python - How to fix character constantly accelerating in both directions after deceleration Pygame?

Question

I was able to successfully implement acceleration into my Pygame game, but there is a bit of a problem. Problem is: After the character decelerates, the character will start accelerating in one direction, decelerate and start accelerating in another direction, decelerate and start accelerating in opposite direction, repeating those two things endlessly. How would i get this to stop?

Heres the code i wrote for acceleration:

if move_R == True:
    accel = PLAYER_ACCEL
if move_L == True:
    accel = -PLAYER_ACCEL

accel += veloc * PLAYER_FRICT
veloc += accel
player_xy[0] += veloc + 0.5 * accel

Answer 1

The acceleration depends on the input only, if there is no input, the acceleration is zero:

accel = 0
if move_R:
    accel += PLAYER_ACCEL
if move_L:
    accel -= PLAYER_ACCEL

The velocity changes by the acceleration, but it is reduced by the friction.

veloc = (veloc + accel) * (1 - PLAYER_FRICT)

Note, at a certain point, the acceleration can not compensate the friction. The complete acceleration energy is consumed by the friction.
If veloc / (veloc + accel) is equal 1 - PLAYER_FRICT, the movement is uniform.

The position of the player changes by the current velocity:

player_xy[0] += veloc

---

Minimal example:

import pygame

pygame.init()

size = 500, 500
window = pygame.display.set_mode(size)
clock = pygame.time.Clock()

border = pygame.Rect(0, 0, size[0]-40, 100)
border.center = [size[0] // 2, size[1] // 2]
player_xy = [size[0] // 2, size[1] // 2]
radius = 10
PLAYER_ACCEL, PLAYER_FRICT = 0.5, 0.02
veloc = 0

run = True
while run:
    clock.tick(120)
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            run = False

    # set acceleration in this frame
    accel = 0
    keys = pygame.key.get_pressed()
    if keys[pygame.K_RIGHT]:
        accel += PLAYER_ACCEL
    if keys[pygame.K_LEFT]:
        accel -= PLAYER_ACCEL
    
    # change velocity by acceleration and reduce dependent on friction
    veloc = (veloc + accel) * (1 - PLAYER_FRICT)

    # change position of player by velocity
    player_xy[0] += veloc

    if player_xy[0] < border.left + radius:
        player_xy[0] = border.left + radius
        veloc = 0
    if player_xy[0] > border.right - radius:
        player_xy[0] = border.right - radius
        veloc = 0

    window.fill(0) 
    pygame.draw.rect(window, (255, 0, 0), border, 1)
    pygame.draw.circle(window, (0, 255, 0), (round(player_xy[0]), round(player_xy[1])), radius)
    pygame.display.flip()