Update ros2_launch_gazebo.md with a Python launch example and typo fixes/formatting for the XML launch file example. (#482)

Signed-off-by: David Dorf <113081373+david-dorf@users.noreply.github.com>
Co-authored-by: Addisu Z. Taddese <addisu@openrobotics.org>

Author: david-dorf

harmonic/ros2_launch_gazebo.md

@@ -26,9 +26,10 @@ to learn about the different parameters that are accepted by each launch file.
 
 ## Using a custom launch file.
 
+### XML
 It's also possible to start Gazebo from your custom launch file. For that
-purpose we have created the custom `<gz_server/>` tag that can be used from you
-XML or YAML launch file. In this case, the arguments are passed as attributes
+purpose we have created the custom `<gz_server/>` tag that can be used from your
+XML launch file. In this case, the arguments are passed as attributes
 within this tag. Here's an example for launching Gazebo server:
 
 ```xml
@@ -49,3 +50,53 @@ within this tag. Here's an example for launching Gazebo server:
 In this case the `<gz_server>` parameters are read from the command line. That's
 an option but not strictly necessary as you could decide to hardcode some of the
 values.
+
+### Python
+Python launch files provide more low-level customization and logic compared to XML launch files. 
+In the following example, the user can specify a world argument to launch an environment for
+the Moon, Mars, or Enceladus. It additionally sets the resource path environment variable and
+sets up empty arrays for topics to be bridged and remapped from Gazebo to ROS 2:
+```python
+from ament_index_python.packages import get_package_share_directory
+from launch import LaunchDescription
+from launch.actions import (DeclareLaunchArgument, SetEnvironmentVariable, 
+                            IncludeLaunchDescription, SetLaunchConfiguration)
+from launch.substitutions import PathJoinSubstitution, LaunchConfiguration, TextSubstitution
+from launch_ros.actions import Node
+from launch.launch_description_sources import PythonLaunchDescriptionSource
+
+
+def generate_launch_description():
+    pkg_ros_gz_sim = get_package_share_directory('ros_gz_sim')
+    pkg_spaceros_gz_sim = get_package_share_directory('spaceros_gz_sim')
+    gz_launch_path = PathJoinSubstitution([pkg_ros_gz_sim, 'launch', 'gz_sim.launch.py'])
+    gz_model_path = PathJoinSubstitution([pkg_spaceros_gz_sim, 'models'])
+
+    return LaunchDescription([
+        DeclareLaunchArgument(
+            'world',
+            default_value='moon',
+            choices=['moon', 'mars', 'enceladus'],
+            description='World to load into Gazebo'
+        ),
+        SetLaunchConfiguration(name='world_file', 
+                               value=[LaunchConfiguration('world'), 
+                                      TextSubstitution(text='.sdf')]),
+        SetEnvironmentVariable('GZ_SIM_RESOURCE_PATH', gz_model_path),
+        IncludeLaunchDescription(
+            PythonLaunchDescriptionSource(gz_launch_path),
+            launch_arguments={
+                'gz_args': [PathJoinSubstitution([pkg_spaceros_gz_sim, 'worlds',
+                                                  LaunchConfiguration('world_file')])],
+                'on_exit_shutdown': 'True'
+            }.items(),
+        ),
+        Node(
+            package='ros_gz_bridge',
+            executable='parameter_bridge',
+            arguments=[],
+            remappings=[],
+            output='screen'
+        ),
+    ])
+```