Python Custom Exception

Summary: in this tutorial, you’ll learn how to define Python custom exception classes.

Introduction to the Python custom exception

To create a custom exception class, you define a class that inherits from the built-in Exception class or one of its subclasses such as ValueError class:

The following example defines a CustomException class that inherits from the Exception class:

class CustomException(Exception):
    """ my custom exception class """Code language: Python (python)

Note that the CustomException class has a docstring that behaves like a statement. Therefore, you don’t need to add the pass statement to make the syntax valid.

To raise the CustomException, you use the raise statement. For example, the following uses the raise statement to raise the CustomException:

class CustomException(Exception):
    """ my custom exception class """


try:
    raise CustomException('This is my custom exception')
except CustomException as ex:
    print(ex)Code language: Python (python)

Output:

This is my custom exceptionCode language: Python (python)

Like standard exception classes, custom exceptions are also classes. Hence, you can add functionality to the custom exception classes like:

• Adding attributes and properties.
• Adding methods e.g., log the exception, format the output, etc.
• Overriding the __str__ and __repr__ methods
• And doing anything else that you can do with regular classes.

In practice, you’ll want to keep the custom exceptions organized by creating a custom exception hierarchy. The custom exception hierarchy allows you to catch exceptions at multiple levels, like the standard exception classes.

Python custom exception example

Suppose you need to develop a program that converts a temperature from Fahrenheit to Celsius.

The minimum and maximum values of a temperature in Fahrenheit are 32 and 212. If users enter a value that is not in this range, you want to raise a custom exception e.g., FahrenheitError.

Define the FahrenheitError custom exception class

The following defines the FahrenheitError exception class:

class FahrenheitError(Exception):
    min_f = 32
    max_f = 212

    def __init__(self, f, *args):
        super().__init__(args)
        self.f = f

    def __str__(self):
        return f'The {self.f} is not in a valid range {self.min_f, self.max_f}'Code language: Python (python)

How it works.

• First, define the FahrenheitError class that inherits from the Exception class.
• Second, add two class attributes min_f and max_f that represent the minimum and maximum Fahrenheit values.
• Third, define the __init__ method that accepts a Fahrenheit value (f) and a number of position arguments (*args). In the __init__ method, call the __init__ method of the base class. Also, assign the f argument to the f instance attribute.
• Finally, override the __str__ method to return a custom string representation of the class instance.

Define the fahrenheit_to_celsius function

The following defines the fahrenheit_to_celsius function that accepts a temperature in Fahrenheit and returns a temperature in Celcius:

def fahrenheit_to_celsius(f: float) -> float:
    if f < FahrenheitError.min_f or f > FahrenheitError.max_f:
        raise FahrenheitError(f)

    return (f - 32) * 5 / 9Code language: Python (python)

The fahrenheit_to_celsius function raises the FahrenheitError excpetion if the input temperature is not in the valid range. Otherwise, it converts the temperature from Fahrenheit to Celcius.

Create the main program

The following main program uses the fahrenheit_to_celsius function and the FahrenheitError custom exception class:

if __name__ == '__main__':
    f = input('Enter a temperature in Fahrenheit:')
    try:
        f = float(f)
    except ValueError as ex:
        print(ex)
    else:
        try:
            c = fahrenheit_to_celsius(float(f))
        except FahrenheitError as ex:
            print(ex)
        else:
            print(f'{f} Fahrenheit = {c:.4f} Celsius')Code language: Python (python)

How it works.

First, prompt users for a temperature in Fahrenheit.

f = input('Enter a temperature in Fahrenheit:')Code language: Python (python)

Second, convert the input value into a float. If the float() cannot convert the input value, the program will raise a ValueError exception. In this case, it displays the error message from the ValueError exception:

try:
    f = float(f)
    # ...
except ValueError as ex:
    print(ex)Code language: Python (python)

Third, convert the temperature to Celsius by calling the fahrenheit_to_celsius function and print the error message if the input value is not a valid Fahrenheit value:

try:
    c = fahrenheit_to_celsius(float(f))
except FahrenheitError as ex:
    print(ex)
else:
    print(f'{f} Fahrenheit = {c:.4f} Celsius')Code language: Python (python)

Put it all together

class FahrenheitError(Exception):
    min_f = 32
    max_f = 212

    def __init__(self, f, *args):
        super().__init__(args)
        self.f = f

    def __str__(self):
        return f'The {self.f} is not in a valid range {self.min_f, self.max_f}'


def fahrenheit_to_celsius(f: float) -> float:
    if f < FahrenheitError.min_f or f > FahrenheitError.max_f:
        raise FahrenheitError(f)

    return (f - 32) * 5 / 9


if __name__ == '__main__':
    f = input('Enter a temperature in Fahrenheit:')
    try:
        f = float(f)
    except ValueError as ex:
        print(ex)
    else:
        try:
            c = fahrenheit_to_celsius(float(f))
        except FahrenheitError as ex:
            print(ex)
        else:
            print(f'{f} Fahrenheit = {c:.4f} Celsius')Code language: Python (python)

Summary

• Subclass the Exception class or one of its subclasses to define a custom exception class.
• Create a exception class hierarchy to make the exception classes more organized and catch exceptions at multiple levels.