Why is pygame displaying the wrong image?

Question

I am making a Guardian game where you fire bullets at enemies and evolve. When you press the r key, your image changes. The image I want the player the turn into when the player hits the r key is an orange triangle with a red outline. The image that the player is before you press the r key is a yellow triangle with an orange outline. However, when I pressed the r key, instead of turning into an orange triangle with a red outline, you turn into a large yellow triangle with an orange outline.

Here is the player class (named Polygon):

class Polygon(Sprite):
    """Is it going to be a triangle, a square, a pentagon, or something else?"""

    def __init__(self, sides, window, x, y, size=None):
        self.size = size
        self.sides = sides
        self.window = window
        self.x = x
        self.y = y
        self.check_image()

        Sprite.__init__(self, self.window, self.x, self.y, self.image, self.size)

    def check_for_movement(self, move_amount=1):
        """Let's get moving!"""

        key = pygame.key.get_pressed()

        if key[K_w]:
            self.y -= move_amount
        if key[K_s]:
            self.y += move_amount
        if key[K_a]:
            self.x -= move_amount
        if key[K_d]:
            self.x += move_amount

    def check_for_power_up(self):
        key = pygame.key.get_pressed()

        if key[K_r]:
            self.upgrade()

    def evolve(self):
        self.sides += 1
        self.check_image()

    def check_image(self):
        if self.sides == 3:  # Is it a triangle?
            self.image = "triangle.png"
        elif self.sides == 4:  # A square?
            self.image = "square.png"
        elif self.sides == 5:  # Or even a pentagon?
            self.image = "pentagon.png"
        elif self.sides == 6:  # A hexagon?
            self.image = "hexagon.png"

    @staticmethod
    def check_bullet_type():
        """Who would win? A bullet or a bomb?"""
        global bullet_type, bullet_type_word

        key = pygame.key.get_pressed()

        if key[K_2]:
            bullet_type = 2
            bullet_type_word = "Bomb"
        if key[K_1]:
            bullet_type = 1
            bullet_type_word = "Bullet"

    def upgrade(self):
        if self.sides == 3:
            self.image = "triangle_plus.png"

        self.check_image()

And here is my loop:

while running:
    """Event loop"""
    for event in pygame.event.get():
        if event.type == QUIT:
            running = False

        """These lines creates a bullet object when the player clicks the screen"""
        if event.type == MOUSEBUTTONDOWN:
            if bullet_type == 1:
                if player.sides == 6:
                    bullet_group.add(Bullet(6, screen, player.x + 3, player.y + 5, size=(270, 140)))
                elif player.sides == 5:
                    bullet_group.add(Bullet(5, screen, player.x + 3, player.y + 5, size=(270, 140)))
                elif player.sides == 4:
                    bullet_group.add(Bullet(4, screen, player.x + 185, player.y + 30, size=(320, 190)))
                elif player.sides == 3:
                    bullet_group.add(Bullet(3, screen, player.x + 4, player.y - 15, size=(170, 100)))

            elif bombs > 0 and bullet_type == 2:
                """This is for bomb firing detection"""
                if player.sides == 3:
                    bullet_group.add(Bomb(screen, player.x + 3, player.y + 2, size=(370, 240)))
                elif player.sides == 4:
                    bullet_group.add(Bomb(screen, player.x + 185, player.y + 30, size=(370, 240)))
                elif player.sides == 5:
                    bullet_group.add(Bomb(screen, player.x + 5, player.y - 15, size=(370, 240)))
                elif player.sides == 6:
                    bullet_group.add(Bomb(screen, player.x - 10, player.y - 15, size=(370, 240)))

                """This will teach the player not to spam bombs!"""
                bombs -= 1

        """This creates a enemy every 2 seconds"""
        if event.type == enemy_event:
            if player.sides == 3:
                enemy_group.add(Enemy(3, screen, random.randint(0, 900), 50, size=(230, 160)))
            elif player.sides == 4:
                enemy_given_score = 20
                enemy_group.add(Enemy(4, screen, random.randint(0, 900), 50, size=(550, 350)))
            elif player.sides == 5:
                enemy_given_score = 30
                enemy_group.add(Enemy(5, screen, random.randint(0, 900), 50, size=(390, 215)))
            elif player.sides == 6:
                enemy_given_score = 40
                enemy_group.add(Enemy(6, screen, random.randint(0, 900), 50, size=(390, 215)))

    screen.fill(THECOLORS["skyblue"])

