More Booleans and Conditionals

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

• Explain truthiness and falsiness — how any Python value can act as True or False
• Use short-circuit evaluation to write safer, faster conditions
• Write compact ternary expressions and use Python's match/case statement
• Combine conditions with and, or, and not to build powerful boolean logic

You've used if statements since Chapter 9. Now it's time to understand how Python's truth system really works — and unlock some powerful shortcuts.

Welcome to Chapter 19!

Did you know Python treats lots of things as True or False — not just True and False? Today we'll peek behind the curtain and discover some clever boolean tricks! Let's code it together!

Truthiness and Falsiness

In Python, every value can be tested in an if statement, not just True and False.

A value is falsy if Python treats it like False in a boolean context:

Falsy values
False
0 (zero integer)
0.0 (zero float)
"" (empty string)
[] (empty list)
{} (empty dict)
None

Everything else is truthy — including non-zero numbers, non-empty strings, and non-empty lists.

▶ Run ↻ Reset

Truthiness lets you write concise checks. Instead of if len(name) > 0:, just write if name:.

Short-Circuit Evaluation

When Python evaluates an and or or expression, it stops as soon as it knows the answer. This is called short-circuit evaluation.

With and: if the left side is falsy, Python returns it immediately without checking the right side. With or: if the left side is truthy, Python returns it immediately.

This matters because it can prevent errors — and provides a useful default-value pattern:

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Functions Returning None

Every Python function that doesn't have a return statement automatically returns None. This catches many beginners off-guard:

def add(a, b):
    result = a + b
    # forgot to write: return result

x = add(3, 4)
print(x)   # None — oops!

Always use return when you want a function to send a value back.

Checking for None with is

To check whether a value is None, use is None rather than == None.

result = get_data()

if result is None:
    print("Nothing came back")
else:
    print("Got:", result)

is checks identity (the exact same object in memory), while == checks equality in value. For None, True, and False, always use is.

Implicit vs Explicit Conversion

Python sometimes converts values automatically — this is implicit conversion. When you convert deliberately using int(), str(), etc., that's explicit conversion.

# Implicit: Python automatically converts int to float in mixed math
result = 3 + 2.5   # 5.5 — int 3 became float 3.0 automatically

# Explicit: you control the conversion
n = int("42")
s = str(3.14)
b = bool(0)

Understanding the difference helps you spot where Python is "helping" you — and when that help might cause surprises.

Reading Multiple Inputs

You can ask for several values on one line using split():

x, y = input("Enter x and y: ").split()
x = int(x)
y = int(y)
print(f"Sum: {x + y}")

The user types something like 3 7, split() returns ["3", "7"], and Python unpacks that into x and y.

The Conditional Ternary Expression

A ternary expression condenses a simple if/else into one line. The format is: value_if_true if condition else value_if_false.

# Normal if/else:
if score >= 70:
    grade = "pass"
else:
    grade = "fail"

# Ternary — same result, one line:
grade = "pass" if score >= 70 else "fail"

What Do You Think Will Happen?

The code below uses a nested ternary expression. What will it print for score = 85? For score = 55? Make your guess — then run it!

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Compound Conditions

Use and, or, and not to combine multiple conditions:

age = 15
has_ticket = True

if age >= 13 and has_ticket:
    print("You may enter.")

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

if not has_ticket:
    print("You need a ticket!")

Operator	Meaning	True when
and	Both must be true	True and True
or	At least one must be true	True or False
not	Flip true/false	not False

match/case — Python's Pattern Matching

Python 3.10 introduced match/case, which compares a value against several patterns — similar to a long chain of if/elif but much cleaner.

command = input("Command (start/stop/pause): ").strip().lower()

match command:
    case "start":
        print("Starting!")
    case "stop":
        print("Stopping!")
    case "pause":
        print("Pausing!")
    case _:
        print("Unknown command.")

The _ case is the wildcard — it matches anything not already handled. Use | inside a case to match multiple values:

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Learning Check

Your Turn — Fix the Default

The program below is supposed to greet a user by name — or say "Hello, stranger!" if nothing is entered. But the or operands are reversed, so it always says "stranger". Swap them to fix it!

▶ Run ↻ Reset

Experiments

Try these changes. Predict what will happen first, then run it to check!

1. Print bool([]), bool([0]), bool(""), and bool(" ") (a space). You'll know it worked when you see False, True, False, True.

2. Write a ternary expression that returns "even" if a number is divisible by 2, else "odd". You'll know it worked when 7 gives "odd" and 8 gives "even".

3. Use match/case to classify a score: 90+ → "A", 80–89 → "B", 70–79 → "C", below 70 → "F". You'll know it worked when entering 85 gives "B".

4. Use a short-circuit or to assign a timeout = user_value or 30 default. You'll know it worked when an empty input gives 30 and "60" gives "60".

5. Write a compound condition that validates a username: truthy AND length >= 3 AND no spaces. You'll know it worked when "ab" fails (too short) and "hello world" fails (has space).

Boolean Master!

You've unlocked truthiness, short-circuits, ternary expressions, and match/case! These tools let you write smarter, cleaner conditional logic in far fewer lines. The more you practice, the more natural they feel. Let's keep coding!

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