[issue25612] nested try..excepts don't work correctly for generators

[STINNER Victor]

STINNER Victor  added the comment:

It seems like this issue introduced a regression in Python 3.7.0: bpo-33996 
"Crash in gen_send_ex(): _PyErr_GetTopmostException() returns freed memory".

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[issue25612] nested try..excepts don't work correctly for generators

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

This change was in top5 breaking changes for 3.7. It broke many projects that 
use Cython until they upgraded Cython to the version that supports 3.7.

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[issue25612] nested try..excepts don't work correctly for generators

[Ned Deily]

Ned Deily  added the comment:

P.S. I added a very minimal What's New in 3.7 entry for this change since it 
has affected some downline projects.  I just copied the NEWS entry.  Feel free 
to expand it and/or move it to a better location in the file.

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[issue25612] nested try..excepts don't work correctly for generators

[Ned Deily]

Ned Deily  added the comment:


New changeset ef5f4ba96e05f61ad3baca502012085f31e99342 by Ned Deily (Miss 
Islington (bot)) in branch '3.7':
bpo-25612: Add minimal What's New in 3.7 entry (GH-7656) (GH-7658)
https://github.com/python/cpython/commit/ef5f4ba96e05f61ad3baca502012085f31e99342


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[issue25612] nested try..excepts don't work correctly for generators

[miss-islington]

Change by miss-islington :


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[issue25612] nested try..excepts don't work correctly for generators

[Ned Deily]

Ned Deily  added the comment:


New changeset 04290cb9945eca1a97f6924495256c15f29fab41 by Ned Deily in branch 
'master':
bpo-25612: Add minimal What's New in 3.7 entry (GH-7656)
https://github.com/python/cpython/commit/04290cb9945eca1a97f6924495256c15f29fab41


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[issue25612] nested try..excepts don't work correctly for generators

[Ned Deily]

Change by Ned Deily :


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[issue25612] nested try..excepts don't work correctly for generators

[Ned Deily]

Ned Deily  added the comment:


New changeset 508d7693bc09affd99fdaa4a321cc3da0638c8a0 by Ned Deily (Miss 
Islington (bot)) in branch '3.7':
bpo-33612: Remove PyThreadState_Clear() assertion (GH-7069) (GH-7074)
https://github.com/python/cpython/commit/508d7693bc09affd99fdaa4a321cc3da0638c8a0


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[issue25612] nested try..excepts don't work correctly for generators

[STINNER Victor]

STINNER Victor  added the comment:


New changeset b6dccf54fd3bac9c87348d96f9d6b571608c15bc by Victor Stinner in 
branch 'master':
bpo-33612: Remove PyThreadState_Clear() assertion (#7069)
https://github.com/python/cpython/commit/b6dccf54fd3bac9c87348d96f9d6b571608c15bc


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[issue25612] nested try..excepts don't work correctly for generators

[miss-islington]

Change by miss-islington :


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[issue25612] nested try..excepts don't work correctly for generators

[STINNER Victor]

Change by STINNER Victor :


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[issue25612] nested try..excepts don't work correctly for generators

[Chris Jerdonek]

Change by Chris Jerdonek :


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[issue25612] nested try..excepts don't work correctly for generators

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

Thank you for the clarification Mark. I agree that the current behavior is well 
justified.

Cython was fixed in 
https://github.com/cython/cython/commit/2d3392463b77eb550bd54a69bd28cc161947acb5.

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[issue25612] nested try..excepts don't work correctly for generators

[Mark Shannon]

Mark Shannon  added the comment:

Looking at the docs:

https://docs.python.org/3.6/library/sys.html#sys.exc_info states:
 If the current stack frame is not handling an exception, the information 
is taken from the calling stack frame, or its caller, and so on until a stack 
frame is found that is handling an exception.

And https://docs.python.org/3/reference/simple_stmts.html#the-raise-statement

If no expressions are present, raise re-raises the last exception that was 
active in the current scope. If no exception is active in the current scope, a 
RuntimeError exception is raised indicating that this is an error.