    """Checks the bullet type"""
    player.check_bullet_type()

    """Updates the player position and drawing the player"""
    player.update()
    player.check_for_movement(9)

    """Allows the bullets to appear and fire towards the top of the screen"""
    bullet_group.draw(screen)
    bullet_group.update()

    """
    Updates the enemy sprite(s) and allows the enemy to fly 
    toward the bottem of the screen
    """
    enemy_group.draw(screen)
    enemy_group.update()

    """Draws the CollectableBombs"""
    bomb_group.draw(screen)
    bomb_group.update()

    """Updates and blits the score text"""
    score_text_surface = font.render("Score: {:,}".format(score), True, (0, 0, 0))
    screen.blit(score_text_surface, (650, 10))

    """Updates and blits the lives text"""
    lives_text_surface = font.render("Lives: {:,}".format(lives), True, (0, 0, 0))
    screen.blit(lives_text_surface, (650, 60))

    """Updates and blits the bombs text"""
    bombs_text_surface = font.render("Bombs: {:,}".format(bombs), True, (0, 0, 0))
    screen.blit(bombs_text_surface, (650, 110))

    """Updates and blits the bullet type text"""
    bullet_type_text_surface = font.render("Bullet Type: {0}".format(bullet_type_word),
                                           True, (0, 0, 0))
    if bullet_type == 1:
        screen.blit(bullet_type_text_surface, (550, 160))
    elif bullet_type == 2:
        screen.blit(bullet_type_text_surface, (555, 160))

    """This blits the game over image if it's game over"""
    if lives <= 0:
        game_over_bg = Image(screen, 390, 300, "game_over.png")
        screen.blit(game_over_bg.image, game_over_bg.rect)

    """
    If a bullet and a enemy collide, delete them both and increase score by 1.
    If you are lucky enough, you may even get a bomb!
    """
    for i in enemy_group:
        if pygame.sprite.spritecollide(i, bullet_group, True):
            score += enemy_given_score
            if random.choice([0, 1]) == 1:
                bombs += 1
            i.kill()

    """POWER UPPPPPPPPPP!"""
    player.check_for_power_up()

    """If a enemy and the player collide, decrease lives by 1"""
    if pygame.sprite.spritecollide(player, enemy_group, True):
        lives -= 1

    """Evolves the player into a square if score is in the 100's"""
    if keep_checking_for_square and 100 <= score < 201:
        player.evolve()
        keep_checking_for_square = False

    """Evolves the player into a pentagon if score is in the 200's"""
    if keep_checking_for_pentagon and 200 <= score < 301:
        player.evolve()
        player.__init__(5, screen, player.x, player.y, size=(385, 230))  # Yeah, this line is for changing the size of the player
        keep_checking_for_pentagon = False

    """Evolves the player into a hexagon if score is from 300 to 410"""
    if keep_checking_for_hexagon and 300 <= score < 411:
        player.evolve()
        player.__init__(6, screen, player.x, player.y, size=(475, 280))  # This line also changes the size of the player
        keep_checking_for_hexagon = False

    """If score is 1000 or more, victory!"""
    if score >= 1000:
        cake = Sprite(screen, SCREEN_X // 2 + 10, SCREEN_Y // 2 + 30, "cake.png")
        screen.fill(Color(71, 181, 245))
        screen.blit(cake.image, cake.rect)

        win_text_surface_upper = font.render("CONGRATULATIONS!", True, (0, 0, 0))  # WINNER WINNER
        win_text_surface_lower = font.render("YOU SAVED THE INFINITE SIDED POLYGON!", True, (0, 0, 0))  # CHICKEN DINNER

        screen.blit(win_text_surface_upper, (SCREEN_X // 2 - 155, SCREEN_Y // 2 - 165))
        screen.blit(win_text_surface_lower, (SCREEN_X // 2 - 275, SCREEN_Y // 2 + 75))

    pygame.display.flip()
    clock.tick(40)

Answer 1

In your Sprite.upgrade() function, you first assign self.image = "triangle_plus.png" but immediately afterwards call check_image() which then overwrites self.image, undoing your previous change. You will probably want to return once you updated the image.