Why is my pygame application loop not working properly?

Question

I am working on a very rough topdown 2d adventure game after skimming around the pygame documentation. However, I have hit a bit of a roadblock after not being able to find anything on a camera system and found that most tutorials for a camera are 5+ years old and don't seem to work anymore. Can anybody help me build a camera?

This is my main executed script

import sys, pygame
from PlayerObject import Player

pygame.init()

screen_height = 180
screen_width = 320
map_height = 1080
map_width = 1920

num_objects = 5

screen = pygame.display.set_mode((screen_width, screen_height))
player_image = pygame.image.load('models/hero.bmp').convert()
background = pygame.image.load('models/lobby.bmp').convert()
screen.blit(background, (0, 0))
objects = []

# randomly generates 5 entities with the 1st one being the controlled character
for i in range(num_objects):
    o = Player(player_image, random.randint(0, screen_width), random.randint(0, screen_height), 10)
    objects.append(o)

while 1:

    keys = pygame.key.get_pressed()
    if keys[pygame.K_LEFT]:
        screen.blit(background, objects[0].pos, objects[0].pos)
        objects[0].move_left()
        screen.blit(objects[0].image, objects[0].pos)
    if keys[pygame.K_RIGHT]:
        screen.blit(background, objects[0].pos, objects[0].pos)
        objects[0].move_right()
        screen.blit(objects[0].image, objects[0].pos)
    if keys[pygame.K_UP]:
        screen.blit(background, objects[0].pos, objects[0].pos)
        objects[0].move_up()
        screen.blit(objects[0].image, objects[0].pos)
    if keys[pygame.K_DOWN]:
        screen.blit(background, objects[0].pos, objects[0].pos)
        objects[0].move_down()
        screen.blit(objects[0].image, objects[0].pos)
        screen.blit(background)
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            sys.exit()

    for o in objects:
        screen.blit(background, o.pos, o.pos)
    for num in range(num_objects - 1):
        objects[num + 1].rand_move()
    for o in objects:
        screen.blit(o.image, o.pos)

    pygame.display.update()
    pygame.time.delay(100)

This is my Player class

import random

map_height = 180
map_width = 320

class Player:
    def __init__(self, image, width, height, speed):
        self.speed = speed
        self.image = image
        self.pos = image.get_rect().move(width, height)
        self.image = image

    def set_speed(self, speed):
        self.speed = speed

    def rand_move(self):
        x = random.randint(-self.speed, self.speed)
        y = random.randint(-self.speed, self.speed)
        self.pos = self.pos.move(x, y)

        # keeps player in boundaries
        if self.pos.left < 0:
            self.pos.left = 0
        if self.pos.right > map_width:
            self.pos.right = map_width
        if self.pos.top < 0:
            self.pos.top = 0
        if self.pos.bottom > map_height:
            self.pos.bottom = map_height

    def move_left(self):
        speed = self.speed
        self.pos = self.pos.move(-speed, 0)
        if self.pos.left < 0:
            self.pos.left = 0

    def move_right(self):
        speed = self.speed
        self.pos = self.pos.move(speed, 0)
        if self.pos.right > map_width:
            self.pos.right = map_width

    def move_up(self):
        speed = self.speed
        self.pos = self.pos.move(0, -speed)
        if self.pos.top < 0:
            self.pos.top = 0

    def move_down(self):
        speed = self.speed
        self.pos = self.pos.move(0, speed)
        if self.pos.bottom > map_height:
            self.pos.bottom = map_height

Answer 1

Your basic misunderstanding, is that you try to draw the background at the position of an object, then you move the object and blit it finally on its new position. That all is not necessary.
In common the entire scene is drawn in each frame in the main application loop. It is sufficient to draw the background to the entire window and blit each object on top of it. Note, you do not see the changes of the window surface immediately. The changes become visible, when the display is updated by pygame.display.update() or pygame.display.flip():

The main application loop has to:

• handle the events by either pygame.event.pump() or pygame.event.get().
• update the game states and positions of objects dependent on the input events and time (respectively frames)
• clear the entire display or draw the background
• draw the entire scene (blit all the objects)
• update the display by either pygame.display.update() or pygame.display.flip()

e.g.:

while 1:

    # handle events
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            sys.exit()

    # update objects (depends on input events and frames)
    keys = pygame.key.get_pressed()
    if keys[pygame.K_LEFT]:
        objects[0].move_left()    
    if keys[pygame.K_RIGHT]:
        objects[0].move_right()
    if keys[pygame.K_UP]:
        objects[0].move_up()
    if keys[pygame.K_DOWN]:
        objects[0].move_down()

    for num in range(num_objects - 1):
        objects[num + 1].rand_move()

    # draw background
    screen.blit(background, (0, 0))

    # draw scene
    for o in objects:
        screen.blit(o.image, o.pos)

    # update dispaly
    pygame.display.update()
    pygame.time.delay(100)

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Minimal example: repl.it/@Rabbid76/PyGame-MinimalApplicationLoop