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Session 3: Loops in Python — for and range

Phase 2 — Python with Turtle · Session 3 of 8

What we’re learning today

Last week your code was getting long and repetitive — eight lines to draw a square, lots of similar lines for the scene. Today that’s about to change. We’ll learn the Python version of Scratch’s repeat block, called a for loop. By the end of class, you’ll draw a square in three lines instead of eight, and you’ll be making patterns that would take hundreds of lines without loops.

You’ll need to remember from last time

  • Open Thonny, type code, save with .py, click Run.
  • Every program starts with import turtle and t = turtle.Turtle().
  • t.forward(N), t.right(N), t.left(N), t.color(), t.pensize().
  • # starts a comment. Everything after it on the line is ignored.
  • Loops from Scratch (repeat 4, forever). Same idea, new syntax.

Part A: The for loop

Open Thonny and start a new file. Save it as loops.py.

The square, the long way

Let’s start by typing the square the way we did in Session 1:

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)

Run it. A square. Eight lines.

Now look at those eight lines. Two lines repeated four times. That’s exactly what a loop is for: doing the same thing N times without writing it N times.

The square, the loop way

Replace your eight lines with this:

import turtle
t = turtle.Turtle()

for i in range(4):
    t.forward(100)
    t.right(90)

Run it. Same square. Three lines instead of eight.

Let’s break down what’s happening:

  • for i in range(4): — this means “do the next part 4 times.” The range(4) produces a sequence of numbers from 0 to 3 (which is four numbers — Python starts counting at 0, not 1).
  • i is the loop variable. Each time through the loop, i takes the next value from the sequence (0, then 1, then 2, then 3). We don’t have to use i in this loop — but the syntax requires we name it. Most programmers use i by default.
  • The colon at the end of the for line is required. Don’t forget it.
  • The lines underneath the for are indented — pushed in by four spaces. That’s how Python knows they’re part of the loop.

Indentation matters in Python

This is the big new thing about Python that didn’t exist in Scratch. Indentation is part of the grammar.

In Scratch, you snapped blocks inside the repeat block’s mouth. Python uses indentation instead. Lines that are indented under for i in range(N): are inside the loop. Lines that are not indented are outside the loop.

Try this — change the file so the second line isn’t indented:

for i in range(4):
    t.forward(100)
t.right(90)

Now run it. The turtle moves forward 4 times in a straight line, then turns once. That’s because only t.forward(100) is inside the loop. The t.right(90) is outside — it runs once, after the loop is done.

Indentation isn’t decoration. It’s part of what your program does. Be careful with it.

Good news: Thonny indents for you. When you type for ... : and press Enter, Thonny moves your cursor in 4 spaces automatically. Trust it.

Heads up: If you ever copy-paste code from somewhere, the indentation may not match. Mixed tabs and spaces is a common cause of confusing errors. If your code looks right but won’t run, check the indentation by clicking each line and looking at the cursor position.

Try other shapes

Now try changing the loop count and the turn angle:

for i in range(3):
    t.forward(100)
    t.right(120)

Run it. A triangle.

Try range(5) and t.right(72). A pentagon.

Try range(6) and t.right(60). A hexagon.

You probably noticed: number-of-sides times turn-angle always equals 360. Same pattern as Phase 1 Session 3. Loops just made it way easier to test.

Checkpoint: You’ve replaced a manually-typed shape with a for loop, and you’ve used range(N) with a different number to draw at least one different shape (triangle, pentagon, etc.). This is the natural stop point if class is cut short.

Part B: Patterns that loops make easy

Now we’ll do things that would have been ridiculous to type by hand.

A spiral

Start a new file. Save it as spiral.py. Type:

import turtle
t = turtle.Turtle()
t.speed(0)
t.color("blue")

for i in range(50):
    t.forward(i * 2)
    t.right(15)

Run it. A blue spiral that grows outward.

Look at what’s happening: each time through the loop, i is different (0, 1, 2, 3, … 49). And i * 2 (i times 2) is the distance the turtle walks. So the first time the turtle walks 0 steps, then 2, then 4, then 6… longer and longer. Combined with a 15-degree turn each time, it spirals.