Note that `sys.exc_info()` explicitly mentions scanning the stack, but `raise` 
just says "active in the current scope". Testing on 3.5 shows that "active in 
the current scope" does scan the stack (for simple calls at least).

Which means that the newly implemented behaviour is correct.



> Note that removing exc_type, exc_value and exc_traceback from PyThreadState 
> breaks Cython.

Is there a matching Cython issue?

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[issue25612] nested try..excepts don't work correctly for generators

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

The problem I mentioned in msg304117 has been resolved in backward direction: 
"raise" outside of an except block don't raise a RuntimeError.

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[issue25612] nested try..excepts don't work correctly for generators

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

Note that removing exc_type, exc_value and exc_traceback from PyThreadState 
breaks Cython.

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[issue25612] nested try..excepts don't work correctly for generators

[Antoine Pitrou]

Antoine Pitrou  added the comment:

Thank you Mark for the fix!

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[issue25612] nested try..excepts don't work correctly for generators

[Antoine Pitrou]

Antoine Pitrou  added the comment:


New changeset ae3087c6382011c47db82fea4d05f8bbf514265d by Antoine Pitrou (Mark 
Shannon) in branch 'master':
Move exc state to generator. Fixes bpo-25612 (#1773)
https://github.com/python/cpython/commit/ae3087c6382011c47db82fea4d05f8bbf514265d


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[issue25612] nested try..excepts don't work correctly for generators

[Mark Shannon]

Mark Shannon  added the comment:

Thanks Serhiy for spotting that.

'raise' should raise the same exception as sys.exc_info() returns.
I'll update the PR.

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[issue25612] nested try..excepts don't work correctly for generators

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

I tried the following code:

def g():
yield 1
raise
yield 2

i = g()
try:
1/0
except:
next(i)
next(i)

Currently it raises:

Traceback (most recent call last):
  File "", line 5, in 
  File "", line 2, in 
ZeroDivisionError: division by zero

With PR 1773 applied it raises:

Traceback (most recent call last):
  File "", line 2, in 
ZeroDivisionError: division by zero

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "", line 5, in 
  File "", line 3, in g
RuntimeError: No active exception to reraise

And this looks more correct.

But if replace raise with print(sys.exc_info()) the patched and unpatched 
interpreters both print:

(, ZeroDivisionError('division by zero',), 
)

Is it correct that sys.exc_info() return an exception while raise can't reraise 
it?

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[issue25612] nested try..excepts don't work correctly for generators

[Roundup Robot]

Changes by Roundup Robot :


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[issue25612] nested try..excepts don't work correctly for generators

[Guido van Rossum]

Changes by Guido van Rossum :


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[issue25612] nested try..excepts don't work correctly for generators

[Nathaniel Smith]

Nathaniel Smith added the comment:

(Issue 29587 is a duplicate of this one, but it has some more information on 
where the !throwflag check came from and why it might be possible to remove it 
now.)

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[issue25612] nested try..excepts don't work correctly for generators

[Nathaniel Smith]

Nathaniel Smith added the comment:

I read the patch but I don't understand the logic behind it :-). Why should the 
value of tstate->exc_type affect whether we save/restore exc_info? Won't this 
mean that things are still broken for code like:

-
def f():
try:
raise KeyError
except Exception:
try:
yield
except Exception:
pass
raise

try:
raise NameError
except Exception:
gen = f()
gen.send(None)
gen.throw(ValueError)
-

?

Conceptually I would have thought the fix would be to remove the '!throwflag' 
check, but I haven't actually tried it...

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[issue25612] nested try..excepts don't work correctly for generators

[Andrew Svetlov]

Changes by Andrew Svetlov :


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[issue25612] nested try..excepts don't work correctly for generators

[Yury Selivanov]

Yury Selivanov added the comment:

I closed issue #25683 as "not a bug".  So it's just this issue that we need to 
fix.

Anyone wants to review the patch? :)  Since the code involved here is quite 
complex, I don't want to commit this patch without a thorough review.

