[Python-Dev] PEP for allowing 'raise NewException from None'

Ethan Furman ethan at stoneleaf.us
Sun Jan 29 08:44:32 CET 2012

For those not on the nosy list, here's the latest post
to http://bugs.python.org/issue6210:


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>

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