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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>
ValueError
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
 File "<stdin>", line 4, in <module>
NameError