From Scratch Programming Class

Session 1: Welcome to Python — typing instead of dragging

Phase 2 — Python with Turtle · Session 1 of 8

What we’re learning today

You spent eight weeks learning how to think like a programmer using Scratch. Today we move into a real programming language called Python — the kind of language used by people who write apps, games, and websites for a living. The blocks are gone. You’ll be typing code instead of dragging it. By the end of class, you’ll have typed your first Python program, watched a turtle draw on your screen, and made (and fixed!) your very first error message.

You’ll need to remember from last time

• Sequences — code runs top to bottom. Order matters.
• The peanut butter rule — computers do exactly what you say, not what you mean. (This rule applies more to Python, not less. You’ll see why.)
• That you finished Phase 1 and built your own game. You can do hard things.

Part A: Meet Thonny and your first turtle

Open Thonny on your computer. It’s already installed — look for a friendly green icon (sometimes it looks like a snail). If you can’t find it, ask your buddy or Mr. Eric.

Thonny is what we’ll use instead of Scratch. It’s called an IDE (which stands for Integrated Development Environment, but nobody says that out loud). For now, it’s the place where you’ll type your Python code.

A tour of Thonny

When Thonny opens, you’ll see two main areas:

• The editor (the top half, big white area). This is where you type your code.
• The shell (the bottom half). This is where Python tells you things — what your program printed, what error it ran into, things like that.

At the top there’s a row of buttons. The one we care about today is the green Run button (or you can press the F5 key on your keyboard — same thing).

Type your first Python program

Click into the editor area at the top. Type this — exactly:

import turtle
t = turtle.Turtle()
t.forward(100)

Three lines. Pay attention to the symbols — the parentheses, the period after t, the period after turtle. Type them all. Type them carefully.

Now save your file. Press Ctrl-S (or File → Save). Thonny will ask where to save it and what to call it. Save it somewhere you’ll remember (the desktop is fine for now), and call it first_turtle.py. The .py part is important — that’s how your computer knows it’s a Python file.

Now click the green Run button (or press F5).

A new window pops up — it’s mostly white, with a small arrow-shape in the middle. That arrow is your turtle. Watch what happens: the turtle moves forward 100 pixels and stops.

Congratulations. You just wrote and ran a Python program.

What just happened

Let’s break those three lines down:

• import turtle — this tells Python: “I want to use the turtle toolbox.” Python comes with lots of toolboxes; you have to ask for the ones you want. We just asked for the one that does drawing with a turtle.
• t = turtle.Turtle() — this creates a new turtle and gives it the name t. (You can name it anything you want, but t is short and quick to type. Some people name it bob. Some name it george. The turtle doesn’t care.)
• t.forward(100) — this tells the turtle named t to move forward by 100 pixels. The number 100 is called an argument — it’s the information you’re passing to the command.

Compare that to Scratch:

Same idea. Different way of telling the computer.

Make a typo on purpose

Now let’s break it. Add a fourth line, but spell forward wrong:

import turtle
t = turtle.Turtle()
t.forward(100)
t.forwerd(100)

Click Run.

The turtle does the first forward. Then it stops, and the shell at the bottom turns red and shows you something like this:

AttributeError: 'Turtle' object has no attribute 'forwerd'

This is your first error message. Don’t panic. Error messages are trying to help. Read it carefully.

Translation: “I tried to find a thing called forwerd for the turtle, but the turtle doesn’t have anything called forwerd. You probably meant something else.”

Fix the typo (change forwerd back to forward) and run again. The error goes away. The turtle moves twice.

The peanut butter rule strikes again. You meant forward. You typed forwerd. The computer doesn’t know what you meant — it only knows what you wrote. Welcome to typing precision.

Draw a square

Now let’s make the turtle draw a real shape. Replace your code with this:

import turtle
t = turtle.Turtle()
t.forward(100)
t.right(90)
t.forward(100)
t.right(90)
t.forward(100)
t.right(90)
t.forward(100)
t.right(90)

Save (Ctrl-S). Run (F5).

The turtle walks forward 100, turns right 90 degrees, walks forward 100, turns again, four times. A square.

Look familiar? In Session 3 of Phase 1, you made the cat draw a square in Scratch with repeat 4. We don’t have repeat yet in Python — that’s next week. For now, four times by hand.

Checkpoint: You have a Python program saved as first_turtle.py that draws a square when you press Run. This is the natural stop point if class is cut short.

Part B: Colors, pen control, and shapes

Now let’s make our drawings look better.

Colors

Add this line before you start drawing the square:

t.color("red")

Save and run. The square is red.

Try other colors: "blue", "green", "purple", "orange", "pink", "yellow", "black". Anything you’d guess, Python probably knows.

Thicker lines

Add this line too:

t.pensize(5)

Now the lines are five pixels thick instead of one. Try pensize(10) or pensize(20).

Lifting the pen

Sometimes you want to move the turtle without drawing. Use these:

• t.penup() — lift the pen, no more drawing
• t.pendown() — drop the pen, drawing again

Try this — two squares side by side:

import turtle
t = turtle.Turtle()
t.color("blue")
t.pensize(3)

t.forward(100)
t.right(90)
t.forward(100)
t.right(90)
t.forward(100)
t.right(90)
t.forward(100)
t.right(90)

t.penup()
t.forward(150)
t.pendown()

t.forward(100)
t.right(90)
t.forward(100)
t.right(90)
t.forward(100)
t.right(90)
t.forward(100)
t.right(90)

Two squares with a gap between them. The middle three lines (penup, forward 150, pendown) move the turtle without drawing.

Try this on your own

Pick one of these:

• Base goal: Draw a triangle. (A triangle is three sides. The turn at each corner is not 90 degrees — figure out what it should be. Hint: remember the polygon pattern from Session 3 of Phase 1.)
• Stretch: Draw your initials. Each letter as a few lines. Use penup() and pendown() to skip between letters.
• Extension: Draw a simple house. A square for the body, a triangle for the roof on top. Use color.

Wrap-up

Before we leave, let’s share:

• What was different from Scratch?
• What was harder?
• What was easier?
• Did you get any error messages? What were they?

You did something today that’s a big deal. You typed real Python code, ran it, made a typo, fixed it, and made the computer draw something. That’s the entire programming workflow. Every programmer in the world does that exact loop, every day.

The hard part wasn’t the typing. The hard part was getting used to being precise — every character matters, every parenthesis matters, every period matters. That precision will get easier fast.

If you missed this session

Open Thonny on your computer. Type these three lines exactly:

import turtle
t = turtle.Turtle()
t.forward(100)

Save the file as first_turtle.py. Click the green Run button or press F5. A window opens; the turtle moves.

Now extend it to draw a square. You’ll need four t.forward(100) lines and four t.right(90) lines, alternating.

Then add t.color("red") and t.pensize(3) near the top to make the square colorful and thick. Save, run.

About 30 minutes of work. If you get stuck, ask your buddy at the start of next class.

Stretch and extension ideas

• Try t.left(90) instead of t.right(90) and see what happens.
• Try t.backward(50) to walk the turtle backward.
• Try t.shape("turtle") (right after the t = ... line). The arrow becomes an actual little turtle shape. Then try "square", "circle", "triangle", "arrow".
• Try t.speed(1) (slow) or t.speed(10) (fast). Where does the speed matter? Where doesn’t it?

What’s next

Next week we’ll learn about all the things the turtle can do — moving in different ways, going to specific positions, drawing patterns. By the end of next week’s class, your drawings will look way fancier.