Disable exception chaining in python 3

Question

There is a new feature that was introduced in python3 - exception chaining. For some reasons I need to disable it for certain exceptions in my code.

Here is sample code:

try:     print(10/0) except ZeroDivisionError as e:     sys.exc_info()     raise AssertionError(str(e)) 

what I see:

Traceback (most recent call last):   File "draft.py", line 19, in main     print(10/0) ZeroDivisionError: division by zero  During handling of the above exception, another exception occurred:  Traceback (most recent call last):   File "draft.py", line 26, in <module>     main()   File "draft.py", line 22, in main     raise AssertionError(str(e)) AssertionError: division by zero 

what I want to see:

Traceback (most recent call last):   File "draft.py", line 26, in <module>     main()   File "draft.py", line 22, in main     raise AssertionError(str(e)) AssertionError: division by zero 

I tried to use sys.exc_clear(), but this method is removed from python 3 too. I can use workaround that works

exc = None try:     print(10/0) except ZeroDivisionError as e:     exc = e if exc:     raise AssertionError(str(exc)) 

but I believe that there is better solution.

Answer 1

Simple Answer

try:     print(10/0) except ZeroDivisionError as e:     raise AssertionError(str(e)) from None 

However, you probably actually want:

try:     print(10/0) except ZeroDivisionError as e:     raise AssertionError(str(e)) from e 

Explanation

__cause__

Implicit exception chaining happens through __context__ when there isn't an explicit cause exception set.

Explicit exception chaining works through __cause__ so if you set __cause__ to the exception itself, it should stop the chaining. If __cause__ is set, Python will suppress the implicit message.

try:     print(10/0) except ZeroDivisionError as e:     exc = AssertionError(str(e))     exc.__cause__ = exc     raise exc 

raise from

We can use "raise from" to do the same thing:

try:     print(10/0) except ZeroDivisionError as e:     exc = AssertionError(str(e))     raise exc from exc 

None __cause__

Setting __cause__ to None actually does the same thing:

try:     print(10/0) except ZeroDivisionError as e:     exc = AssertionError(str(e))     exc.__cause__ = None     raise exc 

raise from None

So that brings us to the most elegant way to do this which is to raise from None:

try:     print(10/0) except ZeroDivisionError as e:     raise AssertionError(str(e)) from None 

But I would argue that you usually want to explicitly raise your exception from the cause exception so the traceback is preserved:

try:     print(10/0) except ZeroDivisionError as e:     raise AssertionError(str(e)) from e 

This will give us a slightly different message that states that the first exception was the direct cause of the second:

Traceback (most recent call last):   File "<stdin>", line 2, in <module> ZeroDivisionError: division by zero  The above exception was the direct cause of the following exception:  Traceback (most recent call last):   File "<stdin>", line 4, in <module> AssertionError: division by zero