Raising Exceptions in Python

Let’s go step by step to understand how to raise exceptions in Python, why we use it, and with practical examples.

What Does “Raising an Exception” Mean?

When we raise an exception, we explicitly tell Python: Stop execution here because something invalid or unexpected happened.

Why Raise Exceptions?

To stop incorrect program execution.
To validate user input.
To enforce business logic rules.
To prevent bad data in ML pipelines.

Key Idea

Instead of waiting for Python to crash naturally, we manually trigger an error using the raise keyword.

Basic Syntax of raise

raise ExceptionType("Custom error message")

Example: Built-in Exception (Without raise)

Python

def divide(a, b):
    return a / b

# Catching built-in exception
try:
    result = divide(10, 0)
except ZeroDivisionError as e:
    print("Error occurred:", e)
else:
    print("Result:", result)

If we run the above code, we will see the output:

Output

Error occurred: division by zero

In the above example, Python automatically raises the built-in ZeroDivisionError, which we handle in the except block.

Example: Raising Exceptions in a Function

Python

def divide(a, b):
    # Check if denominator is zero
    if b == 0:
        raise ZeroDivisionError("Denominator cannot be zero!")
    return a / b

# Example usage
try:
    result = divide(10, 0)
except ZeroDivisionError as e:
    print("Error occurred:", e)
else:
    print("Result:", result)

Output:

Output

Error occurred: Denominator cannot be zero!

Here, we manually raised the exception with a custom error message, which makes debugging more meaningful and user-friendly.

Raising Exceptions with if Statements

Sometimes we need to raise exceptions based on our own business logic, even if Python does not consider it an error.

Example: Check the age input.

Python

age = int(input("Enter your age: "))

if age < 0:
    raise ValueError("Age cannot be negative!")
elif age < 18:
    raise PermissionError("You must be 18 or older.")
else:
    print("You are eligible.")

Output Example:

Output

ValueError: Age cannot be negative!

In this example, Python allows negative numbers, but logically an age cannot be negative. So we manually raise a ValueError to enforce our business rule.

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