This is the first time we’ve actually used the loop variable i for something. It’s not just a counter — it’s a value we can compute with. Variables are next week’s lesson, so don’t worry about the math yet — just notice that i does something.

t.speed(0) makes the turtle draw without animation (instant). On a long loop like this, the animation would take forever.

Try the base goal

Pick one:

  • A colorful shape: Use a loop to draw a many-sided shape (like a 12-sided dodecagon, with range(12) and t.right(30)). Make it any color you want.
  • A row of squares: Use a loop to draw 5 squares in a row, each one a small distance apart. (Hint: inside the loop, draw a square the long way — without a loop — then move the turtle to the next position.)

Stretch — nested loops

You can put a loop inside another loop. The inside loop runs all the way through, then the outside loop moves to its next iteration, then the inside loop runs all the way through again, and so on.

Try this — the flower of squares from Phase 1:

import turtle
t = turtle.Turtle()
t.speed(0)
t.color("purple")

for i in range(12):
    for j in range(4):
        t.forward(50)
        t.right(90)
    t.right(30)

Run it. A flower-shaped pattern made of 12 rotated squares.

The outer loop runs 12 times. Each time, the inner loop draws a square (4 sides), and then the outer loop turns 30 degrees. 12 squares × 30 degrees = 360 degrees, so they go all the way around.

Notice: the inner loop’s body is indented twice (8 spaces from the left). Indentation level says how nested you are. The t.right(30) is indented once (4 spaces) because it’s inside the outer loop but outside the inner loop.

This is genuinely confusing the first time. Read it carefully. Look at the indentation. Run it. Change the numbers and see what changes.

Extension

Make a colorful spiral that changes color as it grows. Add this to the spiral:

import turtle
t = turtle.Turtle()
t.speed(0)

colors = ["red", "orange", "yellow", "green", "blue", "purple"]

for i in range(60):
    t.color(colors[i % 6])
    t.forward(i * 2)
    t.right(15)

The colors[i % 6] part picks a color from the list based on which iteration we’re in. The % operator gives the remainder when dividing — so i % 6 cycles through 0, 1, 2, 3, 4, 5, 0, 1, 2, 3… and that picks the colors in order, repeating.

If that’s confusing right now, that’s fine. Just type it and run it. Lists, indexing, and the % operator are things we’ll cover properly in later sessions. For now, enjoy the rainbow.

Wrap-up

Before we leave, share with the room:

  • What was the most-sides shape anyone drew?
  • Did anyone try the nested-loops flower? What did it look like?
  • Did anyone get a confusing indentation error? What fixed it?

You did something today that genuinely changes what you can build. Loops are how programmers make patterns, animations, and anything that involves “do this many times.” From here on out, you’ll use them constantly.

You also met indentation as a real grammar rule. That’s the single weirdest thing about Python compared to most other programming languages. After a couple of sessions it’ll feel normal — for now, just remember: indented = inside, not indented = outside.

If you missed this session

Open Thonny and start a new file. Save it as loops.py. Then:

  1. Build a for loop that draws a square:

    import turtle
t = turtle.Turtle()

for i in range(4):
    t.forward(100)
    t.right(90)

    Save, run. Watch the square draw.

  2. Change range(4) and t.right(90) to draw a triangle (3 sides, 120 degrees) and a pentagon (5 sides, 72 degrees).

  3. Try the spiral from Part B above.

About 30 minutes. Pay attention to indentation — the lines inside the loop need to be pushed in by 4 spaces (Thonny does this for you when you press Enter after the colon).

If you get stuck, ask your buddy at the start of next class.

Stretch and extension ideas

  • Try range(N) with a different starting number: range(5, 15) produces 5 through 14. range(0, 20, 2) produces 0, 2, 4, 6, …, 18 (counts by twos).
  • Try a for i in range(360) loop where the turtle moves 1 step and turns 1 degree each iteration. What shape does that draw?
  • Try multiple loops in one program. Draw five different shapes, one after another, with penup / goto between them to space them out.

What’s next

Next week we’ll learn about functions — how to take a chunk of code (like “draw a square”) and give it a name, so you can just say draw_square() instead of writing the whole loop every time. It’s the biggest new idea in Phase 2.