io.UnsupportedOperation.__new__(io.UnsupportedOperation) fails

[markshannon]

BPO	14241
Nosy	@ncoghlan, @pitrou, @larryhastings, @benjaminp, @tarekziade, @merwok, @markshannon
Files	io_new.patch

^{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 = 'https://github.com/merwok'
closed_at = <Date 2012-07-07.14:03:22.177>
created_at = <Date 2012-03-09.12:39:15.420>
labels = ['type-bug']
title = 'io.UnsupportedOperation.__new__(io.UnsupportedOperation) fails'
updated_at = <Date 2012-07-07.14:48:49.410>
user = 'https://github.com/markshannon'

bugs.python.org fields:

activity = <Date 2012-07-07.14:48:49.410>
actor = 'benjamin.peterson'
assignee = 'eric.araujo'
closed = True
closed_date = <Date 2012-07-07.14:03:22.177>
closer = 'Mark.Shannon'
components = ['Distutils2']
creation = <Date 2012-03-09.12:39:15.420>
creator = 'Mark.Shannon'
dependencies = []
files = ['24768']
hgrepos = []
issue_num = 14241
keywords = ['patch']
message_count = 2.0
messages = ['155227', '164849']
nosy_count = 8.0
nosy_names = ['ncoghlan', 'pitrou', 'larry', 'benjamin.peterson', 'tarek', 'eric.araujo', 'alexis', 'Mark.Shannon']
pr_nums = []
priority = 'normal'
resolution = 'out of date'
stage = 'patch review'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue14241'
versions = ['Python 3.3']

[markshannon]

>>> io.UnsupportedOperation.__new__(io.UnsupportedOperation)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: ValueError.__new__(UnsupportedOperation) is not safe, use OSError.__new__()

Looking at the mro of io.UnsupportedOperation
(io.UnsupportedOperation, ValueError, OSError, Exception, BaseException, object'>)

Shows that ValueError.__new__ is getting called, but will not construct
an instance of a subtype of OSError

Switching the order of ValueError and OSError fixes this error (patch attached), but doesn't fix the underlying problem.

Why doesn't io.UnsupportedOperation() fail, when
UnsupportedOperation.__new__ does?
io.UnsupportedOperation() actually calls OSError.__new__
via the internal tp_new slot.
In other words UnsupportedOperation->tp_new != UnsupportedOperation.__new__
This should not happen, it looks like the logic in inherit_special()
in typeobject.c may be faulty.

[markshannon]

>>> io.UnsupportedOperation.__new__(io.UnsupportedOperation)
behaves correctly now (rev d9c98730e2e8)

This bug was (I believe) caused somehow by an error in OSError_new() which did not initialize self->args if OSError_init() was not called.

Here is a crash which exploited the lack of initialization:
(Works on rev 2a142141e5fd)

>>> class C(ValueError, OSError):pass
... 
>>> c = OSError.__new__(C)
>>> str(c)
Segmentation fault (core dumped)

This has already been fixed, so I'm closing this issue.