ImportError: No module named launch

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Hello, I ran ./file_name.py to run a python file on ros2 supported open source . But I am getting the following error: I followed the installation instructions in the Ros2 basic documentation and just run the file.

I'm running on Ubuntu 18.04 eloquent-devel and I added #!/usr/bin/env python to the first line of python.

to_laserscan_launch.py
Traceback (most recent call last):
  File "./sample_pointcloud_to_laserscan_launch.py", line 3, in <module>
    from launch import LaunchDescription
ImportError: No module named launch

The code below is the python code I want to run.

#!/usr/bin/python

from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument
from launch.substitutions import LaunchConfiguration
from launch_ros.actions import Node


def generate_launch_description():
    return LaunchDescription([
        DeclareLaunchArgument(
            name='scanner', default_value='scanner',
            description='Namespace for sample topics'
        ),
        Node(
            package='pointcloud_to_laserscan', executable='dummy_pointcloud_publisher',
            remappings=[('cloud', [LaunchConfiguration(variable_name='scanner'), '/cloud'])],
            parameters=[{'cloud_frame_id': 'cloud', 'cloud_extent': 2.0, 'cloud_size': 500}],
            name='cloud_publisher'
        ),
        Node(
            package='tf2_ros',
            executable='static_transform_publisher',
            name='static_transform_publisher',
            arguments=['0', '0', '0', '0', '0', '0', '1', 'map', 'cloud']
        ),
        Node(
            package='pointcloud_to_laserscan', executable='pointcloud_to_laserscan_node',
            remappings=[('cloud_in', [LaunchConfiguration(variable_name='scanner'), '/cloud']),
                        ('scan', [LaunchConfiguration(variable_name='scanner'), '/scan'])],
            parameters=[{
                'target_frame': 'cloud',
                'transform_tolerance': 0.01,
                'min_height': 0.0,
                'max_height': 1.0,
                'angle_min': -1.5708,  # -M_PI/2
                'angle_max': 1.5708,  # M_PI/2
                'angle_increment': 0.0087,  # M_PI/360.0
                'scan_time': 0.3333,
                'range_min': 0.45,
                'range_max': 4.0,
                'use_inf': True,
                'inf_epsilon': 1.0
            }],
            name='pointcloud_to_laserscan'
        )
    ])