04-control-flow

Control Flow

Objective

Learn how to make your programs make decisions using if, elif, else, and pattern matching with match/case.

Concepts Covered

• if statements and indentation
• Comparison operators (==, !=, <, >, <=, >=)
• elif and else
• Logical operators (and, or, not)
• Truthy and falsy values revisited
• Nested if statements
• Ternary expressions (conditional expressions)
• match/case (Python 3.10+ structural pattern matching)
• Common pitfalls

Prerequisites

• Variables and Types — you should know how variables, types, and booleans work

Lesson

The if Statement

Up until now, every line of your code has run from top to bottom, no exceptions. That changes here. An if statement lets Python decide whether to run a block of code:

age = 18

if age >= 18:
    print("You can vote!")

Two things to notice:

• The colon : at the end of the if line. It's required. Forget it and Python will yell at you.
• The indentation. The indented line belongs to the if block. Python uses whitespace (4 spaces by convention) to define code blocks — not curly braces {} like JavaScript or C. This is one of Python's most distinctive features.

If the condition is True, the indented code runs. If it's False, Python skips it entirely.

Comparison Operators

You need these to build conditions:

x = 10

x == 10    # True   (equal to — note the DOUBLE equals sign)
x != 5     # True   (not equal to)
x > 5      # True   (greater than)
x < 20     # True   (less than)
x >= 10    # True   (greater than or equal to)
x <= 9     # False  (less than or equal to)

Big warning: = is assignment, == is comparison. Mixing them up is one of the most common beginner mistakes:

x = 10     # This SETS x to 10
x == 10    # This CHECKS if x equals 10

elif and else

What if you need to check multiple conditions? That's where elif (short for "else if") and else come in:

score = 85

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

print(f"Your grade: {grade}")   # Your grade: B

Here's how it works:

• Python checks each condition from top to bottom
• The first condition that's True wins — its block runs and everything else is skipped
• else is the catch-all — it runs only if nothing above matched
• else is optional. You can have if/elif without an else

Logical Operators

Combine multiple conditions with and, or, and not:

age = 25
has_license = True

# and — both must be True
if age >= 16 and has_license:
    print("You can drive!")

# or — at least one must be True
if age < 13 or age > 65:
    print("You get a discount!")

# not — flips True to False and vice versa
is_banned = False
if not is_banned:
    print("Welcome in!")

You can combine as many as you want, but keep it readable. If your condition is getting long, consider breaking it into variables:

# Hard to read
if age >= 18 and has_license and not is_banned and country == "US":
    print("OK")

# Much clearer
is_eligible = age >= 18 and has_license
is_allowed = not is_banned and country == "US"
if is_eligible and is_allowed:
    print("OK")

Truthy and Falsy Values Revisited

You learned about truthy/falsy in the variables lesson. Here's where it really matters. Python doesn't require conditions to be explicit booleans — any value works:

name = "Alice"

# Instead of this:
if name != "":
    print("Name is set")

# You can just write this:
if name:
    print("Name is set")

Quick reminder of what's falsy in Python:

• False
• 0, 0.0
• "" (empty string)
• None
• [], {}, () (empty collections — you'll learn these later)

Everything else is truthy. This is a very Pythonic pattern you'll see everywhere.

Nested if Statements

You can put if statements inside other if statements. Each level gets another indent:

has_ticket = True
age = 15

if has_ticket:
    if age >= 18:
        print("Welcome to the R-rated movie!")
    else:
        print("Sorry, you must be 18+")
else:
    print("You need a ticket first!")

Nesting works, but don't go too deep — more than 2-3 levels usually means you should rethink your approach. Often you can flatten nested ifs with and:

# Same logic, flattened
if has_ticket and age >= 18:
    print("Welcome to the R-rated movie!")
elif has_ticket:
    print("Sorry, you must be 18+")
else:
    print("You need a ticket first!")

Ternary Expressions (Conditional Expressions)

Sometimes you just need a quick "this or that" assignment. Python has a one-liner for that:

age = 20
status = "adult" if age >= 18 else "minor"
print(status)   # adult

The format is: value_if_true if condition else value_if_false

It's great for simple cases but don't overuse it. If the logic is complex, a regular if/else is much clearer.

# Fine — short and obvious
label = "even" if x % 2 == 0 else "odd"

# Don't do this — hard to read
result = "A" if score >= 90 else "B" if score >= 80 else "C" if score >= 70 else "F"

match/case (Python 3.10+)

Python 3.10 introduced structural pattern matching with match/case. Think of it like a cleaner alternative to long if/elif chains when you're checking a single value:

command = "quit"

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

The _ is a wildcard — it matches anything, like else in an if chain.

match/case can do much more (matching patterns, destructuring, etc.), but for now just know it exists as an option when you're comparing one value against several possibilities.

Common Pitfalls

1. = vs ==

# WRONG — this assigns, doesn't compare
# if x = 10:

# RIGHT — double equals for comparison
if x == 10:
    print("x is 10")

2. Forgetting the colon

# WRONG — missing colon
# if x > 5
#     print("big")

# RIGHT
if x > 5:
    print("big")

3. Wrong indentation

# WRONG — inconsistent indentation causes IndentationError
# if x > 5:
#     print("big")
#       print("number")     # Too much indent!

# RIGHT — all lines in a block use the same indent
if x > 5:
    print("big")
    print("number")

4. Using or incorrectly

# WRONG — this doesn't do what you think!
# "red" is truthy, so this is always True
# if color == "red" or "blue":

# RIGHT
if color == "red" or color == "blue":
    print("Primary color")

# EVEN BETTER
if color in ("red", "blue"):
    print("Primary color")

Code Example

Check out example.py for a complete working example that demonstrates everything above.

Exercises

Try the practice problems in exercises.py to test your understanding.

Key Takeaways

• if, elif, and else let your program make decisions
• Python uses indentation (4 spaces) to define code blocks — no braces needed
• Comparison operators: ==, !=, <, >, <=, >=
• Combine conditions with and, or, not
• Any value can be used as a condition — Python evaluates its "truthiness"
• Ternary expressions (x if condition else y) are handy for simple one-liners
• match/case (Python 3.10+) is great for matching a value against multiple options
• Watch out for = vs ==, missing colons, and inconsistent indentation