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[issue25612] nested try..excepts don't work correctly for generators

[Martin Panter]

Martin Panter added the comment:

Nick, your scenario seems to behave no differently with and without the patch:

Traceback (most recent call last):
  File "", line 2, in 
__main__.MainError

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[issue25612] nested try..excepts don't work correctly for generators

[Guido van Rossum]

Guido van Rossum added the comment:

I'm sorry, I'm no help here -- I don't know how __context__ is
supposed to work. :-(

On Fri, Nov 20, 2015 at 8:14 AM, Yury Selivanov  wrote:
>
> Yury Selivanov added the comment:
>
> Martin, you might be right here.  Guido, do you think it's OK that SubError 
> doesn't have MainError in its __context__? 
> http://bugs.python.org/issue25612#msg254576
>
> --
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[issue25612] nested try..excepts don't work correctly for generators

[Yury Selivanov]

Yury Selivanov added the comment:

Anyways, here's a separate issue for the __context__ problem: 
http://bugs.python.org/issue25683

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[issue25612] nested try..excepts don't work correctly for generators

[Yury Selivanov]

Yury Selivanov added the comment:

Guido, Martin, I've just posted to python-dev about this ticket.  Let's see 
what others think.

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[issue25612] nested try..excepts don't work correctly for generators

[Yury Selivanov]

Yury Selivanov added the comment:

Martin, you might be right here.  Guido, do you think it's OK that SubError 
doesn't have MainError in its __context__? 
http://bugs.python.org/issue25612#msg254576

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[issue25612] nested try..excepts don't work correctly for generators

[Nick Coghlan]

Nick Coghlan added the comment:

Interrogating the thread state like that makes me wonder how this patch behaves 
in the following scenario:

class MainError(Exception): pass
class SubError(Exception): pass

def yield_coro():
yield
coro = yield_coro()
coro.send(None)

try:
raise MainError
except MainError:
try:
coro.throw(SubError)
except SubError:
pass
raise

Also potentially worth exploring is this proposed architectural change from 
Mark Shannon to move all exception related state out of frame objects: 
http://bugs.python.org/issue13897

While we couldn't do the latter in a maintenance release, I'd be interested to 
know what effect it has on this edge case.

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[issue25612] nested try..excepts don't work correctly for generators

[Yury Selivanov]

Yury Selivanov added the comment:

> Thinking about the __context__ thing some more, I guess it might make sense 
> for __context__ to be overwritten by the generator context. The same way it 
> gets overwritten when you execute “raise” inside an exception handler.

Not sure I understand what you're saying here.

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[issue25612] nested try..excepts don't work correctly for generators

[Larry Hastings]

Larry Hastings added the comment:

I don't plan to hold up 3.5.1 for this.

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[issue25612] nested try..excepts don't work correctly for generators

[Martin Panter]

Martin Panter added the comment:

I was making an analogy between how the “raise” statement works, and how the 
throw() method could work. In this example, there are three exception objects 
(MainError, SubError, and ValueError). I was suggesting that it is okay for the 
context to be set to the MainError instance, because that is how the analogous 
version using a “raise” statement works.

def main():
try:
raise MainError("Context inside generator")
except MainError:
yield  # __context__ could be changed to MainError

coro = main()
coro.send(None)
try:
try: raise ValueError("Context outside of generator")
except ValueError: raise SubError()
except SubError as ex:
coro.throw(ex)  # __context__ is ValueError

# raise analogy:

try:
try: raise ValueError("Context outside of generator")
except ValueError as ex: raise SubError()
except SubError as ex:
saved = ex  # __context__ is ValueError

try:
raise MainError("Context inside generator")
except MainError:
raise saved  # Changes __context__ to MainError

===

Sorry I’m not really familiar with the code to quickly propose a patch or 
review your change though.

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[issue25612] nested try..excepts don't work correctly for generators

[Yury Selivanov]

Yury Selivanov added the comment:

Can someone work with me on fixing this issue?  I think it's important to ship 
3.5.1 with this resolved (and 3.4.4 FWIW).

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[issue25612] nested try..excepts don't work correctly for generators

[Guido van Rossum]

Guido van Rossum added the comment:

You might have to ping python-dev. But in terms of priorities I think
it's not a blocker -- it's been broken for quite some time now, and
it's a fairly odd corner of the language.

