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In the following code, the original traceback attached to the exception thrown into the generator is ignored:
def gen():
 try:
 yield
 except:
 raise
g = gen()
try:
 1/0
except ZeroDivisionError as v:
 g.throw(v)
But if you replace the last line with:
 g.throw(type(v), v, v.__traceback__)
then the original traceback gets appended.
g.throw() should have fetched the __traceback__ attribute by itself.

Attached a patch that fixes this. The test case is a bit ugly, as it only checks that the traceback's depth is correct.

Thank you! There is a memory leak somewhere:
$ ./python -m test -R 3:2 test_generators
[1/1] test_generators
beginning 5 repetitions
12345
.....
test_generators leaked [1945, 1945] references, sum=3890
1 test failed:
 test_generators
Also, since the patch is so short, a stylistic nit:
+ if(tb == NULL) {
There should be a space after the "if".
(see PEP 7: http://www.python.org/dev/peps/pep-0007/)

It leaks because `PyException_GetTraceback()` already returns a new reference, hence the "Py_XINCREF(tb)" is wrong.

Attached an updated patch.
I incref'd the return value of PyErr_GetTraceback() because PyErr_Restore() steals the reference. The documentation of PyErr_GetTraceback() doesn't tell whether it returns a new or borrowed reference, but apparently a new reference then?

Actually, it is documented:
"Return the traceback associated with the exception as a new reference (...)"
http://docs.python.org/dev/c-api/exceptions.html#PyException_GetTraceback
Thanks for the patch, will apply!

New changeset dcf5cc88d5c9 by Antoine Pitrou in branch '3.2':
Issue #13188: When called without an explicit traceback argument,
http://hg.python.org/cpython/rev/dcf5cc88d5c9
New changeset f4e3db1194e4 by Antoine Pitrou in branch 'default':
Issue #13188: When called without an explicit traceback argument,
http://hg.python.org/cpython/rev/f4e3db1194e4 

Antoine Pitrou wrote:
> Actually, it is documented:
> "Return the traceback associated with the exception as a new reference (...)"
Ah, you're right. It just doesn't have the green "Return value: New
reference" note.
Thanks for committing!

BTW, shouldn't this be applied to 2.7 too?

The same issue exists on 2.7, working on a patch.

It seems this cannot be achieved the same way in 2.7 as the traceback is not directly associated with the exception. However, if we're currently in an exception handler and sys.exc_info() corresponds to the exception passed to generator.throw(), we could use the current traceback. Would this approach be too complicated and not worth it?

I don't think this should be applied to 2.7.
In 2.x, the full exception info consists of the (type, value, traceback)-trio. Programmer is expected to pass this around to retain full exception info. Re-raising just the value (exception instance) using "raise value" starts a fresh traceback. Currently "raise" and gen.throw() behave the same in that regard and there's no reason to change that.
Python 3 reduces the exception info to one object by adding __traceback__ to the exception instance so it's reasonable to expect that "raise" and gen.throw() will be able to extract the full info from just the instance.
"raise" does that today, gen.throw() didn't, hence this bug report.

I agree with Arkadiusz, this doesn't seem to match Python 2's exception semantics, where you always specify the traceback explicitly if you want to.

Ok, I think we have reached a consensus. Closing.