问surface_obj.fill('color')函数正在引起错误

EN

import pygame , math
pygame.init()
WIDTH , HEIGHT = 900 , 600
win = pygame.display.set_mode((WIDTH , HEIGHT))
pygame.display.set_caption('Hangmane')
RUN = True

# indexing button coordinates and appending in letters(list)
letters = [ ]
RADIUS = 20 
BUTTON_X = 0
BUTTON_Y = 457
for i in range(26):
    BUTTON_X += 60
    if i == 14:
        BUTTON_X = 100
        BUTTON_Y = 520
    #pygame.draw.circle(win,'green',(BUTTON_X,BUTTON_Y),RADIUS,3)
    letters.append([BUTTON_X, BUTTON_Y])                

#drawing button(circle) in pygame window
def button():
    for letter in letters:
        BUTTON_X , BUTTON_Y = letter
        pygame.draw.circle(win,'green',(BUTTON_X,BUTTON_Y),RADIUS,3)

# this will check and remove if button(circle) is clicked
def click_checker(coordinates):
    letters.remove(coordinates)

while RUN:
    win.fill('darkred') #if you comment this than code will not work as intended
    button()
    pygame.display.update()
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            RUN = False
        if event.type == pygame.MOUSEBUTTONDOWN:
                pos_x , pos_y = pygame.mouse.get_pos()
                print(pos_x , pos_y)
                for letter in letters:
                    BUTTON_X , BUTTON_Y = letter
                    dis = math.sqrt((BUTTON_X-pos_x)**2 + (BUTTON_Y-pos_y)**2)
                    if dis < RADIUS: # this will check if distance between mouse Cursor is less than button(circle) radius
                        #print('clicked')
                        click_checker(letter)

pygame.quit()