How to implement a camera and do collision detection for a platformer game?

Question

I need help making the screen follow the player throughout the level as well as handle collisions at the same time. I've done both separately and they work fine but I run into trouble doing both at the same time.

Here is what I've tried. If you need more of the code, I'd be happy to include it.

What am I doing wrong? I've literally been trying to do this for 3 days now.

class Sprite(pygame.sprite.Sprite):
    def __init__(self,img):
        pygame.sprite.Sprite.__init__(self)

        self.image = pygame.image.load(img).convert_alpha()

        self.rect = self.image.get_rect()

class Player(Sprite):
    velocity_y = 0
    onGround = True
    def update(self):
        self.rect.y += self.velocity_y
        self.movePlayer()
        self.checkBarriers()
        self.checkCollision(0, self.velocity_y,terrain_list)
        self.checkCollision(Terrain.velocity_x*-1,0,terrain_list)

    def movePlayer(self):
        if moving_up: # pressing the up arrow key
            if self.onGround == True:
                self.velocity_y = -SPEED
                self.rect.y -= 1
                self.onGround = False
        if not moving_up: self.velocity_y = SPEED

    def checkBarriers(self):
        if self.rect.y >= LOWER_BOUNDARY-GROUND_HEIGHT-ROBOT_HEIGHT:
            self.onGround = True
            self.velocity_y = 0
            self.rect.y = LOWER_BOUNDARY-GROUND_HEIGHT-ROBOT_HEIGHT
        if self.rect.x <= LEFT_BOUNDARY:
            Terrain.velocity_x = 0

    def checkCollision(self,xvel,yvel,blocks):
        for block in blocks:
            if pygame.sprite.collide_rect(block,player):

                if xvel < 0: # handle collisions for moving right. (Everything I've tried didnt work)                                  
                if xvel > 0: # handle collisions for moving left
                if yvel > 0:
                     self.rect.y = block.rect.y-ROBOT_HEIGHT # player moving down
                     self.onGround = True

class Terrain(Sprite):
    velocity_x = 0

    terrain_x = 0
    terrain_y = 0
    terrain_vel_x = 0
    terrain_vel_y = 0

    def update(self):
        self.rect.x += self.velocity_x
        self.movePlayer()

    def movePlayer(self):
        if moving_right: self.velocity_x = -SPEED
        elif not moving_right: self.velocity_x = 0
        if moving_left: self.velocity_x = SPEED

Answer 1

Please check out the following questions as they may answer your question.

Example1

Example 2

Just fyi example 2 is a much better example as I believe it answers your question how you are thinking. It answers the side scrolling and pretty much the following of the camera with your character constantly however Example 1 has some pretty interesting stuff itself.