How to make gravity function in pygame?

Question

I was trying to create a simple physics system in pygame. My main goal was to add a Y gravity function that would move the player when activated. I have tried several times to make it work, but I guess I am missing something.

Here is the code:

class Physic:
    def __init__(self, color, master, type, sizeX, sizeY, pos=[0,0]):
        self.master = master
        self.color = color
        self.type = type
        self.sizeX = sizeX
        self.sizeY = sizeY
        self.x = pos[0]
        self.y = pos[1]

        self.moveX = 0
        self.moveY = 0

        self.create_Object()


    def create_Object(self):
        if self.type == 'rect':
            self.rect()

    def rect(self):
        return pygame.draw.rect(self.master, self.color, (self.x, self.y, self.sizeX, self.sizeY))

    def drag(self):
        for event in pygame.event.get():
            if event.type == pygame.mouse.get_pressed()[0]:
                self.x = pygame.mouse.get_pos()

    def add_gravity(self):
        self.moveY += 3.2
        self.update_pos()

    def update_pos(self):
        self.y += self.moveY
        self.x += self.moveX

---

In the main script I put this:

def game():
    while True:
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                sys.exit()


        screen.fill(WHITE)

        player = object.Physic(BLUE, screen, 'rect', 50, 50, [POS[0], POS[1]])
        player.add_gravity()

        # platform = object.rect(screen, RED, [30, 30], 100, 30)

        # physics.add_collider(player, platform, POS[0])

        pygame.display.update()


game()

Do you know what I am missing?

Answer 1

Your big problem is that you are recreating the player every pass inside the main loop and so it looks like it is frozen in place.

You also need to have a limit on the frame rate so that you control the speed of the game and therefor can properly set the acceleration per frame.

There are a some other minor things that needed to be fixed to run this. I tried to change the minimum possible to run it, since the point was to fix the error rather than rewrite it on you. Obviously I had to add some wrapper code around it

Try this slightly adjusted version:

#!/usr/bin/env python

import traceback
import pygame
import sys

FRAME_RATE = 60
GRAVITY = 32

SCREEN_SIZE = (600, 800)

WHITE = pygame.Color("white")
BLUE = pygame.Color("blue")

POS = (100, 600)

class Physic:
    def __init__(self, color, master, type, sizeX, sizeY, pos=(0,0)):
        self.master = master
        self.color = color
        self.type = type
        self.sizeX = sizeX
        self.sizeY = sizeY
        self.x = pos[0]
        self.y = pos[1]

        self.moveX = 0
        self.moveY = 0

        self.create_Object()


    def create_Object(self):
        if self.type == 'rect':
            self.draw()

    def draw(self):
        return pygame.draw.rect(self.master, self.color, (self.x, self.y, self.sizeX, self.sizeY))

    def drag(self):
        for event in pygame.event.get():
            #if event.type == pygame.mouse.get_pressed()[0]:    <--- remove this
            if event.type == pygame.MOUSEBUTTONDOWN:
                self.x = pygame.mouse.get_pos()

    def add_gravity(self):
        self.moveY += GRAVITY / FRAME_RATE
        self.update_pos()

    def update_pos(self):
        self.y += self.moveY
        self.x += self.moveX

def game():
    
    pygame.init()
    screen = pygame.display.set_mode(SCREEN_SIZE)

    clock = pygame.time.Clock()
    
    player = Physic(BLUE, screen, 'rect', 50, 50, POS)
    # I added this to illustrate the gravity better ... going up and down
    player.moveY = -25
    player.moveX = 2

    while True:
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                return


        screen.fill(WHITE)

        player.add_gravity()

        player.draw()

        # platform = object.rect(screen, RED, [30, 30], 100, 30)


        # physics.add_collider(player, platform, POS[0])

        pygame.display.update()

        clock.tick(FRAME_RATE)



def main():
    
    try:
        game()

    except Exception as ex:
        print(traceback.format_exc())
        raise

    finally:
        # game exit cleanup stuff
        pygame.quit()


if __name__ == '__main__':
    main()

An issue that I want to point out though it is not affecting this code here. You should not use an immutable object (like a list) as an default when defining an optional/named argument in a method. I.e. in the Physic __init__() I changed pos=[0,0] to pos=(0,0). Not a big deal here, but can cause really odd bugs if you had assigned it to a var, then tried to change it. It will have effects on other instances of the object because they actually share the default initialization object and if it gets modified by one of them it happens in all of them!