__context__ for yields inside except clause

[1st1]

BPO	25683
Nosy	@gvanrossum, @vstinner, @benjaminp, @njsmith, @cjerdonek, @1st1, @Vgr255

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2015-12-17.23:07:38.969>
created_at = <Date 2015-11-20.16:29:46.688>
labels = ['interpreter-core', 'type-bug', 'invalid']
title = '__context__ for yields inside except clause'
updated_at = <Date 2017-12-01.06:25:21.619>
user = 'https://github.com/1st1'

bugs.python.org fields:

activity = <Date 2017-12-01.06:25:21.619>
actor = 'chris.jerdonek'
assignee = 'none'
closed = True
closed_date = <Date 2015-12-17.23:07:38.969>
closer = 'yselivanov'
components = ['Interpreter Core']
creation = <Date 2015-11-20.16:29:46.688>
creator = 'yselivanov'
dependencies = []
files = []
hgrepos = []
issue_num = 25683
keywords = []
message_count = 5.0
messages = ['254994', '255348', '255349', '256621', '287993']
nosy_count = 7.0
nosy_names = ['gvanrossum', 'vstinner', 'benjamin.peterson', 'njs', 'chris.jerdonek', 'yselivanov', 'abarry']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue25683'
versions = ['Python 3.3', 'Python 3.4', 'Python 3.5', 'Python 3.6']

[1st1]

In the below snippet, SubError will propagate with __context__ set to None, instead of MainError. Maybe this isn't a 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())

[Vgr255]

This is due to the fact that Python 3 added the ability to define only __eq__ and get a free __ne__ defined. If my memory serves me right, functools.total_ordering was added in 3.2 and then backported to 2.x - where the relationship with __eq__ and __ne__ is not present. total_ordering doesn't do anything to touch __ne__ as it expects Python itself to do so (which it doesn't in 2.x).

[Vgr255]

Oops, that was *completely* the wrong issue. I apologize for the noise.

[1st1]

I now don't think this is a bug.

In the above example, SubError is instantiated outside of main generator. It's also thrown into main from the outside scope. And context should be set for exceptions that were originated by the code that was handling other exceptions.

Closing this issue with 'not a bug'.

[njsmith]

I disagree with the stated reason for closing this, because in general, implicit context chaining doesn't care about where the exception was instantiated, only where it was raised. For example:

-----

err = ValueError()
try:
    raise KeyError
except Exception:
    raise err

---

Prints:

-----

Traceback (most recent call last):
  File "/tmp/bar.py", line 3, in <module>
    raise KeyError
KeyError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/tmp/bar.py", line 5, in <module>
    raise err
ValueError

---

I would expect 'gen.throw(OBJ)' to be equivalent to doing 'raise OBJ' inside the generator, and raise does set __context__.