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Author ethan.furman
Recipients aronacher, catalin.iacob, draghuram, eric.araujo, ethan.furman, ezio.melotti, mrabarnett, ncoghlan, pitrou, poke, rhettinger, steven.daprano
Date 2012-01-29.07:37:23
SpamBayes Score 4.4964e-15
Marked as misclassified No
Message-id <>
It looks like agreement is forming around the

    raise ... from None

method.  It has been mentioned more than once that having the context saved on the exception would be a Good Thing, and for further debugging (or logging or what-have-you) I must agree.

The patch attached now sets __cause__ to True, leaving __context__ unclobbered.  The exception printing routine checks to see if __cause__ is True, and if so simply skips the display of either cause or __context__, but __context__ can still be queried by later code.

One concern raised was that since it is possible to write (even before this patch)

    raise KeyError from NameError

outside of a try block that some would get into the habit of writing

    raise KeyError from None

as a way of preemptively suppressing implicit context chaining;  I am happy to report that this is not an issue, since when that exception is caught and a new exception raised, it is the new exception that controls the display.

In other words:

>>> try:
...   raise ValueError from None
... except:
...   raise NameError
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
Date User Action Args
2012-01-29 07:37:27ethan.furmansetrecipients: + ethan.furman, rhettinger, ncoghlan, pitrou, draghuram, aronacher, ezio.melotti, eric.araujo, mrabarnett, steven.daprano, poke, catalin.iacob
2012-01-29 07:37:27ethan.furmansetmessageid: <>
2012-01-29 07:37:26ethan.furmanlinkissue6210 messages
2012-01-29 07:37:25ethan.furmancreate