Raise exception in except block and suppress first error

Question

I'm trying to catch an exception and raise a more specific error at a point in my code:

try:
  something_crazy()
except SomeReallyVagueError:
  raise ABetterError('message')

This works in Python 2, but in Python 3, it shows both exceptions:

Traceback(most recent call last):
...
SomeReallyVagueError: ...
...

During handling of the above exception, another exception occurred:

Traceback(most recent call last):
...
ABetterError: message
...

Is there some way to get around this, so that SomeReallyVagueError's traceback isn't shown?

score 17 · Accepted Answer · 2014-06-25T21:18:28.520

17

In Python versions 3.3 and greater, you can use the raise <exception> from None syntax to suppress the traceback of the first exception:

>>> try:
...     1/0
... except ZeroDivisionError:
...     raise ValueError
...
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
ZeroDivisionError: division by zero

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
ValueError
>>>
>>>
>>> try:
...     1/0
... except ZeroDivisionError:
...     raise ValueError from None
...
Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
ValueError
>>>

edited Jun 25 '14 at 21:18

answered Jun 25 '14 at 20:21

This does require 3.3 or later. – Ethan Furman Jun 25 '14 at 20:33
It worked! I just wrapped it in an `exec` statement because my code needs to run under Python 2 and 3. – kirbyfan64sos Jun 25 '14 at 20:39
The `raise from ` was added in 3.0, but the `from None` part of that syntax didn't make it in until 3.3 as part of [PEP 409](https://docs.python.org/3.3/whatsnew/3.3.html#pep-409-suppressing-exception-context). – Ethan Furman Jun 25 '14 at 21:16
1

How can this be written to support both Python 2/3? – Guillochon Mar 24 '17 at 12:44

Raise exception in except block and suppress first error

1 Answers1