Python | Flappy Bird game clone for a beginners' programming class

I'll soon begin teaching a beginners' programming class. It's voluntary, so I thought I'd make it interesting by teaching Python programming and then introduce the kids to Pygame, so that they can make their own games. To try out Pygame (I've never used it before) and to see exactly how easy it is to make a game, I made a clone of Flappy Bird.

What do you think? Could this be made simpler/shorter? Is there anything in there that I shouldn't teach my students?

Github repo, with images

#!/usr/bin/env python3

"""Flappy Bird, implemented using Pygame."""

import math
import os
from random import randint

import pygame
from pygame.locals import *


FPS = 60
EVENT_NEWPIPE = USEREVENT + 1  # custom event
PIPE_ADD_INTERVAL = 3000       # milliseconds
FRAME_ANIMATION_WIDTH = 3      # pixels per frame
FRAME_BIRD_DROP_HEIGHT = 3     # pixels per frame
FRAME_BIRD_JUMP_HEIGHT = 5     # pixels per frame
BIRD_JUMP_STEPS = 20           # see get_frame_jump_height docstring
WIN_WIDTH = 284 * 2            # BG image size: 284x512 px; tiled twice
WIN_HEIGHT = 512
PIPE_WIDTH = 80
PIPE_PIECE_HEIGHT = BIRD_WIDTH = BIRD_HEIGHT = 32


class PipePair:
    """Represents an obstacle.

    A PipePair has a top and a bottom pipe, and only between them can
    the bird pass -- if it collides with either part, the game is over.

    Attributes:
    x: The PipePair's X position.  Note that there is no y attribute,
        as it will only ever be 0.
    surface: A pygame.Surface which can be blitted to the main surface
        to display the PipePair.
    top_pieces: The number of pieces, including the end piece, in the
        top pipe.
    bottom_pieces: The number of pieces, including the end piece, in
        the bottom pipe.
    """

    def __init__(self, surface, top_pieces, bottom_pieces):
        """Initialises a new PipePair with the given arguments.

        The new PipePair will automatically be assigned an x attribute of
        WIN_WIDTH.

        Arguments:
        surface: A pygame.Surface which can be blitted to the main
            surface to display the PipePair.  You are responsible for
            converting it, if desired.
        top_pieces: The number of pieces, including the end piece, which
            make up the top pipe.
        bottom_pieces: The number of pieces, including the end piece,
            which make up the bottom pipe.
        """
        self.x = WIN_WIDTH
        self.surface = surface
        self.top_pieces = top_pieces
        self.bottom_pieces = bottom_pieces
        self.score_counted = False

    @property
    def top_height_px(self):
        """Get the top pipe's height, in pixels."""
        return self.top_pieces * PIPE_PIECE_HEIGHT

    @property
    def bottom_height_px(self):
        """Get the bottom pipe's height, in pixels."""
        return self.bottom_pieces * PIPE_PIECE_HEIGHT

    def is_bird_collision(self, bird_position):
        """Get whether the bird crashed into a pipe in this PipePair.

        Arguments:
        bird_position: The bird's position on screen, as a tuple in
            the form (X, Y).
        """
        bx, by = bird_position
        in_x_range = bx + BIRD_WIDTH > self.x and bx < self.x + PIPE_WIDTH
        in_y_range = (by < self.top_height_px or
                      by + BIRD_HEIGHT > WIN_HEIGHT - self.bottom_height_px)
        return in_x_range and in_y_range


def load_images():
    """Load all images required by the game and return a dict of them.

    The returned dict has the following keys:
    background: The game's background image.
    bird-wingup: An image of the bird with its wing pointing upward.
        Use this and bird-wingdown to create a flapping bird.
    bird-wingdown: An image of the bird with its wing pointing downward.
        Use this and bird-wingup to create a flapping bird.
    pipe-end: An image of a pipe's end piece (the slightly wider bit).
        Use this and pipe-body to make pipes.
    pipe-body: An image of a slice of a pipe's body.  Use this and
        pipe-body to make pipes.
    """

    def load_image(img_file_name):
        """Return the loaded pygame image with the specified file name.

        This function looks for images in the game's images folder
        (./images/).  All images are converted before being returned to
        speed up blitting.

        Arguments:
        img_file_name: The file name (including its extension, e.g.
            '.png') of the required image, without a file path.
        """
        file_name = os.path.join('.', 'images', img_file_name)
        img = pygame.image.load(file_name)
        # converting all images before use speeds up blitting
        img.convert()
        return img

    return {'background': load_image('background.png'),
            'pipe-end': load_image('pipe_end.png'),
            'pipe-body': load_image('pipe_body.png'),            
            # images for animating the flapping bird -- animated GIFs are
            # not supported in pygame
            'bird-wingup': load_image('bird_wing_up.png'),
            'bird-wingdown': load_image('bird_wing_down.png')}


def get_frame_jump_height(jump_step):
    """Calculate how high the bird should jump in a particular frame.

    This function uses the cosine function to achieve a smooth jump:
    In the first and last few frames, the bird jumps very little, in the
    middle of the jump, it jumps a lot.
    After a completed jump, the bird will have jumped
    FRAME_BIRD_JUMP_HEIGHT * BIRD_JUMP_STEPS pixels high, thus jumping,
    on average, FRAME_BIRD_JUMP_HEIGHT pixels every step.

    Arguments:
    jump_step: Which frame of the jump this is, where one complete jump
        consists of BIRD_JUMP_STEPS frames.
    """
    frac_jump_done = jump_step / float(BIRD_JUMP_STEPS)
    return (1 - math.cos(frac_jump_done * math.pi)) * FRAME_BIRD_JUMP_HEIGHT


def random_pipe_pair(pipe_end_img, pipe_body_img):
    """Return a PipePair with pipes of random height.

