How do I test for a collision in Pygame?

I'm struggling to work out how collisions work in pygame. I understand it's something to do with pygame.rect.colliderect, and I'm probably pretty close, but I'd appreciate someone who knows a bit more than me having a look! :-)

Here's a simple program I've written to steer a tiny green square onto a larger red square, and I'm trying to implement collision detection when the two meet, but at this stage everything works except collisions.

Thanks in advance!!

pygame.init()

import random #import random

size = (700,500) # set up the screen
screen = pygame.display.set_mode((size))

BLACK = (0,0,0) #define colours
WHITE = (255,255,255)
GREEN = (0,255,0)
RED =   (255, 0, 0)

class player(): #assign a player class
    def __init__(self):
        self.xpos = 450
        self.ypos = 250
        self.rect = pygame.Rect(self.xpos,self.ypos,5,5)
        self.xvel = 0
        self.yvel = 0
        self.colour = GREEN

    def update(self): #define a function to update the payer
        #self.xpos +=self.xvel  Ignore this bit. I implemented velocity, but it quickly flew off the screen
        #self.ypos +=self.yvel
        if player.rect.colliderect(obstacle.rect):    #<--------- this is the bit I think might be wrong?
            print("collision!")

    def draw(self): #define a function to draw the player
        pygame.draw.rect(screen, self.colour,[self.xpos,self.ypos,5,5])

class obstacle(): #define an obstacle class
    def __init__ (self):
        self.xpos = random.uniform(0,700)
        self.ypos = random.uniform(0,500)
        self.rect = pygame.Rect(self.xpos,self.ypos,20,20)
        self.colour = RED

    def draw(self): #define a function to draw the obstacle
        pygame.draw.rect(screen, self.colour,[self.xpos,self.ypos, 20,20])

player = player() #run an instance of the player class
obstacle = obstacle() #run an instance of the obstacle class
clock = pygame.time.Clock()


while True: #game loop
    for event in pygame.event.get(): #quit 
        if event.type == pygame.QUIT:
           pygame.display.quit()

#-----Game logic           
    keys = pygame.key.get_pressed() #check for key presses and do whatever
    if keys[pygame.K_LEFT]:
        player.xpos -= 1
    if keys[pygame.K_RIGHT]:
        player.xpos += 1
    if keys[pygame.K_UP]:
        player.ypos -= 1
    if keys[pygame.K_DOWN]:
        player.ypos += 1


    player.update() #Update the player - obstacle shouldn't need updating

#-----Drawing code

    screen.fill(BLACK) #draw screen black
    obstacle.draw() #draw the obstacle from the function
    player.draw() #draw the player from the function
    pygame.display.flip() #update

    clock.tick(60)'''

Solution

The problem is you are not updating the position of your player's rectangle, which what colliderect looks at when detecting a collision. You were drawing the rectangle as you changed xpos and ypox but the coordinates for your rectangle rect.x and rect.y were not being updated accordingly. I changed the lines

    def draw(self): #define a function to draw the player
        pygame.draw.rect(screen, self.colour,[self.xpos,self.ypos,5,5])

    def draw(self): #define a function to draw the player
        pygame.draw.rect(screen, self.colour,[self.rect.x,self.rect.y,5,5])

and

    if keys[pygame.K_LEFT]:
        player.xpos -= 1
    if keys[pygame.K_RIGHT]:
        player.xpos += 1
    if keys[pygame.K_UP]:
        player.ypos -= 1
    if keys[pygame.K_DOWN]:
        player.ypos += 1

    if keys[pygame.K_LEFT]:
        player.rect.x -= 1
    if keys[pygame.K_RIGHT]:
        player.rect.x += 1
    if keys[pygame.K_UP]:
        player.rect.y -= 1
    if keys[pygame.K_DOWN]:
        player.rect.y += 1

so that the player's rectangle coordinates would be updated.