ErrorHandling Mini Project — BMI Calculator

The problem...

You've built the BMI Calculator three times. Each version was better than the last.

But none of them handled bad input. Type a word — crash. Enter a negative weight — wrong result. Enter zero as height — division by zero.

Time to make it bulletproof.

The idea!

Three functions. Each with one job. Built in the order you need them.

Step 1 — get valid input

def get_input(prompt):
    while True:
        try:
            value = float(input(prompt))
            if value <= 0:
                raise ValueError("Value must be greater than zero.")
            return value
        except ValueError as e:
            print(f"Invalid input: {e}")

Keeps asking until the user provides a valid positive number. Handles both wrong types and wrong values.

Step 2 — calculate BMI

def calculate_bmi(weight, height):
    return weight / height ** 2

Clean and simple — validation already happened in get_input().

Step 3 — get the category

def get_category(bmi):
    if bmi < 18.5:
        return "Underweight"
    elif bmi < 25:
        return "Normal weight"
    elif bmi < 30:
        return "Overweight"
    else:
        return "Obese"

Put it together

weight = get_input("Enter your weight (kg): ")
height = get_input("Enter your height (m): ")

bmi = calculate_bmi(weight, height)
category = get_category(bmi)

print(f"Weight: {weight}kg | Height: {height}m | BMI: {bmi:.1f} | {category}")

Test it

# Enter your weight (kg): abc
# Invalid input: could not convert string to float: 'abc'
# Enter your weight (kg): -5
# Invalid input: Value must be greater than zero.
# Enter your weight (kg): 70
# Enter your height (m): 1.75
# Weight: 70kg | Height: 1.75m | BMI: 22.9 | Normal weight

Go further

• Add a name parameter to the final print — "Bull | BMI: 22.9 | Normal weight"
• Wrap everything in a while True — let the user calculate multiple times
• Add raise inside calculate_bmi() as an extra safety net

What you should understand now

• get_input() — validates before anything else runs
• calculate_bmi() — clean logic, no validation needed
• get_category() — one job, one result
• Three functions, built in order, each feeding the next