Making Decisions with If/Else

Summary

Programs need to make choices — just like Scratch's "if/then/else" blocks. Python's if, elif, and else keywords handle branching. This chapter introduces the Boolean type (True/False), comparison operators (==, !=, <, >), logical operators (and, or, not), chained comparisons, and nested conditionals for multi-way decisions.

Concepts Covered

This chapter covers the following 9 concepts from the learning graph:

1. Boolean Type
2. Comparison Operators
3. Logical Operators
4. Boolean in Conditions
5. Chained Comparisons
6. if Statement
7. if...else Block
8. elif Chains
9. Nested if Statements

Prerequisites

This chapter builds on concepts from:

• Chapter 3: Variables and Numbers
• Chapter 6: Math and Arithmetic

---

By the end of this lesson you'll be able to:

• Create and compare Boolean values using True, False, and comparison operators
• Write if, if/else, and if/elif/else blocks to make your program branch
• Combine conditions with and, or, and not
• Use an if/elif/else chain to draw different shapes based on a variable's value

Every useful program makes decisions. A game needs to know if a player has won. A calculator needs to know if the denominator is zero before dividing. Python's if statement is how programs choose what to do based on what is true or false.

Welcome to Chapter 9!

This is the chapter where your programs stop being one-way streets! Once you know if, elif, and else, your code can respond differently to different situations — just like a real decision-maker. Let's code it together!

The Boolean Type

A Boolean (named after mathematician George Boole) is a value that is either True or False — and nothing else. In Python, True and False are written with a capital first letter.

is_raining = True
has_umbrella = False
print(is_raining)       # True
print(type(True))       # <class 'bool'>

Booleans are produced automatically whenever Python compares two values.

Comparison Operators

A comparison operator compares two values and returns a Boolean. Python has six comparison operators:

Operator	Meaning	Example	Result
==	equal to	5 == 5	True
!=	not equal to	5 != 3	True
<	less than	3 < 5	True
>	greater than	5 > 3	True
<=	less than or equal	5 <= 5	True
>=	greater than or equal	4 >= 5	False

The Most Common Beginner Mistake

= is NOT the same as ==. A single = assigns a value to a variable: x = 5. A double == compares two values: x == 5. Writing if x = 5: will give you a SyntaxError. This mistake happens to every beginner — now you know to look for it!

Scratch Bridge

In Scratch, the green Operators blocks include =, <, and > for comparisons, and the Control category has the "if/then" and "if/then/else" blocks. Python's if statement and comparison operators do exactly the same jobs — you just type them instead of dragging!

The program below uses comparisons to describe a score. Before clicking Run, think about which print() calls will actually run.

What Do You Think Will Happen?

The variable score is 75. Look at each if statement carefully. Which conditions are True for a score of 75? How many lines do you think will actually print? Make your guess — then click Run!

Try It Now

Edit the code below and click Run to see the result.

▶ Run ↻ Reset

Were you right? The line "Perfect score!" did NOT print because 75 == 100 is False. The if body only runs when its condition is True.

The if Statement

An if statement runs a block of code only when a condition is True. The structure is:

if condition:
    # this block runs only when condition is True
    do_something()

The colon : at the end of the if line is required. The indented lines below it form the if body — they only run when the condition is True.

The if…else Block

An if/else block adds a second branch: code that runs when the condition is False.

temperature = 15

if temperature > 20:
    print("It's warm — wear a t-shirt!")
else:
    print("It's cool — bring a jacket!")

Every if/else runs exactly one branch — never both, never neither. When temperature is 15, the condition 15 > 20 is False, so the else branch runs.

elif Chains

When you need more than two choices, use elif (short for "else if"):

score = 83

if score >= 90:
    print("A")
elif score >= 80:
    print("B")
elif score >= 70:
    print("C")
elif score >= 60:
    print("D")
else:
    print("F")

Python checks conditions from top to bottom and runs the first branch whose condition is True, then skips all the rest. For score = 83, the first True condition is score >= 80, so Python prints "B" and ignores the remaining branches.

