Terry Reedy wrote:
> It sounds like you are asking for a special class
> __NoException__(BaseException) to use as the marker.
Guido van Rossum wrote:
So what did you think of Terry Reedy's idea of using a special exception class?
Our table would then look like:
__context__ __cause__
raise None __NoException__
reraise previous __NoException__
reraise from previous None | exception
It is certainly simpler than trying to force the use of both True and
False. :)
The purist side of me thinks it's still slightly awkward; the practical
side recognizes that there probably is not a perfect solution and thinks
this is workable, and is willing to deal with the slight awkwardness to
get 'from None' up and running. :)
The main reason for the effort in keeping the previous exception in
__context__ instead of just clobbering it is for custom error handlers, yes?
So here is a brief comparison of the two:
def complete_traceback():
....
Actually, I got about three lines into that and realized that whatever
__cause__ is set to is completely irrelevant for that function: if
*__context__* is not None, follow the chain; the only relevance
__cause__ has is when would it print? If it is a (valid) exception.
And how do we know if it's valid?
# True, False, None
if isinstance(exc.__cause__, BaseException):
print_exc(exc)
or
if exc.__cause__ not in (True, False, None):
print_exc(exc)
vs
# None, __NoException__ (forced to be an instance)
if (exc.__cause__ is not None
and not isinstance(exc.__cause__, __NoException__):
print_exc(exc)
or
# None, __NoException__ (forced to stay a class)
if exc.__cause__ not in (None, __NoException__):
print_exc(exc)
Having gone through all that, I'm equally willing to go either way
(True/False/None or __NoException__).
Implementation questions for the __NoException__ route:
1) Do we want double underscores, or just a single one?
I'm thinking double to mark it as special as opposed
to private.
2) This is a new exception class -- do we want to store the
class itself in __context__, or it's instance? If its
class, should we somehow disallow instantiation of it?
3) Should it be an exception, or just inherit from object?
Is it worth worrying about somebody trying to raise it, or
raise from it?
4) Is the name '__NoException__' confusing?
~Ethan~
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