ModuleNotFoundError: No module named 'rospy'

Course Support ROS Prerequisites Exam
1

I just got started on the Python section from the ROS Prerequisites Exam. Everytime i try to execute something that uses the robot_control_class.py i get this error, that it connot find rospy

user:~$ /usr/bin/python3 /home/user/catkin_ws/src/python_exam/robot_control_class.py
Traceback (most recent call last):
  File "/home/user/catkin_ws/src/python_exam/robot_control_class.py", line 3, in <module>
    import rospy
ModuleNotFoundError: No module named 'rospy'

or from another file with import:

user:~$ /usr/bin/python3 /home/user/catkin_ws/src/python_exam/task1.py
Traceback (most recent call last):
  File "/home/user/catkin_ws/src/python_exam/task1.py", line 1, in <module>
    from robot_control_class import RobotControl
  File "/home/user/catkin_ws/src/python_exam/robot_control_class.py", line 3, in <module>
    import rospy
ModuleNotFoundError: No module named 'rospy'
2

rospy is the Python client library, which is installed in the system when ROS is installed. Try sourcing the ROS installation with

source /opt/ros/noetic/setup.bash

and run again