Broken Snake Code

from OpenGL.GL import *
from OpenGL.GLUT import *
from OpenGL.GLU import *
from random import randint

window = 0
width,height = 500,500
field_width, field_height = 50,50
snake = [(20,20)]
snake_dir = (1,0)
interval = 200
food = []

def refresh2d_custom(width,height, internal_width, internal_height):
    glViewport(0,0,width, height)
    glMatrixMode(GL_PROJECTION)
    glLoadIdentity()
    glOrtho(0.0, internal_width, 0.0, internal_height, 0.0, 1.0)
    glMatrixMode(GL_MODELVIEW)
    glLoadIdentity()
    
def draw_rect(x,y,width,height):
    glBegin(GL_QUADS)
    glVertex2f(x,y)
    glVertex2f(x+width, y)
    glVertex2f(x+width, y + height)
    glVertex2f(x, y + height)
    glEnd()
    
def draw():
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT)
    glLoadIdentity()
    refresh2d_custom(width, height, field_width, field_height)
    draw_food()
    draw_snake()
    glutSwapBuffers()

def draw_snake():
    glColor3f(1.0,1.0,1.0)
    for x,y in snake:
        draw_rect(x,y, 1,1)

def draw_food():
    glColor3f(0.5,0.5, 1.0)
    for x,y in food:
        draw_rect(x,y,1,1)
        
def update(value):
    snake.insert(0, vec_add[0,0], snake_dir)
    snake.pop()
    
    r = randint(0,20)
    if r == 0:
        x,y = randint(0, field_width), randint(0,field_height)
        food.append((x,y))
        
    (hx, hy) = snake[0]
    for x,y in food:
        if hx == x and hy == y:
            snake.append((x,y))
            food.remove((x,y))
    
    for x,y in snake:
        if hx == x and hy == y:
            snake = [(20,20)]
    if hx > 50 or hx < 0:
        snake = [(20,20)]
    if hy > 50 or hy < 0:
        snake = [(20,20)]
        
    glutTimerFunc(interval, update, 0)

def vec_add((x1,y1),(x2, y2)):
    return(x1 + x2, y1 + y2)
    
def keyboard(*args):
    global snake_dir
    
    if args[0] == 'w':
        snake_dir = (0,1)
    if args[0] == 's':
        snake_dir = (0,-1)
    if args[0] == 'a':
        snake_dir = (-1,0)
    if args[0] == 'd':
        snake_dir = (1,0)
        
glutInit()
glutInitDisplayMode(GLUT_RGBA | GLUT_DOUBLE | GLUT_ALPHA | GLUT_DEPTH)
glutInitWindowSize(width,height)
glutInitWindowPosition(0,0)
window = glutCreateWindow(b"snake.py") 
glutDisplayFunc(draw)
glutIdleFunc(draw)
glutTimerFunc(interval, update, 0)
glutKeyboardFunc(keyboard)
glutMainLoop()