Python Workshop

CH4HS Dec 2012

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

© Peter Andreae

People

Tutors:

• Peter Andreae, VUW

• Robert Sheehan, U Auckland

• Heidi Newton, VUW (prev Canty)

• Tim Bell U Canterbury

• Edward Dalley U Canterbury

Teachers:Sarah Hailes, Daniel Greenwood, Chris Dillon, Paul Curry, Irena Krchnavy, Alistair West, Giuseppe Cugilari, Trish McKay, Rowena Bullock, Alison Chester, Tim Curran, Steve Rodkiss, Karen Clark, Paul Akula

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Where are you?

No programming experience at all (not even scratch)

Have done some Scratch/Alice, but no text languages

Have done a little bit of programming in another text-based language

Have done quite a bit of programmingbut not in Python

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The Programming Standards:• Level 1

• Variables and values• Conditionals (if..else), loops (while… for…)• Input and output

• What's added at level 2?• Program decomposed into modules (functions)• Functions with parameters (rules out Scratch)• Lists/Arrays/Sequences

• What's added at level 3?• Simple GUI: event-based input • Simple object oriented: defining a class of objects• Staged development• Must be a text based language• Design and implementation integrated

© Peter Andreae

Text-based vs Drag&Drop

Text-based Languages ( eg Python, Java, ….)

vs

Drag and Drop Languages (eg Scratch, Alice)

• Just the same

• Text-based is harder for learners because• There is much more to remember (no menu of options)• More detail to get right (or wrong)

© Peter Andreae

Python vs other textual languages:• Plus

• simple syntax rules (fewer details to get wrong)• untyped variables and parameters (less syntax to get wrong)• interpreter (allows testing and experimenting)• lots of libraries (including turtle)

• Minus• untyped variables and parameters (computer can't help

debug as much)• interpreter (computer doesn't help debug as much)• some inconsistencies, especially with lists.• non-standard structures as well as common ones.

© Peter Andreae

Python syntax.

(If you’ve used other text-based languages:)

• Don't have to specify types of variables

• Indentation is meaningful - specifies nesting

• Very little required punctuation• if, while, for, def use a :• " … " or ' … ' or ' ' ' ….. ' ' ' for strings

• [ ] for lists

• ( ) for function arguments/parameters and for "tuples"

• { } for dictionaries (mappings)

© Peter Andreae

Invitation: input/output,

print(“Hi”)

name = input(“What’s your name: ")

print(“Hello ", name)

print(“You’re invited to my party on Friday“)

• print(string , ...) action that does something

• input(promptString ) action that also returns a value

• variable = value store a value in a variable

• using the variable use the value that was stored

• sequence of actions

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Let’s do it:• Start up the Wing-101 IDE

• Type the program in main window

• Click the “bug” button to run the program.

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To Do• Temperature converter (F -> C)

• ask for temperature in fahrenheit and store in variable• compute and print out temperature in celcius

• You will need to calculate values• +, -, *, / work as normal

• You need to be able to convert strings into numbers• float(string ) number (with fractional part) • int(val ) number (with no fractional part)

© Peter Andreae

Guess my name: if / conditionals

guess = input(“Can you guess my name? ")

if guess == "pam" :

print("Yes, you got it right!")

print("How did you guess?")

else :

print("no, I’m not ", guess, "My name is pam")

• if condition : actionselse : other actions

• indentation is important• == tests if two values are the same

• To Do – try it out. Modify to guess my height

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Guess the word: while / iteration• (This is a silly program, but it illustrates some python)

print(“Lets play Guess the Word, ok?”)

word = input(“What is your guess: ")

while word != "pancake" :

print("no, ", word, " is not the right answer")

word = input("guess again: ")

print("Yes, you guessed the magic word!")

