How can I stop my player from glitching through my platform?

Question

So I'm making a small project and I can't understand why my player keeps going up and down when it collides with my platform even when I want it to be static. The problem is in the code below and I can't find a fix.

def Collision_Manager():
    if player.rect.colliderect(platform.rect):
        if abs(player.rect.bottom >= platform.rect.top):
            player.y = platform.y - player.height

I cant understand why it's not a seamless animation like my border system which I did by doing this.

def player_borders():
    if player.x <= 0:
        player.x = 0
    elif player.x >= 870:
        player.x = 870
    if player.y <= 0:
        player.y = 0
    elif player.y >= 570:
        player.y = 570

Answer 1

player.y and platform.y are floating point coordinates. pygame.Rect objects however can just store integral coordinates.

The issue is caused when the .rect attribute is updated with the player's position and the coordinates are truncated. e.g.:

self.rect.topleft = int(self.x), int(self.y)

A possible solution might be to round the coordinates:

self.rect.topleft = round(self.x), round(self.y)

Or round up the y coordinate:

self.rect.topleft = round(self.x), int(math.ceil(self.y))

---

Depending on the control flow of your code, it may also be necessary to update the .rect attribute when a collision is detected:

if player.rect.colliderect(platform.rect):
    if player.rect.bottom >= platform.rect.top:
        platform.rect.bottom = platform.rect.top
        player.y = platform.rect.y

By the way, abs() is completely useless here:

if abs(player.rect.bottom >= platform.rect.top):

if player.rect.bottom >= platform.rect.top:

It doesn't any harm, however, since it's abs(True) or abs(False), which results in 1 or 0.

---

Minimal example (see also Platform collision):

import pygame
import random

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

class Player(pygame.sprite.Sprite):
    JUMP_ACCELERATION = -7
    def __init__(self, midbottom_pos):
        super().__init__() 
        self.image = pygame.Surface((30, 30), pygame.SRCALPHA)
        self.image.fill("red") 
        self.rect = self.image.get_rect(midbottom = midbottom_pos)
        self.y = self.rect.bottom
        self.vel_y = 0
        self.acc_y = 0
    def jump(self):
        self.acc_y = Player.JUMP_ACCELERATION
    def update(self, platforms):
        self.vel_y += GRAVITY
        self.y += self.vel_y
        self.rect.bottom = round(self.y + 1)
        colliders = pygame.sprite.spritecollide(self, platforms, False)
        if colliders:
            self.y = colliders[0].rect.top
            self.vel_y = self.acc_y
            self.y += self.vel_y
        elif self.y > window.get_height():
            self.y = 100
        self.acc_y = 0
        self.rect.bottom = round(self.y)

class Platform(pygame.sprite.Sprite):
    PLATFORM_SHIFT = 1.5
    def __init__(self, topleft_pos):
        super().__init__() 
        self.image = pygame.Surface((200, 10), pygame.SRCALPHA)
        self.image.fill("gray") 
        self.rect = self.image.get_rect(topleft = topleft_pos)
        self.x = self.rect.x
    def update(self):
        self.x -= Platform.PLATFORM_SHIFT
        self.rect.x = round(self.x)
        if self.rect.right <= 0:
            self.kill()

GRAVITY = 0.2
platfrom_group = pygame.sprite.Group(Platform((0, 250)))
player = Player((50, 100))
plyer_sprites = pygame.sprite.GroupSingle(player)

run = True
while run:
    clock.tick(100)
    acc_y = GRAVITY
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            run = False
        if event.type == pygame.KEYDOWN: 
            if event.key == pygame.K_SPACE:
                player.jump()

    platfrom_group.update()
    while platfrom_group.sprites()[-1].rect.right < window.get_width():
        last_platform_rect = platfrom_group.sprites()[-1].rect
        if last_platform_rect.top - 50 < 150:
            new_y = last_platform_rect.top + 50
        elif last_platform_rect.top + 50 > window.get_height() - 50:
            new_y = last_platform_rect.top - 50
        else:
            new_y = last_platform_rect.top + (50 if random.random() > 0.5 else -50)
        platfrom_group.add(Platform((last_platform_rect.right, new_y)))
        print(len(platfrom_group.sprites()))
    player.update(platfrom_group)

    window.fill((0, 0, 64))
    platfrom_group.draw(window)
    plyer_sprites.draw(window)
    pygame.display.flip()

pygame.quit()
exit()