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Control Flow

Learn how to make your Python programs make decisions and repeat tasks so they can process data in smarter ways.

Skill Level: Beginner
Prerequisites: Variables, Data Types, Collections
Estimated Time: 30 minutes

Story Time​

So far, you have learned how to store values, work with different data types, and use collections such as lists. Now you are ready for something more powerful: telling Python how to make decisions and repeat tasks.

Imagine you are working with student exam scores. You may want to check whether a score is above a target, print only selected values, or repeat the same task for every student in a list. Control flow gives your program that ability.

What You’ll Learn​

By the end of this lesson, you will be able to:

  • Explain what control flow means in Python.
  • Use comparison operators to compare values.
  • Write if, elif, and else statements.
  • Use for loops to repeat tasks over a sequence.
  • Use while loops to repeat tasks while a condition is true.
  • Use break and continue to control loop behavior.

Comparison Operators​

Comparison operators compare two values and return a Boolean value: either True or False.

OperatorMeaningExample
==Equal to5 == 5
!=Not equal to5 != 3
>Greater than10 > 4
<Less than8 < 12
>=Greater than or equal to18 >= 18
<=Less than or equal to7 <= 9

Example​

age = 20
print(age >= 18)

Output:

True

Conditional Statements​

Conditional statements allow a program to choose what to do based on a condition.

The if Statement​

Use if when you want code to run only when a condition is true.

age = 20

if age >= 18:
    print("Eligible to vote")

Output:

Eligible to vote

Indentation matters in Python. The indented line belongs to the if block.

The if...else Statement​

Use else when you want one action if the condition is true and another action if it is false.

age = 16

if age >= 18:
    print("Eligible")
else:
    print("Not eligible")

Output:

Not eligible

The if...elif...else Statement​

Use elif when you need to check multiple conditions in order. The word elif is short for “else if”.

marks = 84

if marks >= 90:
    print("Grade A")
elif marks >= 75:
    print("Grade B")
else:
    print("Grade C")

Output:

Grade B

Repeating Tasks with Loops​

Loops repeat a block of code. This is useful when you want to perform the same action many times.

Instead of writing this:

print("Python")
print("Python")
print("Python")

you can use a loop.

The for Loop​

A for loop is useful when you want to go through a sequence, such as a list or a range of numbers.

for i in range(5):
    print(i)

Output:

0
1
2
3
4

Looping Through a List​

students = ["Emma", "Alex", "Sophia"]

for student in students:
    print(student)

Output:

Emma
Alex
Sophia

Use a for loop when you already know the collection or sequence you want to move through.

The while Loop​

A while loop repeats as long as a condition stays true.

count = 1

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

Output:

1
2
3
4
5

Use a while loop when repetition depends on a condition rather than a fixed list or range.

Using break​

The break statement stops a loop immediately, even if the loop has more work to do.

for number in range(10):
    if number == 5:
        break

    print(number)

Output:

0
1
2
3
4

Using continue​

The continue statement skips the current iteration and moves to the next one.

for number in range(6):
    if number == 3:
        continue

    print(number)

Output:

0
1
2
4
5

Worked Example​

Suppose you have a list of student math scores and you want to display only scores above 80.

scores = 

for score in scores:
    if score > 80:
        print(score)

Output:

82
91
88

This is a simple example of how control flow supports data analysis. Your program repeats a task for every value in the list and applies a condition to decide what to display.

Practice in the Notebook​

Notebook

Open in Google Colab

Practice this lesson in a live notebook.

After completing the activity in Google Colab, mark it as complete below.

Try this in Google Colab:

scores =

Complete these tasks:

  1. Display every score.
  2. Display only scores greater than 80.
  3. Count how many scores are greater than 80.

One possible starting point:

scores = 

count_above_80 = 0

for score in scores:
    print(score)

for score in scores:
    if score > 80:
        print(score)
        count_above_80 += 1

print("Number of scores above 80:", count_above_80)

Challenge​

Imagine each value in the list below is a student’s reading score.

reading_scores =

Write a program that:

  1. Displays only the scores greater than or equal to 80.
  2. Counts how many scores are greater than or equal to 80.
  3. Prints "Strong performance" if the count is 3 or more, otherwise prints "Needs more review".

Common Mistakes​

Forgetting indentation​

Incorrect:

if age >= 18:
print("Eligible")

Correct:

if age >= 18:
    print("Eligible")

Using = instead of ==​

Incorrect:

if age = 18:

Correct:

if age == 18:

Infinite while loops​

Incorrect:

count = 1

while count <= 5:
    print(count)

This loop never ends because count never changes.

Correct:

count = 1

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

Key Takeaways​

  • Comparison operators return True or False.
  • if, elif, and else help programs make decisions.
  • for loops are useful for iterating over sequences.
  • while loops repeat while a condition remains true.
  • break stops a loop, and continue skips to the next iteration.

Self Evaluation​

Check Your Understanding

1. What does a comparison operator return in Python?

2. Which statement is used to check multiple conditions in sequence?

3. Which loop is best when you want to go through every item in a list?

4. What does break do inside a loop?

5. What does continue do inside a loop?

Continue Your Journey​

Next, you will learn about Functions, which help you organize code into reusable blocks.