Raising Exceptions

Introduction

You can raise your own exceptions to signal that something has gone wrong in your code. This makes your functions self-documenting and prevents bad data from propagating silently.

raise

def set_salary(salary):
    if salary < 0:
        raise ValueError("Salary cannot be negative.")
    return salary

set_salary(-5000)
# ValueError: Salary cannot be negative.

Raising with a Custom Message

def get_employee(employee_id, employees):
    if employee_id not in employees:
        raise KeyError(f"No employee found with ID {employee_id}")
    return employees[employee_id]

Custom Exception Classes

For larger projects, define your own exception types by subclassing Exception.

class InvalidSalaryError(Exception):
    """Raised when a salary value is invalid."""
    pass

class EmployeeNotFoundError(Exception):
    """Raised when an employee cannot be found."""
    pass

def set_salary(salary):
    if not isinstance(salary, (int, float)):
        raise TypeError("Salary must be a number.")
    if salary < 0:
        raise InvalidSalaryError(f"Salary {salary} cannot be negative.")
    return salary

Callers can then catch your specific exception:

try:
    set_salary(-5000)
except InvalidSalaryError as e:
    print(f"Payroll error: {e}")

Practice Exercises

• Write a function set_age(age) that raises a ValueError if age is below 0 or above 150.
• Create a custom exception class InsufficientStockError. Write a function that raises it when stock goes below zero.
• Write a function that raises an exception and then call it inside a try/except that catches and prints the message.