Help Generating Gravity

import pygame, sys, glob
from pygame import *

h = 400
w = 800

screen = pygame.display.set_mode((w,h))

clock = pygame.time.Clock()

class player:
    def __init__(self):
        self.x = 200
        self.y = 300
        self.initial = 300
        self.ani_speed_init = 3
        self.ani_speed = self.ani_speed_init
        self.ani = glob.glob("p1_walk0*.png")
        self.ani.sort()
        self.ani_pos = 0
        self.ani_max = len(self.ani) - 1
        self.img = pygame.image.load(self.ani[0])
        self.update(0)
        self.max = 380

    def update(self, pos):
        if pos == 1 or pos == -1:
            self.ani_speed -= 1
            self.x += pos
            if self.ani_speed == 0:
                self.img = pygame.image.load(self.ani[self.ani_pos])
                self.ani_speed = self.ani_speed_init
                if self.ani_pos == self.ani_max:
                    self.ani_pos = 0
                else:
                    self.ani_pos += 1
        if pos == 2:
            self.y = max(200, self.y - 80)
        if pos == -2:
            self.y = min(self.initial, self.y + 5)



        screen.blit(self.img,(self.x,self.y))

player1 = player()
pos = 0
testkey = 0

while 1:
    screen.fill((0,0,0))
    clock.tick(60)

    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            pygame.quit()
            sys.exit()
        elif event.type == KEYDOWN and event.key == K_LEFT:
            pos = -1
        elif event.type == KEYUP and event.key == K_LEFT:
            pos = 0
        elif event.type == KEYDOWN and event.key == K_RIGHT:
            pos = 1
        elif event.type == KEYUP and event.key == K_RIGHT:
            pos = 0
        elif event.type == KEYDOWN and event.key == K_SPACE:
            pos = 2
        elif event.type == KEYUP and event.key == K_SPACE:
            pos = -2



    player1.update(pos)
    pygame.display.update()