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 :

Python3 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 :

Python3 1==

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 :

Python3 1==

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

Python3 1==

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

Python | Raising an Exception to Another Exception

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Python | Raising an Exception to Another Exception

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