Py_LIMITED_API breaks most PySomething_Check() functions

[petere]

BPO	11067
Nosy	@loewis, @birkenfeld, @pitrou, @asvetlov, @petere
Files	unisub.diff tp_flags.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 2011-02-05.20:36:14.380>
created_at = <Date 2011-01-29.21:34:46.259>
labels = ['interpreter-core', 'type-bug', 'release-blocker']
title = 'Py_LIMITED_API breaks most PySomething_Check() functions'
updated_at = <Date 2011-02-05.20:36:14.379>
user = 'https://github.com/petere'

bugs.python.org fields:

activity = <Date 2011-02-05.20:36:14.379>
actor = 'loewis'
assignee = 'none'
closed = True
closed_date = <Date 2011-02-05.20:36:14.380>
closer = 'loewis'
components = ['Interpreter Core']
creation = <Date 2011-01-29.21:34:46.259>
creator = 'petere'
dependencies = []
files = ['20646', '20678']
hgrepos = []
issue_num = 11067
keywords = ['patch']
message_count = 6.0
messages = ['127488', '127727', '127729', '127924', '127939', '128023']
nosy_count = 5.0
nosy_names = ['loewis', 'georg.brandl', 'pitrou', 'asvetlov', 'petere']
pr_nums = []
priority = 'release blocker'
resolution = 'accepted'
stage = 'commit review'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue11067'
versions = ['Python 3.2']

[petere]

When setting Py_LIMITED_API, functions such as PyUnicode_Check() can no longer be used. Example:

#define Py_LIMITED_API

#include <Python.h>

void foo()
{
    PyObject *o;

PyUnicode_Check(o); 

}

test.c: In function ‘foo’:
test.c:9: error: dereferencing pointer to incomplete type

PEP-384 contains some nested language that suggests that the _Check macros should be available under the limited API. And it seems to me that they easily could be, if Py_TYPE were implemented as a function instead of a macro.

[loewis]

The fast subtype checks cannot be supported in the limited API, as the layout of type objects is intentionally not exposed. It would be possible to expose a getter for the type flags; the individual flag values *are* part of the ABI.

I propose the attached patch, which replaces the fast subtype check with a slow one in the limited API. The patch would need to be extended to all subtype checks.

It's not strictly necessary to include this patch into the 3.2 release; users could also explicitly call PyObject_IsSubclass as a work-around. The patch could then be added for 3.2.1.

As another work-around, users can also use PyUnicode_CheckExact, which is even faster and might be correct in many cases as well.

[loewis]

Reconsidering: it's better to use PyType_IsSubtype, as this matches better the fast check.

[loewis]

Here is the patch I'd like to add to 3.2: this adds PyType_GetFlags, so that the fast checks remain available.

[pitrou]

The doc needs a versionadded attribute. Otherwise, looks good to me.

[loewis]

Thanks for the review. Committed as r88351.