Python try…except…else

Summary: in this tutorial, you’ll learn how to use the Python try...except...else statement.

Introduction to the Python try…except…else statement

The try statement has an optional else clause with the following syntax:

try:
    # code that may cause errors
except:
    # code that handle exceptions
else:
    # code that executes when no exception occurs
Code language: PHP (php)

The try...except...else statement works as follows:

• If an exception occurs in the try clause, Python skips the rest of the statements in the try clause and the except statement execute.
• In case no exception occurs in the try clause, the else clause will execute.

When you include the finally clause, the else clause executes after the try clause and before the finally clause.

Python try…except…else statement examples

Let’s take some examples of using the try...except...else statement.

1) Using try…except…else statement for control flow

The following example illustrates how to use the try...except...else clause develop a program that calculates the body mass index (BMI).

First, define a function for calculating the (BMI) based on height and weight:

def calculate_bmi(height, weight):
    """ calculate body mass index (BMI) """
    return weight / height**2
Code language: Python (python)

Second, define another function for evaluating BMI:

def evaluate_bmi(bmi):
    """ evaluate the bmi """
    if 18.5 <= bmi <= 24.9:
        return 'healthy'

    if bmi >= 25:
        return 'overweight'

    return 'underweight'
Code language: Python (python)

Third, define a new main() function that prompts users for entering height and weight, and prints out the BMI result:

def main():
    try:
        height = float(input('Enter your height (meters):'))
        weight = float(input('Enter your weight (kilograms):'))

    except ValueError as error:
        print('Error! please enter a valid number.')
    else:
        bmi = round(calculate_bmi(height, weight), 1)
        evaluation = evaluate_bmi(bmi)

        print(f'Your body mass index is {bmi}')
        print(f'This is considered {evaluation}!')
Code language: Python (python)

The main() function uses the try...except...else statement to control its flow. If you enter height and weight that cannot be converted to numbers, the ValueError exception will occur.

If no exception occurs, the else clause will execute. It calculates the BMI index and displays the evaluation.

Put it all together.

def calculate_bmi(height, weight):
    """ calculate body mass index (BMI) """
    return weight / height**2


def evaluate_bmi(bmi):
    """ evaluate the bmi """
    if 18.5 <= bmi <= 24.9:
        return 'healthy'

    if bmi >= 25:
        return 'overweight'

    return 'underweight'


def main():
    try:
        height = float(input('Enter your height (meters):'))
        weight = float(input('Enter your weight (kilograms):'))

    except ValueError as error:
        print(error)
    else:
        bmi = round(calculate_bmi(height, weight), 1)
        evaluation = evaluate_bmi(bmi)

        print(f'Your body mass index is {bmi}')
        print(f'This is considered {evaluation}!')

main()Code language: Python (python)

2) Using Python try…except…else and finally example

The else clause executes right before the finally clause if no exception occurs in the try clause.

The following example shows how to use the try...except...else...finally clause:

fruits = {
    'apple': 10,
    'orange': 20,
    'banana': 30
}

key = None
while True:
    try:
        key = input('Enter a key to lookup:')
        fruit = fruits[key.lower()]
    except KeyError:
        print(f'Error! {key} does not exist.')
    except KeyboardInterrupt:
        break
    else:
        print(fruit)
    finally:
        print('Press Ctrl-C to exit.')Code language: Python (python)

How it works.

• First, define the fruits dictionary that contains three elements.
• Second, use a while loop to repeatedly get inputs from users. It stops the loop until the users press Ctrl-C.
• Third, use the try...except...else...finally clause inside the while loop. We use the user input to find for the element in the dictionary.

If the key doesn’t exist, the KeyError exception occurs, the except clause will execute.

If users press Ctrl-C, the KeyboardInterrupt exception occurs that executes the break statement to terminate the loop.

If the key is found in the fruits dictionary, the program prints out the found element.

The finally clause always executes. It shows the reminder to the users that they should press Ctrl-C to exit.

Summary

• Use the Python try...except...else statement provides you with a way to control the flow of the program in case of exceptions.
• The else clause executes if no exception occurs in the try clause.
• If so, the else clause executes after the try clause and before the finally clause.