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
