Raise an Exception to another Exception in Python

By Ranjeet V

In this tutorial, we will learn how to raise an exception in response to another exception in Python. This is also called exception chaining.

In the below example code, we have a try-except block. The try block contains a statement with ZeroDivisionError. This is handled in the except block and in response, the RuntimeError is thrown as we used a raise from statement as shown below. Have a look at the code.

try:
    a = 5/0
except ZeroDivisionError as e:
    raise RuntimeError('Error') from e

Output:

Traceback (most recent call last):
File "err.py", line 2, in <module>
a = 5/0
ZeroDivisionError: division by zero

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "err.py", line 4, in <module>
raise RuntimeError('Error') from e
RuntimeError: Error

Chained Exception can also occur when a statement in the except block contains an error as shown in the given code. This is an example of implicit exception chaining.

try:
    a = 5/0
except ZeroDivisionError as e:
    print(b)

Output:

Traceback (most recent call last):
File "err.py", line 2, in <module>
a = 5/0
ZeroDivisionError: division by zero

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "err.py", line 4, in <module>
print(b)
NameError: name 'b' is not defined

We can prevent exception chaining using raise from None statement. The below code demonstrates how we can suppress exception chaining.

try:
    a = 5/0
except ZeroDivisionError:
    raise RuntimeError('Error') from None

Output:

Traceback (most recent call last):
File "err.py", line 4, in <module>
raise RuntimeError('Error') from None
RuntimeError: Error

Thank you.

Also read: Handling Exceptions using try and except in Python 3.x. or earlier