• while condition : Loop repeatedly, testing each time actions

• != test for inequality

• To Do: modify to guess the number. Say if their guess is too low or too high

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Multiplication: from random import *

base = 5other = randomint(1,10)ans = input("what is " + str(base)+ " x " + str(other) + " : ")ans = int(ans)if ans == base * other : print(" yes, that’s right! ")else : print ("no, that’s not the right answer “) print (base, " X ", other, " is “, (base * other) )

• random numbers, need to import “library”• adding strings, converting numbers to strings• doing arithmetic

To Do: change it to test addition or subtraction

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Multiplication: repeating n times

from random import *

base = 5for i in range(10) :

other = randomint(1,10)ans = int(input("what is " + str(base)+ " x " + str(other) + " : "))if ans == base * other : print(" yes, that’s right! ")else : print ("no, that’s not the right answer “) print (base, " x ", other, " is “, (base * other) )

print(" Good bye “)

• For loop for repeating fixed number of times• range(num) range(from, to) list of numbers

To Do: change it to test addition or subtraction

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To Do• Times table printer

• asks for number and prints out 1 x up to 10 x

choose number for table: 81 x 8 = 82 x 8 = 163 x 8 = 244 x 8 = 325 x 8 = 406 x 8 = 487 x 8 = 568 x 8 = 649 x 8 = 7210 x 8 = 80

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Turtle Graphics• Lets you write programs with simple graphical

patterns with minimal maths

• Can also do standard 2D graphics, but requires coordinates – some students find it tricky

Turtle graphics programsimport turtle

make a screen to move around on

make a Turtle (or more than one!)

tell the turtle to move around and draw stuff.

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Turtle Graphicsimport turtle

window = turtle.Screen()pet = turtle.Turtle()pet.pencolor(“red”)pet.forward(50)pet.left(120)pet.forward(50)pet.left(120)pet.forward(50)pet.left(120)

• Making new objects• calling functions on objects, and passing arguments

To Do Make a turtle that draws a house.

Draw star shape; draw letter shapes

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import turtle

window = turtle.Screen()

sam = turtle.Turtle()

sam.pencolor(“blue”)

for i in range(3) :

sam.forward(50)

sam.left(120)

© Peter Andreae

Defining functions• Breaking a program up into modules

• easier to manage• easier to understand• easier to reuse.

• Each module is a function• has a name• has parameters – specifying the information it needs

def draw_triangle (turtle, size) :for i in range(3) :

turtle.forward(size)turtle.left(120)

• when you call the function, you provide values for each parameter

draw_triangle(sam, 80)

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To Do• Define functions for drawing stars and moons

• Write a program that uses those functions to draw a sky.

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Lists• Lists are a collection of items in order

• can act on individual elements, indexed by position (0,1,2….) • Like arrays, but better• can be extended or shortened• can be acted on as a whole

• eg, can be stored in variable, passed to function, printed out, …

• Written with square brackets, comma separated, • eg names = ["peter", "james", "justine"]

ages = [ ]

• Access using index inside [ ]: print(names[ 2 ] )

names[ i ] = “jim"

• Append to end:• names.append("jillian")

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Using a listdef main () :

allowed = ["peter", "james", "justine"]

name = input(“Please enter name ")

for i in range(len(allowed)) :

if name == allowed[i] :print("Yes, you may enter")

else :print(“Sorry, you are not allowed”)

main()

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Using a listdef main2 () :

allowed = ["peter", "james", "justine"]name = input(“Please enter name ") for n in allowed :

if name == n :print("Yes, you may enter")

else :print(“Sorry, you are not allowed”)

def main3 () :allowed = ["peter", "james", "justine"]name = input(“Please enter name ") if name in allowed :

print("Yes, you may enter")else :

print(“Sorry, you are not allowed”)

• New for loop construct• New condition construct

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Input the allowed listdef readAllowedNames () : names = [ ] print("Please enter the allowed names. End with 'done'") while True : name = input("Name:") if name == "done" : break; names.append(name) return names

def main () : allowed = readAllowedNames() while True : name = input("Please enter name ") if name in allowed : print("Yes, you may enter") else : print("Sorry, you are not allowed")

main()

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To Do:• stats:

• read a line of numbers into a list then find min, max, average• list all the numbers less than the average.

• wordList• read a list of words from user• print out shortest and longest word ( len(“word”) 4 )• any word starting with a vowel ( “word”[0] “w” )

• scrabble arranger:• read a list of letters• repeatedly :

• ask user for index of a letter to move to the left• swap the letter with its neighbour• print out the list