Peter Andreae

Python for Level 3

CS4HS

see website: ecs.vuw.ac.nz/Main/PythonForSchools

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Intro• Quick overview of what’s new in level 3 Programming

• Go through key ideas of L3 in python, making little programs

• Discussion of ideas for the classroom.

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Level 3:• GUI

• program must use some kind of event-driven input

• Object-oriented: • defining a Class, making & using Objects of that class

• Staged development• repeated cycles of design, code, test, debug on increasing

scope of the program

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Level 3:GUI• Program must have its own window

• Not inside the “shell”• Might have graphical output, but not critical

• Must respond to some kind of event-driven input • eg buttons, text fields, mouse clicks • Event-driven input changes the design and structure of the

program• The program is no longer in control.

• Fancy layout and good UI design is not required• Code for fancy layout is tricky and fiddly and not important• Programs should not be so complicated that UI design is

significant• two or three buttons would be sufficient (at least for

achieved) • Autogenerated code (drag and drop) is not banned, but

doesn’t demonstrate that they understand it.

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Example• Peg puzzle

• has three buttons to control the actions• has graphical output (draws the puzzle on the screen)

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Level 3:using Classes and Objects• An object is a way of wrapping up multiple bits of

data and functions that use that data.• “Fields” are variables in the object to hold values• Each object of the class has its own copies of the fields• Functions in the class operate on the particular object they

were called on, access the fields of that object

turtle1.forward(20)

turtle2.turn(30)

• Using objects of predefined classes is unavoidable • eg calling append on a list: names.append(“mary”)• eg calling forward on a turtle:

sam = turtle.Turtle() sam.forward(20)

• Standard expects them to define new classes, and create and use them

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Example• Bouncing balls:

• Each ball is represented by an object, storing the size, colour, and current speed and position of the ball.

• There is a list of the ball objects

• Reset creates a new list of ball objects

• The main loop runs through the list, telling each ball to move then redraws them all.

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Level 3:• Staged development

• design, code, test, debug small part of program• repeatedly

• extend the design and the code to cover more features/aspects

• test and debug • requires a task for which there is a simple “core”, but which

can be extended in a series of stages

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Graphical output and GUI• Need tkinter library to do any graphical input/output

or GUIfrom tkinter import *

• Simple graphical output:• set up window and canvas

def setup () :

global canvas

window = Tk()

canvas = Canvas(window, width=600, height=450, bg='white')

canvas.pack()

• call drawing commands on the canvas

canvas.create_rectangle(left, top, right, bot,fill=“blue”)

• call canvas.update() to make changes visible.

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To Do• Write program to draw a flower on the canvas:

(the center circle is optional )

• Ensure you define a function to draw it at a given position, and call it from your main() function.

def main() :

set up the window, canvas, etc

draw a flower at 100 100

def drawFlower(x, y) :

draw it

main()

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Event driven input• Buttons

• Create button and specify function to call when button is pressed.

Button(window, text=“Flower", command=draw).pack()Button(window, text=“Clear”, command= clear).pack()

• Tell tk to listen for button/mouse events

window.mainloop()

• define function that the button calls (no parameters)def draw() : def clear() :

• Mouse• bind mouse event to function called when mouse event

happens:

canvas.bind("<ButtonRelease-1>", command=plant)

• define function that responds to the mouse event:def plant(event) :

….. event.x …. event.y

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To Do• Extend the flower program

• Make a button that draws a flower in the middle of the canvas• Make the program respond to the mouse by drawing a flower

where it was clicked.

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Objects• Whenever you have some kind of entity in your

program• more than just a string• more than just a number• especially if your program has more than one of them

Create a class describing the entity• work out what information needs to be stored about the

entity• work out what actions can be performed on the entity

Eg, flower:• need to store its position, and possibly its colour• need to be able to draw it

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To create a class• Define the name of the class

• convention: capitalised name

class Flower() :

• Define the constructor• The function that is called to set up /initialise an object of the

class• typically sets up the fields

def __init__(self, …..other parameters it needs)

self.

• Define the other functions that act on the objects• all the functions have a first parameter of self.• they can access the fields via self

def grow(self, …. other parameters)

self.size = self.size+5

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Flowerclass Flower () :

def __init__(self, )

def draw(self, )

def bloom (self )

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Example Programs• Garden :

• contains a Flower class• has a list of Flower objects• respond to mouse by adding a new flower to the garden• button to make all the flowers turn red

• Walk through the program.

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To Do:

Write program for planning a banquet layout

• Shows a collection of tables,

• Allow the user to place or move tables with the mouse

• Design: • A Table class• A main program with canvas which responds to mouse

• Extensions:• buttons to allow the tables to be lined up• Extend the table to have a list of names

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Ideas for the classroom18