    The returned PipePair's surface will contain one bottom-up pipe
    and one top-down pipe.  The pipes will have a distance of
    BIRD_HEIGHT*3.
    Both passed images are assumed to have a size of (PIPE_WIDTH,
    PIPE_PIECE_HEIGHT).

    Arguments:
    pipe_end_img: The image to use to represent a pipe's endpiece.
    pipe_body_img: The image to use to represent one horizontal slice
        of a pipe's body.
    """
    surface = pygame.Surface((PIPE_WIDTH, WIN_HEIGHT), SRCALPHA)
    surface.convert()   # speeds up blitting
    surface.fill((0, 0, 0, 0))
    max_pipe_body_pieces = int(
        (WIN_HEIGHT -            # fill window from top to bottom
        3 * BIRD_HEIGHT -        # make room for bird to fit through
        3 * PIPE_PIECE_HEIGHT) / # 2 end pieces and 1 body piece for top pipe
        PIPE_PIECE_HEIGHT        # to get number of pipe pieces
    )
    bottom_pipe_pieces = randint(1, max_pipe_body_pieces)
    top_pipe_pieces = max_pipe_body_pieces - bottom_pipe_pieces
    # bottom pipe
    for i in range(1, bottom_pipe_pieces + 1):
        surface.blit(pipe_body_img, (0, WIN_HEIGHT - i*PIPE_PIECE_HEIGHT))
    bottom_pipe_end_y = WIN_HEIGHT - bottom_pipe_pieces*PIPE_PIECE_HEIGHT
    surface.blit(pipe_end_img, (0, bottom_pipe_end_y - PIPE_PIECE_HEIGHT))
    # top pipe
    for i in range(top_pipe_pieces):
        surface.blit(pipe_body_img, (0, i * PIPE_PIECE_HEIGHT))
    top_pipe_end_y = top_pipe_pieces * PIPE_PIECE_HEIGHT
    surface.blit(pipe_end_img, (0, top_pipe_end_y))
    # compensate for added end pieces
    top_pipe_pieces += 1
    bottom_pipe_pieces += 1
    return PipePair(surface, top_pipe_pieces, bottom_pipe_pieces)


def main():
    """The application's entry point.

    If someone executes this module (instead of importing it, for
    example), this function is called.
    """

    pygame.init()

    display_surface = pygame.display.set_mode((WIN_WIDTH, WIN_HEIGHT))
    pygame.display.set_caption('Pygame Flappy Bird')

    clock = pygame.time.Clock()

    score_font = pygame.font.SysFont(None, 32, bold=True)  # default font

    # the bird stays in the same x position, so BIRD_X is a constant
    BIRD_X = 50
    bird_y = int(WIN_HEIGHT/2 - BIRD_HEIGHT/2)  # center bird on screen

    images = load_images()

    # timer for adding new pipes
    pygame.time.set_timer(EVENT_NEWPIPE, PIPE_ADD_INTERVAL)
    pipes = []

    steps_to_jump = 2
    score = 0
    done = paused = False
    while not done:
        for e in pygame.event.get():
            if e.type == QUIT or (e.type == KEYUP and e.key == K_ESCAPE):
                done = True
                break
            elif e.type == KEYUP and e.key in (K_PAUSE, K_p):
                paused = not paused
            elif e.type == MOUSEBUTTONUP or (e.type == KEYUP and
                    e.key in (K_UP, K_RETURN, K_SPACE)):
                steps_to_jump = BIRD_JUMP_STEPS
            elif e.type == EVENT_NEWPIPE:
                pp = random_pipe_pair(images['pipe-end'], images['pipe-body'])
                pipes.append(pp)

        clock.tick(FPS)
        if paused:
            continue  # don't draw anything

        for x in (0, WIN_WIDTH / 2):
            display_surface.blit(images['background'], (x, 0))

        for p in pipes:
            p.x -= FRAME_ANIMATION_WIDTH
            if p.x <= -PIPE_WIDTH:  # PipePair is off screen
                pipes.remove(p)
            else:
                display_surface.blit(p.surface, (p.x, 0))

        # calculate position of jumping bird
        if steps_to_jump > 0:
            bird_y -= get_frame_jump_height(BIRD_JUMP_STEPS - steps_to_jump)
            steps_to_jump -= 1
        else:
            bird_y += FRAME_BIRD_DROP_HEIGHT

        # because pygame doesn't support animated GIFs, we have to
        # animate the flapping bird ourselves
        if pygame.time.get_ticks() % 500 >= 250:
            display_surface.blit(images['bird-wingup'], (BIRD_X, bird_y))
        else:
            display_surface.blit(images['bird-wingdown'], (BIRD_X, bird_y))

        # update and display score
        for p in pipes:
            if p.x + PIPE_WIDTH < BIRD_X and not p.score_counted:
                score += 1
                p.score_counted = True

        score_surface = score_font.render(str(score), True, (255, 255, 255))
        score_x = WIN_WIDTH/2 - score_surface.get_width()/2
        display_surface.blit(score_surface, (score_x, PIPE_PIECE_HEIGHT))

        pygame.display.update()

        # check for collisions
        pipe_collisions = [p.is_bird_collision((BIRD_X, bird_y)) for p in pipes]
        if (0 >= bird_y or bird_y >= WIN_HEIGHT - BIRD_HEIGHT or
                True in pipe_collisions):
            print('You crashed! Score: %i' % score)
            break
    pygame.quit()


if __name__ == '__main__':
    # If this module had been imported, __name__ would be 'flappybird'.
    # It was executed (e.g. by double-clicking the file), so call main.
    main()