README.md

Py01-Ball

Student handout

The aim of this exercise is to talk the kids through some basic Python syntax, especially in relation to PyGame which is what subsequent exercises will use.

It's the simplest possible thing using PyGame.

You should get them to type it up following along, whilst you explain everything.

Finally, there's one challenge to make sure they understood everything. Or at least, some of it.

Setup:

• You should start with a version of ball.py with lots of carriage returns everywhere that you can then delete, to reveal a bit of the program a bit at a time.
• The students should start with a blank editor, viewing the (neearly empty) ball.py so that the editor (e.g. Sublime text) knows it's Python and does suitable syntax highlighting etc.
• The directory in which they're editing ball.py must have the data and codeclub.py entries.

Before starting you should set expectations:

• This exercise is boring and terrifying.
• They won't understand everything.
• But from next week, we will be back to real games.

Things to explain then reveal.

Perhaps tick each item off to make sure you don't miss some...

• What's a comment.

#######################################################################################
# CodeClub Ball exercise
#######################################################################################

• In Scratch programs triggered other programs. e.g. "Say 'Hello'" knew how to draw speech bubble.
• But options limited. In Python there are many options but we have to choose what we want.
• That's what import does.

import pygame
import codeclub
from pygame.locals import *

• What pygame does.
• We have to tell it to set up (equivalent of 'when green flag clicked')
• What something.somethingelse() means - like a broadcast, but sent to something specific. In this case pygame. Emphasise that the brackets are important.
• It's called "calling a method".

pygame.init()

• Variables now. Easier than Scratch - no need to say "create" variables - we just put something into them.
• In Scratch variables can be numbers or writing. In Python they can be lots of things. This one will be the size and shape of a rectangle

screen_rect = Rect(0, 0, 640, 480)

• The next variable represents the whole window where the game occurs.
• Only pygame knows how to make the value we want to store in this variable.
• We call a method on pygame and it replies with the value.
• We also have to pass the screen size into it. So this is kind of like a broadcast where:
  • It's sent only to pygame
  • We send some information with the message
  • It sends some information back
• But we don't send this just to pygame. We send it to the display variable which belongs to pygame.
• Note that the students may be able to use return-completion here to automatically type things like pygame and screen_rect

screen = pygame.display.set_mode(screen_rect.size)

• Warn that this is the most complicated bit.
• In Scratch there is just one of each sprite. In Python there are multiple instances of each sprite, so we call each type of sprite a "class".
• Also, they're not necessarily created at the start of the game. They can be created or destroyed later.
• We're going to call our sprite Ball and say that it's a type of CodeClub sprite. This second bit means it knows how to do lots of the typical things which CodeClub sprites need to do.

class Ball(codeclub.CodeClubSprite):

• Each script on a class is called a 'method' and starts with def.
• The first method, __init__ is special and runs when a new Ball is created.
• It stands for "initialise"

	def __init__(self):
		super(Ball, self).__init__() # ignore this line, it's complicated.

• When our __init__ method is called, we want to ask ourselves to set a costume.
• This is possible only because we've said we're a CodeClub sprites, and all CodeClub sprites know how to do this.
• We need to tell ourself just what sort of costume we need - the name of an image file and how big we want to be.

		self.set_costume('ball.png', 50)

• Now we set up some more variables.
• Talk through the lines.

background = pygame.Surface(screen_rect.size)
ball = Ball()
all = pygame.sprite.RenderUpdates()

• The all variable contains a list of all the different sprites. pygame will draw these. So we need t make sure that the ball we've made is within that list.
• So we send a message ("call a method") to the 'all' list, asking it to add the ball to itself.

all.add(ball)

• Now we actually run the game.
• We keep running as long as the variable running is 'yes'.
• 'yes' is True

running = True

while running:

• We stop the game running when someone presses escape or the close box on the window is pressed.
• Talk through the lines.

	for event in pygame.event.get():
		if event.type == QUIT or (event.type == KEYDOWN and event.key == K_ESCAPE):
			running = False

• We want to move the ball whenever left or right is pressed.
• We set up a new variable containing the state of all the keys, which pygame tells us.

	keys_pressed = pygame.key.get_pressed()

• keys_pressed[name of key] is 0 if the key is not pressed, or 1 if it is pressed
• We tell our ball to point in a certain direction and move. Again, all CodeClubSprites know how to do this - not just our Ball.

	if keys_pressed[K_RIGHT]:
		ball.point_in_direction(180)
		ball.move(5)
	if keys_pressed[K_LEFT]:
		ball.point_in_direction(0)
		ball.move(5)

• Finally we do some things to draw the ball on a blank background.

	all.clear(screen, background)

	dirty = all.draw(screen)
	pygame.display.update(dirty)

Challenge

This is all going to be very terrifying for them so it's something simple where they copy existing code. Mostly.

TODO

Figure out a further challenge involving pitch.png.

Acknowledgements

pitch.png licensed under CC BY-SA by Xhandler from http://commons.wikimedia.org/wiki/File:Football_pitch_v2.svg.