Suppressing __context__

[Chris Rebert]

On Fri, Oct 29, 2010 at 4:02 AM, Antoine Pitrou <solipsis at pitrou.net> wrote:
> On Sun, 24 Oct 2010 10:48:23 +0200
> "Martin v. Loewis" <martin at v.loewis.de> wrote:
>>
>> You may now wonder whether it is possible to set __context__ to None
>> somehow. See PEP 3134:
>>
>> Open Issue: Suppressing Context
>>
>>     As written, this PEP makes it impossible to suppress '__context__',
>>     since setting exc.__context__ to None in an 'except' or 'finally'
>>     clause will only result in it being set again when exc is raised.
>
> It is not easily discoverable, but it is possible to suppress
> __context__ by using a bare re-raise afterwards:
>
>>>> try:
> ...   try: 1/0
> ...   except ZeroDivisionError: raise KeyError
> ... except BaseException as e:
> ...   e.__context__ = None
> ...   raise
> ...
> Traceback (most recent call last):
>  File "<stdin>", line 3, in <module>
> KeyError

So, how/why does that work?

Cheers,
Chris