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[issue25612] nested try..excepts don't work correctly for generators

[STINNER Victor]

STINNER Victor added the comment:

> Can someone work with me on fixing this issue?  I think it's important to 
> ship 3.5.1 with this resolved (and 3.4.4 FWIW).

It don't consider this issue as a blocker for Python 3.5.1. This
release already contains a *lot* of bugfixes! It's important to get it
as soon as possible.

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[issue25612] nested try..excepts don't work correctly for generators

[Martin Panter]

Martin Panter added the comment:

Thinking about the __context__ thing some more, I guess it might make sense for 
__context__ to be overwritten by the generator context. The same way it gets 
overwritten when you execute “raise” inside an exception handler.

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[issue25612] nested try..excepts don't work correctly for generators

[Martin Panter]

Martin Panter added the comment:

Regarding the second bug, did you consider that the exception thrown to the 
generator can already have __context__ set?

try:
try: raise ValueError("Context outside of generator")
except ValueError as ex: raise SubError() from ex
except SubError as ex:
coro.throw(ex)  # ex.__context__ is a ValueError

I guess either one context could trump the other, or we could to follow the 
chain of contexts and append the other chain at the end. Both these ideas seem 
a bit ugly though.

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[issue25612] nested try..excepts don't work correctly for generators

[A. Jesse Jiryu Davis]

Changes by A. Jesse Jiryu Davis :


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[issue25612] nested try..excepts don't work correctly for generators

[Yury Selivanov]

New submission from Yury Selivanov:

Nested try..except statements with yields can loose reference to the current 
exception.

The following code:

class MainError(Exception):
pass

class SubError(Exception):
pass

def main():
try:
raise MainError()
except MainError:
try:
yield
except SubError:
print('got SubError')
raise

coro = main()
coro.send(None)
coro.throw(SubError())

prints:

got SubError
Traceback (most recent call last):
  File "t.py", line 19, in 
coro.throw(SubError())
  File "t.py", line 15, in main
raise
RuntimeError: No active exception to reraise

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components: Interpreter Core
messages: 254557
nosy: benjamin.peterson, gvanrossum, haypo, larry, ncoghlan, yselivanov
priority: release blocker
severity: normal
status: open
title: nested try..excepts don't work correctly for generators
versions: Python 3.4, Python 3.5, Python 3.6

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[issue25612] nested try..excepts don't work correctly for generators

[Yury Selivanov]

Changes by Yury Selivanov :


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[issue25612] nested try..excepts don't work correctly for generators

[STINNER Victor]

STINNER Victor added the comment:

Is this issue related to the issue #23353?

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[issue25612] nested try..excepts don't work correctly for generators

[Yury Selivanov]

Changes by Yury Selivanov :


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[issue25612] nested try..excepts don't work correctly for generators

[STINNER Victor]

Changes by STINNER Victor :


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[issue25612] nested try..excepts don't work correctly for generators

[Yury Selivanov]

Yury Selivanov added the comment:

Here's a patch the fixes the first problem (but __context__ bug is still open).

I'm not sure that the patch is correct :(  But at least I've added new 
unittests (one still failing)

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Added file: http://bugs.python.org/file41026/gen_exc_1.patch

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[issue25612] nested try..excepts don't work correctly for generators

[Yury Selivanov]

Yury Selivanov added the comment:

This was originally discovered here: 
https://github.com/python/asyncio/issues/287

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[issue25612] nested try..excepts don't work correctly for generators

[STINNER Victor]

STINNER Victor added the comment:

I reverted the change of the issue #23353 but it doesn't fix this example, so 
it looks like these issues are not related. Cool.

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[issue25612] nested try..excepts don't work correctly for generators

[Yury Selivanov]

Yury Selivanov added the comment:

Another bug:

class MainError(Exception):
pass

class SubError(Exception):
pass

def main():
try:
raise MainError()
except MainError:
yield

coro = main()
coro.send(None)
coro.throw(SubError())


SubError will propagate, but won't have MainError in its __context__

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