Exceptions

When something goes wrong an exception is raised. For example, if you try to divide by zero, ZeroDivisionError is raised or if you try to access a nonexistent key in a dictionary, KeyError is raised.

empty_dict = {}
# empty_dict['key']  # Uncomment to see the traceback

try-except structure

If you know that a block of code can fail in some manner, you can use try-except structure to handle potential exceptions in a desired way.

# Let's try to open a file that does not exist
file_name = "not_existing.txt"

try:
    with open(file_name) as my_file:
        print("File is successfully open")

except FileNotFoundError as e:
    print(f"Uups, file: {file_name} not found")
    print(f"Exception: {e} was raised")

Uups, file: not_existing.txt not found
Exception: [Errno 2] No such file or directory: 'not_existing.txt' was raised

If you don't know the type of exceptions that a code block can possibly raise, you can use Exception which catches all exceptions. In addition, you can have multiple except statements.

def calculate_division(var1, var2):
    result = 0

    try:
        result = var1 / var2
    except ZeroDivisionError as ex1:
        print("Can't divide by zero")
    except Exception as ex2:
        print(f"Exception: {ex2}")

    return result


result1 = calculate_division(3, 3)
print(f"result1: {result1}")

result2 = calculate_division(3, "3")
print(f"result2: {result2}")

result3 = calculate_division(3, 0)
print(f"result3: {result3}")

result1: 1.0
Exception: unsupported operand type(s) for /: 'int' and 'str'
result2: 0
Can't divide by zero
result3: 0

try-except can be also in outer scope:

def calculate_division(var1, var2):
    return var1 / var2


try:
    result = calculate_division(3, "3")
except Exception as e:
    print(e)

unsupported operand type(s) for /: 'int' and 'str'

Creating your custom exceptions

In your own applications, you can use custom exceptions for signaling users about errors which occur during your application run time.

import math


# Define your own exception
class NegativeNumbersNotSupported(Exception):
    pass


# Dummy example how to use your custom exception
def secret_calculation(number1, number2):
    if number1 < 0 or number2 < 0:
        msg = f"Negative number in at least one of the parameters: {number1}, {number2}"
        raise NegativeNumbersNotSupported(msg)

    return math.sqrt(number1) + math.sqrt(number2)


# Uncomment to see the traceback
# result = secret_calculation(-1, 1)