elif event.type == KEYDOWN: if event.key == K_q: clickedButton = YELLOW elif event.key == K_w: clickedButton = BLUE elif event.key == K_a: clickedButton = RED elif event.key == K_s: clickedButton = GREEN
Lines 1  to 9  check for any
KEYDOWN events (created when the user presses a key on the keyboard). The Q, W, A, and S keys correspond to the buttons because they are arranged in a square shape on the keyboard.
The Q key is in the upper left of the four keyboard keys, just like the yellow button on the screen is in the upper left, so we will make pressing the Q key the same as clicking on the yellow button.
We can do this by setting the
clickedButton variable to the value in the constant variable
YELLOW. We can do the same for the three other keys. This way, the user can play Simulate with either the mouse or keyboard.