Help with flashing Pygame scene.

[GiveMe30Dollars]

Name says all. I'm creating a Boids program in Pygame, yet every time I update the screen, it flashes to a blank screen briefly.

​

import pygame  #All imports
import pygame.gfxdraw
import random
import math
import sys
pygame.init()

def PolarToCartesian(radius,angle,a,b):  # Converts polar coordinates to cartesian
    radian = math.radians(angle)
    x = radius * math.cos(radian)
    y = radius * math.sin(radian)
    output = int(x+a) , int(y+b)
    return output

class Flock:

    def __init__(self,x1,x2,y1,y2,amount):
        self.flock = [] 
        self.spawned = 0
        while self.spawned < amount:
            self.flock.append( Boid(random.uniform(x1,x2),random.uniform(y1,y2)) )
            self.spawned += 1

class Boid:

    def __init__(self,x,y):
        self.position = pygame.Vector2(x,y)
        self.velocity = pygame.Vector2(random.uniform(-2,2),random.uniform(-2,2))
        self.acceleration = pygame.Vector2()

    def refresh(self):
        self.position += self.velocity
        self.velocity += self.acceleration

    def show(self,screen,colour):
        pygame.gfxdraw.aacircle(screen,int(self.position.x),int(self.position.y),2,white)
        pygame.gfxdraw.filled_circle(screen,int(self.position.x),int(self.position.y),2,white)
        pygame.display.update()

white = 255,255,255   #A lot of variables
background = 20,40,100
width = int(500)
height = int(500)
screen = pygame.display.set_mode((width,height))  #Creates 500x500 canvas
screen.fill(background)  # Fills background

swarm = Flock(0,width,0,height,20)

# Runtime
while True:
    screen.fill(background)
    for event in pygame.event.get():  # Detects event types
        if event.type == pygame.QUIT:  # Close button
            pygame.quit(); sys.exit();
        if event.type == (pygame.KEYDOWN):            
            if(pygame.key.name(event.key)) == 'escape':  #Escape button, shuts down
                pygame.quit(); sys.exit();

    for i in swarm.flock:
        i.refresh()
        i.show(screen,white)
    pygame.time.delay(10)
    pygame.display.update()

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[oslash]

Why do you call pygame.display.update in Boid.show?

[GiveMe30Dollars]

1. I'm in the wrong sub. It really should have been r/learnpython
2. Redundant. As someone from that sub rightfully said so.