Raise a Py3K warning for catching nested tuples with non-BaseException exceptions

[abalkin]

BPO	2380
Nosy	@abalkin, @pitrou, @benjaminp, @ezio-melotti, @zware
Files	issue2380.diff

^{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 2020-05-04.13:55:06.718>
created_at = <Date 2008-03-18.03:51:02.944>
labels = ['interpreter-core', 'easy', 'type-bug']
title = 'Raise a Py3K warning for catching nested tuples with non-BaseException exceptions'
updated_at = <Date 2020-05-04.13:55:06.715>
user = 'https://github.com/abalkin'

bugs.python.org fields:

activity = <Date 2020-05-04.13:55:06.715>
actor = 'zach.ware'
assignee = 'none'
closed = True
closed_date = <Date 2020-05-04.13:55:06.718>
closer = 'zach.ware'
components = ['Interpreter Core']
creation = <Date 2008-03-18.03:51:02.944>
creator = 'belopolsky'
dependencies = []
files = ['9720']
hgrepos = []
issue_num = 2380
keywords = ['patch', 'easy', '26backport']
message_count = 5.0
messages = ['63875', '66580', '123030', '123248', '368051']
nosy_count = 6.0
nosy_names = ['belopolsky', 'pitrou', 'benjamin.peterson', 'brunogola', 'ezio.melotti', 'zach.ware']
pr_nums = []
priority = 'low'
resolution = 'out of date'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue2380'
versions = []

[abalkin]

As of r61478, the following code fails to produce a py3k warning:

try:
raise ValueError
except ((ValueError,object),):
pass

which is an error for in py3k:

Traceback (most recent call last):
  File "x.py", line 3, in <module>
    except ((ValueError,object),):
TypeError: catching classes that do not inherit from BaseException is 
not allowed

[brunogola]

Should nested tuples in an except clause be a valid syntax?
like:
...
except ((ValueError, Exception), TypeError, (...)):
pass

I don't see any benefit and because of this behavior the implementation
(in Python/errors.c and in the patch attached) will use recursive function.

I think that any objects inside a tuple in an except clause should be
treated equally (as an object), even a tuple, so the except clause above
should be an error in py3k and should produce a warning because the list
objects does not inherits BaseException.

Please, see my comment in http://bugs.python.org/issue2345, it is
related to this issue.

[abalkin]

bpo-2380.diff does not apply anymore.

[pitrou]

Not all incompatibilities have to raise a py3k warnings; AFAIK, only those which are not handled by 2to3 do.

[zware]

With 2.7 out of support, closing.