AttributeError: 'str' object has no attribute 'update'

I am making a sandbox(with sand) and I stumbled on an error in this line:

for grain in sand:
            grain.update()

The sand variable is a dictionary, and I don’t know what went wrong. If you need further clarity, comment.
The full program is:

import pygame
import random

WIDTH, HEIGHT = 640, 360
FPS = 60

WHITE = (255, 255, 255)
SAND = (194, 178, 128)

GRAVITY = 0.5 # increased gravity

def main_program():
    global sand
    pygame.init()

    game_display = pygame.display.set_mode((WIDTH, HEIGHT))
    pygame.display.set_caption("Sandbox")

    is_crashed = False
    clock = pygame.time.Clock()

    font = pygame.font.SysFont("Arial", 32)

    sand_grain = pygame.Surface((4, 4))
    pygame.draw.rect(sand_grain, SAND, (0, 0, 4, 4))

    sand = {}
    sand_id = 0

    while not is_crashed:
        mouse_x, mouse_y = pygame.mouse.get_pos()
        mouse_held = pygame.mouse.get_pressed()[0]

        if mouse_held:
            sand[str(sand_id)] = Grain(mouse_x, mouse_y)
            sand_id += 1
        for grain in sand:
            grain.update()
        
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                is_crashed = True
            # handle mouse button down event
            if event.type == pygame.MOUSEBUTTONDOWN:
                # get the position and button of the mouse event
                pos = event.pos
                button = event.button
                # get a list of all grains that are under the mouse cursor
                clicked_grains = [g for g in sand.values() if g.rect.collidepoint(pos)]
                # if there are any grains clicked
                if clicked_grains:
                    # get the topmost grain
                    top_grain = clicked_grains[-1]
                    # if the left mouse button is pressed
                    if button == 1:
                        # set the dragged attribute of the grain to True
                        top_grain.dragged = True
            # handle mouse button up event
            if event.type == pygame.MOUSEBUTTONUP:
                # get the position and button of the mouse event
                pos = event.pos
                button = event.button
                # get a list of all grains that are under the mouse cursor
                clicked_grains = [g for g in sand.values() if g.rect.collidepoint(pos)]
                # if there are any grains clicked
                if clicked_grains:
                    # get the topmost grain
                    top_grain = clicked_grains[-1]
                    # if the left mouse button is released
                    if button == 1:
                        # set the dragged attribute of the grain to False
                        top_grain.dragged = False
        game_display.fill(WHITE)

        for grain in sand.values():
            grain.draw(game_display, sand_grain)

        pygame.display.flip()
        clock.tick(FPS)
    pygame.quit()

class Grain:
    def __init__(self, x, y):
        self.x = x
        self.y = y

        self.yv = 0
        self.xv = random.randint(-2, 2) # added horizontal velocity

        self.rect = pygame.Rect(self.x, self.y, 4, 4)
        self.dragged = False # added dragged attribute
    def update(self):
        global sand
        if self.y > HEIGHT - 4: # check for bottom edge collision
            self.y = HEIGHT - 4
            self.yv = 0
        elif self.y < 0: # check for top edge collision
            self.y = 0
            self.yv = 0
        else:
            self.yv += GRAVITY
        if self.x > WIDTH - 4: # check for right edge collision
            self.x = WIDTH - 4
            self.xv = 0
        elif self.x < 0: # check for left edge collision
            self.x = 0
            self.xv = 0
        else:
            self.xv += random.randint(-1, 1) # add some randomness to horizontal movement
        self.x += self.xv
        self.y += self.yv

        self.rect = pygame.Rect(self.x, self.y, 4, 4)

        for other in sand.values(): # check for collisions with other grains
            if other is not self and self.rect.colliderect(other.rect): # use pygame.Rect.colliderect method
                self.yv = 0
                self.xv = 0
                
                if self.y < other.y:
                    self.y = other.y - 4
                elif self.y > other.y:
                    self.y = other.y + 4
                if self.x < other.x:
                    self.x = other.x - 4
                elif self.x > other.x:
                    self.x = other.x + 4
                break
        # check if the grain is being dragged by the mouse
        if self.dragged:
            # get the current mouse position and movement
            mouse_x, mouse_y = pygame.mouse.get_pos()
            mouse_rel_x, mouse_rel_y = pygame.mouse.get_rel()
            # set the position and velocity of the grain to match the mouse
            self.x = mouse_x
            self.y = mouse_y
            self.xv = mouse_rel_x
            self.yv = mouse_rel_y
            # update the rect of the grain
            self.rect = pygame.Rect(self.x, self.y, 4, 4)
    def draw(self, display, grain):
        display.blit(grain, self.rect)

if __name__ == "__main__":
    main_program()

>Solution :

Try this

for grain in sand.values():
    grain.update()

I think with sand you are accessing the keys and with sand.values you will access the correct instances