How to make a character stop falling after hitting a certain platform

Question

I am writing a game using python's Pygame module. In this game you need to jump on platforms while avoiding falling out of the screen. I borrowed the code from a previous game so some of the variables don't match up. Nevertheless, I want to make it so that if you are on top of a platform, you can stand on it, but I cant get the character to stand on the platforms, it just falls.

Here is my code:

    # Import modules and initialize
import pygame as p
import sys
import random
p.init()
# Define variables
player_movement = 0
GRAVITY = 0.25
asteroid_height = [650, 625]
score = 0
high_score = 0
game_font = p.font.Font('fonts/PressStart2P-Regular.ttf',20)
game_active = False
FPS = p.time.Clock()
# Make some functions
def create_asteroid():
    random_asteroid_pos = random.choice(asteroid_height)
    new_asteroid = asteroid_surface.get_rect(midtop = (650, random_asteroid_pos))
    return new_asteroid
def move(asteroids):
    for asteroid in asteroids:
        asteroid.centerx -= 5
    return asteroids
def draw(asteroids):
    for asteroid in asteroids:
        sc.blit(asteroid_surface, asteroid)
def collision(asteroids):
    for asteroid in asteroids:
        if player_rect.colliderect(asteroid):
            GRAVITY = 0
            player_movement += 1
        else:
            GRAVITY = 0.25
            player_movement = 0
        if player_rect.bottom >= 600:
            GRAVITY = -1
        else:
            GRAVITY = 0.25
    return True
def display_score(game_state):
    if game_state == 'in_game':
        score_surface = game_font.render(f'Score: {int(score)}',True, (255,255,255))
        score_rect = score_surface.get_rect(center = (110, 30))
        sc.blit(score_surface, score_rect)
    elif game_state == 'game_over':
        high_score_surface = game_font.render(f'High Score: {int(high_score)}',True, (255,255,255))
        high_score_rect = high_score_surface.get_rect(center = (320, 370))
        sc.blit(high_score_surface, high_score_rect)
def update_score(score, high_score):
    if score > high_score:
        high_score = score
    return high_score
# Make a screen
sc = p.display.set_mode((700, 700))
p.display.set_caption('The Adventures of Cubeman')
# Make a backround
bg = p.image.load('images/backround.png')
bg = p.transform.scale2x(bg)
# Put in main music
# Play main music
# Import character
ship = p.image.load('images/cube.png').convert()
# Import asteroid
asteroid_surface = p.image.load('images/platform.png').convert()
# Asteroid logic
asteroid_list = []
SPAWN = p.USEREVENT
p.time.set_timer(SPAWN, 1500)
# Hitbox variables
player_rect = ship.get_rect(center = (100, 350))
# Sound FX
jump_sound = p.mixer.Sound('sound/jump.wav')
hit_sound = p.mixer.Sound('sound/impact.wav')
# Begin code logic
while True:
    # Make the game exit cleanly
    for event in p.event.get():
        if event.type == p.QUIT:
            p.quit()
            sys.exit()
        if event.type == p.KEYDOWN:
            if event.key == p.K_SPACE and game_active:
                player_movement = 0
                player_movement -= 6
                jump_sound.play()
            if event.type == p.KEYDOWN:
                if event.key == p.K_SPACE and game_active == False:
                    game_active = True
                    asteroid_list.clear()
                    player_movement = 0
                    score = 0
                    player_rect.center = (100, 350)
        if event.type == SPAWN:
            asteroid_list.append(create_asteroid())
        # INDENTATION

    #<--|

    sc.blit(bg, (0, 0))
    if game_active:
        player_movement += GRAVITY
        player_rect.centery += player_movement
        # Draw asteroids and player
        sc.blit(ship,player_rect)
        asteroid_list = move(asteroid_list)
        draw(asteroid_list)
        game_active = collision(asteroid_list)
        # Score system
        score += 0.01
        display_score('in_game')
        # Game info
        info_surface = game_font.render('SPACE to move!',True, (255,255,255))
        info_rect = info_surface.get_rect(center = (550, 30))
        sc.blit(info_surface, info_rect)
    else:
        high_score = update_score(score, high_score)
        display_score('game_over')
        # Make the main menu/game over screen
        game_over_surface = game_font.render('The Adventures of Cubeman',True, (255,255,255))
        game_over_rect = game_over_surface.get_rect(center = (320, 330))
        sc.blit(game_over_surface, game_over_rect)
        game_over_surface1 = game_font.render('SPACE to play!',True, (255,255,255))
        game_over_rect1 = game_over_surface1.get_rect(center = (320, 350))
        sc.blit(game_over_surface1, game_over_rect1)
    FPS.tick(120)
    p.display.update()

Any help is appreciated!