One dictionary per star

Classroom resources

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Students will be able to

• I can make a dictionary, and read and change a value by its key.
• I can loop over a list of dictionaries.
• I can explain why one dictionary per thing is safer than lists side by side.

Timing

Predict and Run 10 min · Investigate 15 min · Modify 10 min · Make 15 min

Before class

Read the unit primer first. A dictionary pairs keys with values: {"speed": 5, "points": 2}. star["speed"] reads the value; star["speed"] = 8 changes it. Keys are text, so they need quotes; star[speed] (the bug task) looks for a variable called speed and stops with a NameError. A list of dictionaries is a table: one row per thing. The Make task repeats last lesson's bad star so students can compare a new list with a new key.

Running the lesson

1. Predict: read the second line from the right: work out star["points"] + 1, then store it under "points".
2. Run: ask students to find where a star's speed is stored and where it is read.
3. Investigate: the colour question is the lesson's point. One key, made in one place.
4. Bug task: quotes make it text. Without them Python looks for a variable.
5. Make: compare with last lesson's bad star. No list to keep in step.

The hard part (on purpose)

Keys are text in quotes: star["speed"], not star[speed]. Ask them to read the NameError aloud and say what Python thinks speed is.

When students get stuck

Answers to the checks

1. Predict: What does it print?
   5 3. star["points"] starts at 2; the second line stores 2 + 1 back under "points". star["speed"] is still 5.
2. Match the pieces: Match each piece with what it does.
   {"speed": 5, "points": 1} → A. a dictionary: keys paired with values; star["speed"] → B. the value stored under the key "speed"; star["points"] = 3 → D. changes the value stored under "points"; for star in stars: → C. each dictionary in the list, one at a time
3. A new property: Every star should also remember its colour. What changes?
   One more key in the dictionary where each star is made
   Add "colour": ... where the dictionary is made, then read star["colour"] where it is needed. Nothing else can fall out of step.
4. Spot the bug: This should print each star's speed doubled, but it stops with a NameError. Click the line that causes it.
   line 3
   Keys are text, so they need quotes: star["speed"]. Without quotes Python looks for a variable called speed, which does not exist.
5. Where this leads: A school stores each student as a dictionary: {"name": "Ada", "year": 9}. What is a list of them?
   A table: one row per student, one column per key
   A list of dictionaries is a table, which is how most data is stored: spreadsheets, databases and the data sets in Data detective.

Solutions

One possible solution for each task. Other correct programs are fine.

Modify › Different points

from turtle import *
import random

speed(0)
penup()
goto(0, -180)
left(90)
score = 0

pen = Turtle()
pen.hideturtle()
pen.penup()
pen.goto(-220, 200)

stars = []
for i in range(4):
    s = Turtle()
    s.shape("circle")
    s.penup()
    s.goto(-150 + i * 100, 200)
    s.setheading(270)
    stars.append({
        "turtle": s,
        "speed": random.randint(3, 7),
        "points": random.randint(1, 3),
    })

for star in stars:
    star["turtle"].color("gold")

def show_score():
    pen.clear()
    pen.write("Score: " + str(score), False, "left", ("Arial", 16, "normal"))

def go_left():
    goto(xcor() - 25, ycor())

def go_right():
    goto(xcor() + 25, ycor())

def tick():
    global score
    for star in stars:
        star["turtle"].forward(star["speed"])
        if star["turtle"].distance(xcor(), ycor()) < 25:
            score += star["points"]
            star["turtle"].goto(random.randint(-200, 200), 200)
        if star["turtle"].ycor() < -220:
            star["turtle"].goto(random.randint(-200, 200), 200)
    show_score()
    ontimer(tick, 50)

onkey(go_left, "Left")
onkey(go_right, "Right")
listen()
show_score()
tick()
mainloop()

Modify › Colours

from turtle import *
import random

speed(0)
penup()
goto(0, -180)
left(90)
score = 0

pen = Turtle()
pen.hideturtle()
pen.penup()
pen.goto(-220, 200)

colours = ["gold", "orange", "yellow"]
stars = []
for i in range(4):
    s = Turtle()
    s.shape("circle")
    s.penup()
    s.goto(-150 + i * 100, 200)
    s.setheading(270)
    stars.append({
        "turtle": s,
        "speed": random.randint(3, 7),
        "points": 1,
        "colour": colours[i % 3],
    })

for star in stars:
    star["turtle"].color(star["colour"])

def show_score():
    pen.clear()
    pen.write("Score: " + str(score), False, "left", ("Arial", 16, "normal"))

def go_left():
    goto(xcor() - 25, ycor())

def go_right():
    goto(xcor() + 25, ycor())

def tick():
    global score
    for star in stars:
        star["turtle"].forward(star["speed"])
        if star["turtle"].distance(xcor(), ycor()) < 25:
            score += star["points"]
            star["turtle"].goto(random.randint(-200, 200), 200)
        if star["turtle"].ycor() < -220:
            star["turtle"].goto(random.randint(-200, 200), 200)
    show_score()
    ontimer(tick, 50)

onkey(go_left, "Left")
onkey(go_right, "Right")
listen()
show_score()
tick()
mainloop()

Make a bad star

from turtle import *
import random

speed(0)
penup()
goto(0, -180)
left(90)
score = 0

pen = Turtle()
pen.hideturtle()
pen.penup()
pen.goto(-220, 200)

stars = []
for i in range(4):
    s = Turtle()
    s.shape("circle")
    s.penup()
    s.goto(-150 + i * 100, 200)
    s.setheading(270)
    stars.append({
        "turtle": s,
        "speed": random.randint(3, 7),
        "points": 1,
        "bad": i == 0,
    })

for star in stars:
    if star["bad"]:
        star["turtle"].color("red")
    else:
        star["turtle"].color("gold")

def show_score():
    pen.clear()
    pen.write("Score: " + str(score), False, "left", ("Arial", 16, "normal"))

def go_left():
    goto(xcor() - 25, ycor())

def go_right():
    goto(xcor() + 25, ycor())

def tick():
    global score
    for star in stars:
        star["turtle"].forward(star["speed"])
        if star["turtle"].distance(xcor(), ycor()) < 25:
            if star["bad"]:
                score -= 3
            else:
                score += star["points"]
            star["turtle"].goto(random.randint(-200, 200), 200)
        if star["turtle"].ycor() < -220:
            star["turtle"].goto(random.randint(-200, 200), 200)
    show_score()
    ontimer(tick, 50)

onkey(go_left, "Left")
onkey(go_right, "Right")
listen()
show_score()
tick()
mainloop()

Stretch and support

Stretch: Add a "hits" key that counts how often each star has been caught, and make a star faster every third catch.

Support: Give students the "bad" key already in the dictionary and ask them only to change what happens when a star is caught.

Exit ticket

pet = {"name": "Rex", "legs": 4}. Write the line that changes legs to 3.

pet["legs"] = 3

What each step is for

• Investigate: A dictionary is one record per thing: values are found by name, not by place, so a new property is one more key.
• Modify: Changing what a star stores is one line where it is made; using it is star["key"] where it is needed.
• Make a bad star: The same bad star as last lesson, now one key instead of one more list: compare the two.

Where it leads

Next lesson makes a Star class, so a star's values read star.speed instead of star["speed"], and later the star can do things itself. Data detective uses lists of dictionaries for real data sets.