Part 4 of a beginner-friendly series on learning Python from scratch.
In Part 3, we learned about booleans — values that are either True or False — and the comparison operators that produce them. Now it's time to put that logic to work.
This is where your programs start to come alive: making decisions, repeating tasks, and responding to different situations. Control flow is the backbone of every real program you'll ever write.
Decision-Making: If, Elif, Else
The most fundamental control flow tool is the if statement. It lets your program make a decision: if something is true, do this; otherwise, do that.
The if statement
age = 25
if age >= 18:
print("You are an adult")
The syntax is simple:
- Write
iffollowed by a condition - End the line with a colon
:
- Indent the code block that follows (4 spaces)
Only if the condition is True does the indented code run. If it's False, the code is skipped.
age = 15
if age >= 18:
print("You are an adult")
print("This always prints") # This runs regardless of the condition
Here, the first print runs only if age >= 18. The second print always runs because it's not indented under the if.
The else clause
Often you want to do something different if the condition is false:
age = 15
if age >= 18:
print("You are an adult")
else:
print("You are a minor")
The else block runs when the if condition is False.
The elif clause — multiple conditions
For more than two paths, use elif (short for "else if"). You can chain as many as you need:
score = 85
if score >= 90:
print("Grade: A")
elif score >= 80:
print("Grade: B")
elif score >= 70:
print("Grade: C")
elif score >= 60:
print("Grade: D")
else:
print("Grade: F")
Python evaluates each condition from top to bottom and stops at the first True one. So for score = 85:
- Is 85 >= 90? No.
- Is 85 >= 80? Yes → prints "Grade: B" and stops.
- Everything after is skipped.
Important: Once one condition is True, the rest are not checked. If you have overlapping conditions, order them carefully.
Nested if statements
You can put if statements inside if statements:
age = 25
has_license = True
if age >= 18:
if has_license:
print("You can drive")
else:
print("You need a license first")
else:
print("You must be 18 to drive")
This works, but too much nesting gets hard to read. Keep it shallow when possible.
Ternary Operator — Inline If
For simple, one-line decisions, Python has a compact syntax:
status = "adult" if age >= 18 else "minor"
This is equivalent to:
if age >= 18:
status = "adult"
else:
status = "minor"
Read it as: "Set status to 'adult' if age >= 18, else set it to 'minor'."
The Match Statement (Python 3.10+)
Python 3.10 introduced match, similar to switch in other languages. It's useful when you have one variable with many possible values:
day = "Monday"
match day:
case "Monday":
print("Start of the work week")
case "Friday":
print("Almost the weekend!")
case "Saturday" | "Sunday":
print("It's the weekend!")
case _:
print("Some other day")
The _ (underscore) acts like a default case — it matches anything not caught by previous cases.
Note: If you're using Python < 3.10, match won't work. Use if/elif/else instead. For now, this is nice-to-know, not essential.
Repeating Code: Loops
Loops let you run the same code multiple times without repeating it. There are two main types: while and for.
While loops — repeat until a condition is false
A while loop keeps running as long as its condition is True:
count = 1
while count <= 5:
print(count)
count = count + 1
Output:
1
2
3
4
5
This will print 1–5. Let's trace it:
- Is
count <= 5? (1 <= 5) Yes → print 1, then count becomes 2 - Is
count <= 5? (2 <= 5) Yes → print 2, then count becomes 3 - ... continues until count becomes 6
- Is
count <= 5? (6 <= 5) No → loop exits
⚠️ Infinite loops: If you forget to change the condition, your loop runs forever:
count = 1
while count <= 5:
print(count)
# Oops, forgot to increment count — this runs forever!
If this happens, press Ctrl+C in your terminal to stop it.
For loops — repeat a specific number of times
A for loop is perfect when you know exactly how many times you want to repeat something. The most common pattern is using range():
for i in range(5):
print(i)
Output:
0
1
2
3
4
range(5) generates the numbers 0, 1, 2, 3, 4 (note: 5 is not included). You can customize this:
range(5) # 0, 1, 2, 3, 4
range(1, 6) # 1, 2, 3, 4, 5 (start at 1, stop before 6)
range(0, 10, 2) # 0, 2, 4, 6, 8 (start at 0, stop before 10, step by 2)
range(10, 0, -1) # 10, 9, 8, 7, 6, 5, 4, 3, 2, 1 (count backward)
The variable i (or any name you choose) takes on each value from the range, one per loop iteration.
Iterating over strings and lists
for loops are also perfect for going through each character in a string or each item in a list:
word = "Python"
for letter in word:
print(letter)
Output:
P
y
t
h
o
n
Or with a list:
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
print(fruit)
Output:
apple
banana
cherry
We'll dive deeper into lists in Part 5, but this pattern is so common you'll see it everywhere.
Break and Continue
Sometimes you want to exit a loop early or skip to the next iteration. break and continue let you do this.
Break — exit the loop immediately
count = 1
while count <= 10:
if count == 5:
break # Exit the loop
print(count)
count = count + 1
print("Loop ended")
Output:
1
2
3
4
Loop ended
When count reaches 5, break exits the loop. Nothing after the break in that iteration runs.
Continue — skip to the next iteration
for i in range(1, 6):
if i == 3:
continue # Skip this iteration
print(i)
Output:
1
2
4
5
When i == 3, continue jumps to the next iteration of the loop. The print(3) is never reached, but the loop keeps going.
Practical Examples
Example 1: Guessing game
secret = 42
guess = 0
while guess != secret:
guess = int(input("Guess the number: "))
if guess < secret:
print("Too low!")
elif guess > secret:
print("Too high!")
else:
print("You got it!")
This keeps asking for guesses until the user gets it right.
Example 2: Multiplication table
number = 7
for i in range(1, 11):
print(f"{number} × {i} = {number * i}")
Output:
7 × 1 = 7
7 × 2 = 14
7 × 3 = 21
... (up to 7 × 10 = 70)
Example 3: Finding even numbers
for i in range(1, 21):
if i % 2 == 0:
print(i)
Output:
2
4
6
8
10
12
14
16
18
20
(i % 2 == 0 is True for even numbers)
Why This Matters
Control flow is what separates a calculator from a real program. Every decision your code makes, every task it repeats, every time it reacts to different input — that's control flow at work. Master if statements and loops now, and you'll be amazed how much you can build.
The most common beginner mistakes:
- Forgetting the colon (
:) afterif,else,for,while
- Forgetting to indent the code block
- Using
=(assignment) instead of==(comparison) in conditions - Getting stuck in infinite loops (remember:
Ctrl+C)
Once you internalize these patterns, they become automatic.
This is Part 4 of an 8-part beginner Python series. Catch up on Part 1: Getting Started & Syntax, Part 2: Variables, Data Types & Numbers, and Part 3: Strings & Booleans.