Handling exceptions in any application is a very critical step. Exceptions help to handle any runtime error that can crash your application. In Python, there are many built-in exceptions like:
* **ArithmeticError**
* **SyntaxError** 
* **FileNotFoundError**  
 * **ZeroDivisionError**

But many times, you would need to create your own custom exceptions too. You can define your own custom exceptions by creating a class that is derived from the built-in `Exception` class. Take a look at the code below:

class NotValidAge(Exception):
  """Raised when the age is less than 18"""
  pass
  
def driving_license(age):
  if age < 18:
    raise NotValidAge

driving_license(17)

**Explanation:**

* In **line 1**, we created the `NotValidAge` class, which inherits from the `Exception` class. We will use the `NotValidAge` exception when the age is below 18 years.
* From **lines 5 to 7**, we created a function that raises an exception (`NotValidAge`) if the age is less than 18 years.

* In **line 9**, we called that function and saw that our custom exception was raised.

You can customize the exception message by overriding the `__str__()` method. (This method is used by `Exception` to print the message). Let's take a look at how we can do this.

class NotValidAge(Exception):
  """Raised when the age is less than 18"""
  def __str__(self):
    return "Age should be greater than 18 years."
  
def driving_license(age):
  if age < 18:
    raise NotValidAge

driving_license(17)

**Explanation:**

* In **line 1**, we created the `NotValidAge` class, which inherits from the `Exception` class. We will use the `NotValidAge` exception when the age is below 18 years.
* In **line 3**, we overrode the `__str__()` method and returned our own message. This message is going to be displayed when this exception occurs.
* From **lines 6 to 8**, we created a function that raises an exception (`NotValidAge`) if the age is less than 18 years.

* In **line 10**, we called that function and saw that our custom exception was raised with our custom message.

> In this way, we can create our own custom exceptions with custom messages.

How to create custom exceptions in Python