[New-bugs-announce] [issue34704] Do not access ob_type directly, introduce Py_TP

[Neil Schemenauer]

New submission from Neil Schemenauer <nas-python at arctrix.com>:

My long term goal is to make it possible to make PyObject an opaque structure.  Obviously that would break nearly every 3rd party extension at this point.  However, to allow freedom on certain interpreter implementation strategies, it is helpful if code does not access ob_type, ob_refcnt and ob_size directly.  Cleaning up core Python is not too hard.

There is closed bug #26824 which proposed a similar change.  There was two main issues with that patch.  One, it causes a fair amount of code churn.  This patch does that too.  Second, replace ob->ob_type with Py_TYPE(ob) adds an extra type-cast to PyObject*.  That's not good.

In this patch, I introduce Py_TP() as a non-typecast version of Py_TYPE().  I think the name is nice as it is short and matches the struct field prefix.

This change overlaps with Victor's proposed %t or %T format code change.  The code churn is mostly caused by code that does ob->ob_type->tp_name.  I'm not against the format code idea but personally I think trying to remove borrowed references is a difficult problem and it should not hold up the relatively simple task of allowing PyObject to be opaque.

Also, I doesn't see any immediate need to make PyTypeObject opaque.  The tp_* are used everywhere and so making access macros or functions for those would be hugely disruptive.  PyTypeObject is already opaque for the limited API.  I think that's good enough for now.

If this change gets accepted, I have follow-up patches to fix access to ob_refcnt and ob_size.  Those are much smaller patches since those fields are not often accessed.

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components: Interpreter Core
files: py_tp_macro.txt
keywords: needs review, patch
messages: 325482
nosy: nascheme, serhiy.storchaka, vstinner
priority: normal
severity: normal
stage: patch review
status: open
title: Do not access ob_type directly, introduce Py_TP
type: enhancement
Added file: https://bugs.python.org/file47806/py_tp_macro.txt

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<https://bugs.python.org/issue34704>
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