
    gazebo_models_path = os.path.join(pkg_share_directory, 'models')
    # os.environ["GAZEBO_MODEL_PATH"] = gazebo_models_path

    if 'GAZEBO_MODEL_PATH' in os.environ:
        os.environ['GAZEBO_MODEL_PATH'] = os.environ['GAZEBO_MODEL_PATH'] + \
            ':' + install_dir + '/share' + ':' + gazebo_models_path
    else:
        os.environ['GAZEBO_MODEL_PATH'] = install_dir + \
            "/share" + ':' + gazebo_models_path

    if 'GAZEBO_PLUGIN_PATH' in os.environ:
        os.environ['GAZEBO_PLUGIN_PATH'] = os.environ['GAZEBO_PLUGIN_PATH'] + \
            ':' + install_dir + '/lib'
    else:
        os.environ['GAZEBO_PLUGIN_PATH'] = install_dir + '/lib'

    print("GAZEBO MODELS PATH=="+str(os.environ["GAZEBO_MODEL_PATH"]))
    print("GAZEBO PLUGINS PATH=="+str(os.environ["GAZEBO_PLUGIN_PATH"]))

    # Gazebo launch
    gazebo = IncludeLaunchDescription(
        PythonLaunchDescriptionSource(
            os.path.join(pkg_gazebo_ros, 'launch', 'gazebo.launch.py'),
        )
    )

    return LaunchDescription([
        DeclareLaunchArgument(
            'world',
            default_value=[os.path.join(
                pkg_share_directory, 'worlds', 'my_custom_scene.world'), ''],
            description='SDF world file'),
        gazebo
    ])