Control Flow — Python Course

What you'll learn

So far programs run straight through. Real programs make decisions — doing different things for different inputs. This module is all about control flow in Python: if statements, the ternary expression, and modern match statements.

By the end you'll be able to:

Build conditions and understand Python's truthiness
Branch with if, elif, and else
Write compact choices with the ternary expression
Match against patterns with match / case
Avoid the classic control-flow mistakes

Booleans & truthiness

Every decision rests on a value that's True or False. Comparison operators (Module 2) produce booleans directly. But Python goes further: any value can be tested for truth. This is called truthiness, and it makes code beautifully concise:

Truthy vs falsy values

Falsy — treated as False

00.0""[]{}NoneFalse

Truthy — treated as True

Essentially everything else: any non-zero number, any non-empty string, list, or dictionary.

truthiness.py

name = ""
items = [1, 2, 3]

if name:                 # empty string is falsy
    print("Has a name")
else:
    print("No name given")

if items:                # non-empty list is truthy
    print(f"{len(items)} items")

Tip

Because empty things are falsy, you write if items: rather than if len(items) != 0:. It reads like plain English — "if there are items" — and it's the Pythonic way.

if, elif & else

The if statement runs a block only when its condition is true. Add elif ("else if") to test more conditions in order, and else for the fallback. Two Python essentials: the condition ends with a colon :, and the block is defined by indentation — no braces:

How an if / else branches

start

score >= 60

True ↓

print("Pass")

False ↓

print("Fail")

grades.py

score = 82

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

Note

Only the first matching branch runs. Python checks if, then each elif in order, and stops at the first true one — the rest are skipped.

The ternary expression

When you just need to pick between two values, Python's ternary reads almost like English: value_if_true if condition else value_if_false:

ternary.py

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

a, b = 7, 12
print("bigger:", a if a > b else b)

match / case

Introduced in Python 3.10, match compares a value against several patterns — cleaner than a long elif chain when checking one value against many options. The underscore _ is the wildcard that matches anything (the default):

match.py

command = "start"

match command:
    case "start":
        print("Starting up...")
    case "stop":
        print("Shutting down...")
    case _:                       # _ is the wildcard (default)
        print("Unknown command")

Note

match can do far more than compare literals — it can destructure lists, tuples, and objects (structural pattern matching). For now, just know it's the modern, readable choice for multi-way branching on a single value.

Common mistakes

Forgetting the colon — every if, elif, and else line ends with :.
Inconsistent indentation — mixing tabs and spaces, or the wrong amount, is a real error in Python.
Using = instead of == — = assigns, == compares. (Python catches this in an if, unlike some languages.)

== vs is

== checks whether two values are equal; is checks whether they're the exact same object. Use == for values, and reserve is for None: write if x is None:.

Recap & quick check

Key takeaways

Conditions are booleans, but ANY value is truthy or falsy — empty/zero/None are falsy.
if / elif / else branch in order; the first true branch wins. Use a colon and indentation, not braces.
Write 'if items:' rather than 'if len(items) != 0:' — it's the Pythonic way.
The ternary 'a if condition else b' picks between two values in one line.
match / case (3.10+) is clean multi-way branching; use 'is None', and == (not =) to compare.

Quick check

1. What ends an if/elif/else line in Python?

2. Which of these is falsy in Python?

3. How many branches run in an if/elif/else chain?

4. How should you check if a variable x is None?

5. Which statement is best for matching one value against many options (Python 3.10+)?

Nicely done — your code can now think for itself. Next up: Module 6 — Loops, where you'll make code repeat work automatically.