First light: device, assistant and first program

Below the slides are the tasks you work through in the workshop, to read again and to carry on with at home. You write your program in VS Code and run it in the terminal. You set up both beforehand, as Required software in the LiFi project describes.

Your assistant could write the kitchen timer in ten seconds, but then you would have a program and would have learned nothing. So use it as a teacher: let it explain what a line does, let it read your file ("read my timer.py, is step 2 right?") or run the program itself, and ask it for a hint instead of the solution.

Warm-up 1: your first program

Open your course folder in VS Code with File > Open Folder. On the left, right-click the folder my-code, choose New File and call it hello.py. Type this line into it and save with Ctrl+S (Cmd+S on a Mac):

print("Hello, world!")

Then open a terminal with Terminal > New Terminal. It starts in your course folder, so you change into your folder with cd my-code and run the program with python hello.py, on a Mac with python3 hello.py.

> python hello.py
Hello, world!

Warm-up 2: asking and answering

Change hello.py so that the program asks for your name and greets you with it. Then run it again; the up arrow in the terminal brings back the last command.

> python hello.py
What is your name? Kim
Hello, Kim!

name = input("What is your name? ")
print("Hello, " + name + "!")

The kitchen timer

Write a kitchen timer for the terminal. It asks how many seconds it should run and then counts down, one line per second. A coloured lamp shows how it stands: green while it runs, yellow in the last five seconds and red when time is up. Without a device the lamp is a coloured block in the terminal; with your device it becomes the LED.

> python timer.py
How many seconds? five
Digits only, please: 8
Timer: 8 seconds
████ 8 s
████ 7 s
████ 6 s
████ 5 s
████ 4 s
████ 3 s
████ 2 s
████ 1 s
████ time is up!

Whoever writes it all at once gets lost. So cut the timer into five steps, each with its own tool in Python, and run the program after every step; when something breaks, the fault is in the last step. How to cut a problem like this is shown in Cutting problems.

Step 1: get the number of seconds

Create the file timer.py in my-code. The program asks for the number of seconds, turns it into a whole number and prints it as a test.

text = input("How many seconds? ")
seconds = int(text)
print("Timer:", seconds, "seconds")

Now start the timer and type five instead of a number. The program stops with an error message:

Traceback (most recent call last):
  File "…\my-code\timer.py", line 2, in <module>
    seconds = int(text)
ValueError: invalid literal for int() with base 10: 'five'

Read it from the bottom up. The last line says what went wrong: int() cannot make a number out of "five". The lines above say where it happened, in timer.py in line 2, and show that line. If you are stuck, let your assistant run the program with five and explain the message.

Step 2: check if the input is valid

Make the program ask again until only digits have been typed. Then try five and then 20, and ask your assistant what happens with 0.

text = input("How many seconds? ")
while not text.isdigit():
    text = input("Digits only, please: ")
seconds = int(text)

Step 3: count down each second

Let the program count down from the number typed to 1, one line per second, and print "time is up!" at the end. Then, as a test, move time.sleep(1) out of the indentation and watch what changes.

import time

for remaining in range(seconds, 0, -1):
    print(remaining, "s")
    time.sleep(1)

print("time is up!")

Step 4: show a coloured lamp

You need the lamp three times in three colours, so you give its lines a name. Copy the function lamp(r, g, b) below import time and try it with lamp(0, 200, 0), which is green. The values for red, green and blue each range from 0 to 255.

def lamp(r, g, b):
    on = f"\033[48;2;{r};{g};{b}m"
    off = "\033[0m"
    print(on + " " * 12 + off, end=" ")

The characters with the backslash are instructions to the terminal that switch a background colour on and off again; in between are twelve spaces, which become coloured that way. You do not have to understand them to use lamp(), and that is exactly what a function gives you: it hides the details behind a name, just as print() and input() do. If you see strange characters instead of a colour, your program runs in the old Windows console; ask your assistant then.

Step 5: decide which colour

Show a lamp in every round of the loop, yellow when five seconds or fewer are left and green otherwise, and a red one once after the loop. Try it with 8 seconds.

for remaining in range(seconds, 0, -1):
    if remaining <= 5:
        lamp(255, 200, 0)   # yellow
    else:
        lamp(0, 200, 0)     # green
    print(remaining, "s")
    time.sleep(1)

lamp(255, 0, 0)             # red
print("time is up!")

The whole program

Read your finished program in VS Code from top to bottom, the way Python reads it. That is why import time stands at the very top, then the function, and only after that the part that uses it, because a function has to be defined before it is called. Ask your assistant about every line you cannot explain.

import time

def lamp(r, g, b):
    on = f"\033[48;2;{r};{g};{b}m"
    off = "\033[0m"
    print(on + " " * 12 + off, end=" ")

text = input("How many seconds? ")
while not text.isdigit():
    text = input("Digits only, please: ")
seconds = int(text)
print("Timer:", seconds, "seconds")

for remaining in range(seconds, 0, -1):
    if remaining <= 5:
        lamp(255, 200, 0)   # yellow
    else:
        lamp(0, 200, 0)     # green
    print(remaining, "s")
    time.sleep(1)

lamp(255, 0, 0)             # red
print("time is up!")

For the quick ones

Show the time left in minutes and seconds, so 1:05 instead of 65 s.

Where are you?

When you are asked in the workshop where you are, answer with the line you have reached. Whoever has reached the last one helps the team at the next table or extends the timer.

1. OpenCode is installed and the course folder is open.
2. The assistant answers.
3. /onboarding has run through.
4. hello.py runs.
5. The timer counts down.
6. The timer shows its colours.

With your device, the lamp in the terminal becomes the LED: you swap lamp() for led.set_color(), and your timer lights up on the table.