Python | Raising an Exception to Another Exception

Last Updated : 12 Jun, 2019

Let’s consider a situation where we want to raise an exception in response to catching a different exception but want to include information about both exceptions in the traceback.

To chain exceptions, use the raise from statement instead of a simple raise statement. This will give you information about both errors.

Code #1 :

def example(): 
    try: 
        int('N/A') 
    except ValueError as e: 
        raise RuntimeError('A parsing error occurred') from e... 
  
example() 

Output :

Traceback (most recent call last):
  File "", line 3, in example
ValueError: invalid literal for int() with base 10: 'N/A'

This exception is the direct cause of the following exception –

Traceback (most recent call last):
  File "", line 1, in 
  File "", line 5, in example
RuntimeError: A parsing error occurred

Both exceptions are captured in the traceback. A normal except statement is used to catch such an exception. However, __cause__ attribute of the exception object can be looked to follow the exception chain as explained in the code given below.

Code #2 :

try: 
    example() 
except RuntimeError as e: 
    print("It didn't work:", e) 
    if e.__cause__: 
        print('Cause:', e.__cause__) 

An implicit form of chained exceptions occurs when another exception gets raised inside an except block.

Code #3 :

def example2(): 
    try: 
        int('N/A') 
    except ValueError as e: 
        print("Couldn't parse:", err) 
  
example2() 

Traceback (most recent call last):
    File "", line 3, in example2
ValueError: invalid literal for int() with base 10: 'N / A'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
    File "", line 1, in 
    File "", line 5, in example2
NameError: global name 'err' is not defined

In the code below, the NameError exception is raised as the result of a programming error, not in direct response to the parsing error. For this case, the __cause__ attribute of an exception is not set. Instead, a __context__ attribute is set to the prior exception.

Code #4 : To suppress chaining, use raise from None

def example3(): 
    try: 
        int('N / A') 
    except ValueError: 
        raise RuntimeError('A parsing error occurred') from None... 
  
example3() 

Output :

Traceback (most recent call last):
    File "", line 1, in 
    File "", line 5, in example3
RuntimeError: A parsing error occurred

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