Patrick W. <[email protected]> added the comment:
Nick Coghlan (ncoghlan) at 2010-12-29 08:46 (UTC):
> No, the context must always be included unless explicitly suppressed.
Then there should be some actually working way to suppress it, right?
I think the standard behaviour that automatically sets the `__context__` of an
exception is fine, especially when thinking about exceptions that get raised
inside `except` blocks and are not custom made. However there should be some
way to suppress the context in some way.
Personally, I would prefer if `raise .. from ..` would set the exception's
cause, but would *not* automatically print that cause. But then again, it would
be a problem to get the cause afterwards when the program terminated because of
that exception. So obviously, in such situations, both the cause and the
context of a raised exception cannot be used (because both prints out the full
trackback).
So we obviously need some new mechanism or syntax to ignore the previous
exception. As it seems that the cause has a higher precedence than the context
(which is fine as the cause is changeable), using `None` as the cause of an
exception would be the best solution in my opinion:
try:
x / y
except ZeroDivisionError as e:
raise Exception( 'Invalid value for y' ) from None
While it might be difficult as the cause is `None` before and gets set to
`None` afterwards, Python is totally able to detect that the value was
explicitely set to `None`. Either the raise statement should set some internal
flag, or the setter for `__cause__` should just check when it is first written
to.
If that would be too complicated (although it would be possible and very
logically to do it like that), maybe a different syntax would work:
try:
x / y
except ZeroDivisionError as e:
raise Exception( 'Invalid value for y' ) instead
Something like that.
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