Event handling and a primitive paint application

A variety of keyboard and mouse events are recognized by OpenCV. We can view the list of events by following these instructions. Open the Python 3 Interpreter in interactive mode by running the python3 command on Command Prompt, and then run the following statements:

>>> import cv2
>>> events = [i for i in dir(cv2) if 'EVENT' in i]
>>> print(events)

It will show the following output:

['EVENT_FLAG_ALTKEY', 'EVENT_FLAG_CTRLKEY', 'EVENT_FLAG_LBUTTON', 'EVENT_FLAG_MBUTTON', 'EVENT_FLAG_RBUTTON', 'EVENT_FLAG_SHIFTKEY', 'EVENT_LBUTTONDBLCLK', 'EVENT_LBUTTONDOWN', 'EVENT_LBUTTONUP', 'EVENT_MBUTTONDBLCLK', 'EVENT_MBUTTONDOWN', 'EVENT_MBUTTONUP', 'EVENT_MOUSEHWHEEL', 'EVENT_MOUSEMOVE', 'EVENT_MOUSEWHEEL', 'EVENT_RBUTTONDBLCLK', 'EVENT_RBUTTONDOWN...