Let's create a virtual environment named venv for all this Python magic:
sudo apt install python3-venv
python3 -m venv venv
Now let's activate the Python virtual environment so we can install all sorts of things into it without breaking our system:
. venv/bin/activate
pip install flask python-dotenv
Now run the flask+rclpy demo program in the venv, assuming you still have a shell with the venv activated in it:
. /opt/ros/foxy/setup.bash
./run.sh
In another shell, start publishing stuff onto /topic using a sample rclpy program:
. /opt/ros/foxy/setup.bash
ros2 run examples_rclpy_minimal_publisher publisher_member_function
In another terminal, query the latest message via flask whenever you want:
curl http://localhost:5000/latest_message
In another terminal, echo /flask_pub_topic:
. /opt/ros/foxy/setup.bash
ros2 topic echo /flask_pub_topic
In another terminal, ask Flask to publish a ros2 message onto that topic:
curl http://localhost:5000/publish_message