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 co

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