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Python Custom Exceptions

An Exception is raised whenever there is an error encountered, and it signifies that something went wrong with the program. By default, there are many exceptions that the language defines for us, such as TypeError when the wrong type is passed. In this article, we shall look at how we can create our own Custom Exceptions in Python.

But before we take a look at how custom exceptions are implemented, let us find out how we could raise different types of exceptions in Python.


Raise Exceptions

Python allows the programmer to raise an Exception manually using the raise keyword.

Format: raise ExceptionName

The below function raises different exceptions depending on the input passed to the function.

def exception_raiser(string):
    if isinstance(string, int):
        raise ValueError
    elif isinstance(string, str):
        raise IndexError
    else:
        raise TypeError

Output:

>>> exception_raiser(123)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in exception_raiser
ValueError
>>> exception_raiser('abc')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 5, in exception_raiser
IndexError
>>> exception_raiser([123, 456])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 7, in exception_raiser
TypeError

As you can observe, different types of Exceptions are raised based on the input, at the programmer’s choice. This allows for good flexibility of Error Handling as well, since we can actively predict why an Exception can be raised.


Defining Custom Exceptions

Similarly, Python also allows us to define our own custom Exceptions. We are in complete control of what this Exception can do, and when it can be raised, using the raise keyword. Let us look at how we can define and implement some custom Exceptions.

1. Create a Custom Exception Class

We can create a custom Exception class to define the new Exception. Again, the idea behind using a Class is because Python treats everything as a Class. So it doesn’t seem that outlandish that an Exception can be a class as well!

All Exceptions inherit the parent Exception Class, which we shall also inherit when creating our class.

We shall create a Class called MyException, which raises an Exception only if the input passed to it is a list and the number of elements in the list is odd.

class MyException(Exception):
	pass

def list_check(lst):
    if len(lst) % 2 != 0:
        raise MyException

# MyException will not be raised
list_check([1, 2, 3, 4])

# MyException will be raised
list_check([1, 3, 5])    

Output:

root@AskPython:~# python3 exceptions.py
Traceback (most recent call last):
  File "exceptions.py", line 12, in <module>
    list_check([1, 3, 5])
  File "exceptions.py", line 6, in list_check
    raise MyException
__main__.MyException

2. Add a custom Message and Error

We can add our own error messages and print them to the console for our Custom Exception. This involves passing two other parameters in our MyException class, the message and error parameters.

Let us modify our original code to account for a custom Message and Error for our Exception.

class MyException(Exception):
    def __init__(self, message, errors):
        # Call Exception.__init__(message)
        # to use the same Message header as the parent class
        super().__init__(message)
        self.errors = errors
        # Display the errors
        print('Printing Errors:')
        print(errors)

def list_check(lst):
    if len(lst) % 2 != 0:
        raise MyException('Custom Message', 'Custom Error')

# MyException will not be raised
list_check([1, 2, 3, 4])

# MyException will be raised
list_check([1, 3, 5])

Output:

Printing Errors:
Custom Error
Traceback (most recent call last):
  File "exceptions.py", line 17, in <module>
    list_check([1, 3, 5])
  File "exceptions.py", line 11, in list_check
    raise MyException('Custom Message', 'Custom Error')
__main__.MyException: Custom Message

We have thus successfully implemented our own Custom Exceptions, including adding custom error messages for debugging purposes! This can be very useful if you are building a Library/API and another programmer wants to know what exactly went wrong when the custom Exception is raised.


Conclusion

In this article, we learned how to raise Exceptions using the raise keyword, and also build our own Exceptions using a Class and add error messages to our Exception.

References


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