Assertions

Sometimes you want to check that something must be true.

If it is false, the program should stop and show an error.

Python gives us assert for this.

What Is an Assertion?

An assertion is a quick check.

If the condition is True, the program continues.

If the condition is False, Python raises an error.

Basic Structure

assert condition

Or with a message:

assert condition, "message"

Why Assertions Matter

Assertions help you:

• Catch bugs early
• Check assumptions
• Test code while building
• Find wrong values quickly

Example: Positive Number

number = 5
assert number > 0
print("Valid")

Output

Valid

Example: Failing Assertion

number = -1
assert number > 0, "Number must be positive"

Result

Python shows:

AssertionError: Number must be positive

Example: Function Check

def divide(a, b):
    assert b != 0, "Cannot divide by zero"
    return a / b

print(divide(10, 2))

Output

5.0

Example: String Check

name = "Tom"
assert name != "", "Name cannot be empty"

print("Saved")

Output

Saved

Assertions vs if

Use if when handling user choices.

Use assert when checking something that should always be true during development.

Good Uses of Assertions

• Value should not be empty
• Number should be positive
• List should have items
• Function input should be valid

Code Along

Build a score checker.

Rules:

• Score must be 0 to 100

Use assert.

Mini Challenge

Build a stock checker.

Steps:

• Create:

stock = 5

• Use assert to check stock is greater than 0
• Print:

In stock

Expected output:

In stock

Real World Use Case

Developers use assertions while testing code, checking values, and finding bugs faster.

Quiz

1. What does assert do?
2. What error happens when an assertion fails?
3. When should you use assertions?
4. Can assertions have messages?

Assignment

Create a program that stores an age and uses assert to make sure age is 18 or more.

Summary

You learned how assertions check conditions and help catch bugs during development.