Control Flow Beginner¶

🐍 Core Python Track · Level 1

⏱️ ~3 days 📚 Prerequisite: Python Basics

When you'd use this

if/elif/else, for loops, while loops and match-case.

Use conditionals and loops to make decisions and repeat work — validating input, iterating over data, retrying until a condition holds, dispatching on a value.

Conditionals¶

Branch on a condition with if/elif/else. Use it to validate input, pick a code path, or map a value to a category.

score = 85

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

Ternary (one-liner)¶

score = 85
label = "pass" if score >= 50 else "fail"
print(label)   # pass

Truthy and Falsy values¶

# Falsy: False, None, 0, 0.0, "", [], {}, set()
# Everything else is Truthy

if []:          print("never")     # empty list is falsy
if [1, 2, 3]:   print("yes!")      # non-empty list is truthy
if "":          print("never")     # empty string is falsy
if "hello":     print("yes!")

for Loops¶

Iterate over any sequence or range. Pair with enumerate when you need the index and zip to walk two sequences together.

# Iterate over a sequence
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

# Range
for i in range(5):          # 0, 1, 2, 3, 4
    print(i)

for i in range(2, 10, 2):   # 2, 4, 6, 8
    print(i)

# Enumerate — get index AND value
for i, fruit in enumerate(fruits):
    print(f"{i}: {fruit}")
# 0: apple
# 1: banana
# 2: cherry

# Zip — iterate two lists together
names  = ["Alice", "Bob", "Charlie"]
scores = [95, 87, 72]
for name, score in zip(names, scores):
    print(f"{name}: {score}")
# Alice: 95
# Bob: 87
# Charlie: 72

while Loops¶

Repeat until a condition changes — when you don't know the iteration count up front (input loops, polling, retries). Use break to exit early and continue to skip an iteration.

count = 0
while count < 5:
    print(count)
    count += 1

# break — exit the loop early
while True:
    answer = input("Type 'quit' to exit: ")
    if answer == "quit":
        break
    print(f"You said: {answer}")

# continue — skip to next iteration
for i in range(10):
    if i % 2 == 0:
        continue        # skip even numbers
    print(i)            # prints 1, 3, 5, 7, 9

match-case (Python 3.10+)¶

Structural pattern matching — cleaner than a long if/elif chain when dispatching on a value or the shape of data. See the Pattern Matching deep dive for destructuring.

command = "quit"

match command:
    case "quit":
        print("Quitting...")
    case "help":
        print("Showing help...")
    case "start" | "run":          # multiple patterns
        print("Starting...")
    case _:                        # default (wildcard)
        print(f"Unknown: {command}")

Loop else clause¶

Runs only if the loop finished without hitting break — handy for "searched everything and found nothing" logic.

# else runs only if the loop completed without break
for i in range(5):
    if i == 10:
        break
else:
    print("Loop finished normally")   # this prints

Practice exercises¶

Write a program that prints all numbers 1–100. For multiples of 3 print "Fizz", multiples of 5 print "Buzz", multiples of both print "FizzBuzz".
Use a while loop to find the first power of 2 greater than 1000.
Write a number guessing game: generate a random number 1–100, let the user guess with "higher"/"lower" hints.
Use match-case to build a simple command dispatcher.

💬 Discussion

Have a question about this topic? Found an error? Share your thoughts below.