See It: One Path Lights Up

The explorer below turns an elif chain into a flowchart. Exactly one path glows green, and — this is the important part — conditions after the first True are marked "never checked" because Python does not even look at them. Before you move the slider, predict: with a score of 85, which conditions get checked, and which branch runs?

Explore the Control Flow Explorer MicroSim

Slide the score slowly from 100 down to 0 and watch the green path move down the chain one branch at a time. Then try the Simple if program — what happens to the flowchart when the condition is False and there is no else?

Logical Operators

Logical operators combine two Boolean values into one:

Operator	Meaning	Example	Result
and	Both must be True	5 > 3 and 2 < 4	True
or	At least one must be True	5 > 3 or 2 > 4	True
not	Flips True↔False	not True	False

age = 13
has_ticket = True

if age >= 10 and has_ticket:
    print("Welcome to the show!")

if age < 5 or age > 65:
    print("Free admission!")

See It: The Boolean Logic Lab

Instead of memorizing truth tables, build them yourself. Set inputs A and B in the lab below and watch the result lamp. Before you start, predict: if A is False, does A and B ever light the lamp — no matter what B is?

Explore the Boolean Logic Lab MicroSim

Notice something sneaky: when A alone decides the answer, the B block grays out and says "Python never looked here." That trick is called short-circuit evaluation, and you will meet it again in Chapter 19. The bonus panel at the bottom previews another Chapter 19 idea — every value, even 0 and "", can act as a Boolean.

Chained Comparisons

Python lets you write range checks in a natural, math-like way:

x = 15

# The Python way (chained):
if 10 < x < 20:
    print("x is between 10 and 20")

# The long way (same thing):
if x > 10 and x < 20:
    print("x is between 10 and 20")

Both forms are correct, but chained comparisons read more naturally and look more like how you'd write it in math class.

Nested if Statements

A nested if is an if statement placed inside another if body. Use nesting when you need to check a second condition only after a first one is already True:

is_member = True
age = 14

if is_member:
    if age >= 13:
        print("Full member access!")
    else:
        print("Junior member access!")
else:
    print("Please sign up first.")

Keep nesting to two levels maximum — deeply nested code becomes hard to read quickly.

if/elif/else with Turtle: Draw Different Shapes

Now let's use branching to make the turtle draw different shapes based on a variable. The program below should draw a different shape depending on the value of shape_choice — but the elif chain is incomplete.

Your Turn — Complete the elif Chain

The program checks shape_choice and draws a square when it equals "square". Add two more elif blocks so that "triangle" draws a triangle (3 sides, 120° turns) and "hexagon" draws a hexagon (6 sides, 60° turns).

▶ Run ↻ Reset

Once all three shapes work, try setting shape_choice = "circle" to see the else branch draw the "Unknown shape" message!

Experiments

Try these changes to the programs above. For each one, predict what will happen first, then run it to check!

1. In the first lab, change score = 75 to score = 92. You'll know it worked when the if score >= 60 block still runs but now score >= 90 would also be True.
2. Add an elif score == 75: print("Exactly 75!") block. You'll know it worked when changing score back to 75 prints that message.
3. In the turtle lab, add an elif shape_choice == "pentagon": block that draws 5 sides with 72° turns. You'll know it worked when shape_choice = "pentagon" draws a pentagon.
4. Add t.pencolor("purple") inside the triangle branch only. You'll know it worked when the triangle draws in purple but the square stays black.
5. Try shape_choice = "hexagon". You'll know it worked when a six-sided hexagon appears on the canvas.

Great Work!

Your programs can now make decisions! You know Boolean values, six comparison operators, three logical operators, and four types of branching: if, if/else, elif chains, and nested if. These are the building blocks of every interactive program ever written. Let's keep coding it together!

Take the Chapter Review Quiz

See Annotated References