bpo-40170: Add PyType_SetBase() function

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Add PyType_SetBase() and PyType_SetBaseStatic() functions to set the
base of a type.

PyType_Ready() no longer increment the reference count of the object
type on static types which have no base type and use a borrowed
reference to their base type.

https://bugs.python.org/issue40170

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After Py_SET_TYPE(), here is a new function to abstract access to the PyTypeObject structure: PyType_SetBase() to set the tp_base member.

PyType_SetBase() is safer than a direct access to tp_base since it handles reference count for the caller.

PyType_SetBaseStatic() is also provided for compatibility with extension modules using static types: set tp_base member, but don't touch reference count.

The PR also fix a reference leak in PyType_Ready():

PyType_Ready() no longer increment the reference count of the object
type on static types which have no base type and use a borrowed
reference to their base type.

cc @nascheme @corona10 @shihai1991 @encukou @pablogsal

[shihai1991]

After Py_SET_TYPE(), here is a new function to abstract access to the PyTypeObject structure: PyType_SetBase() to set the tp_base member.

PyType_SetBase() is safer than a direct access to tp_base since it handles reference count for the caller.

PyType_SetBaseStatic() is also provided for compatibility with extension modules using static types: set tp_base member, but don't touch reference count.

The PR also fix a reference leak in PyType_Ready():

PyType_Ready() no longer increment the reference count of the object
type on static types which have no base type and use a borrowed
reference to their base type.

cc @nascheme @corona10 @shihai1991 @encukou @pablogsal

Nice improvement.
Can we make PyType_SetBase() more universal? something like: PyType_SetSlot(type, Py_tp_base, base)?

[shihai1991]

If Line 2995 can be replaced, why Line 842 can not do the same operation?
https://github.com/python/cpython/blob/master/Objects/typeobject.c#L842

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type_set_bases() is very complex, I prefer to leave it unchanged to reduce the risk of any regression. IMHO the current type_set_bases() code is simpler to follow without an indirection like PyType_SetBase() function call.

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Can we make PyType_SetBase() more universal? something like: PyType_SetSlot(type, Py_tp_base, base)?

It would be possible, but I prefer one function per PyTypeObject member.

For example, PyType_SetBase() cannot fail. A function which would accept a parameter to choose the slot would introduce error handling, to handle invalid/unknown slot.

[Pixmew]

hello sir i accidently removed my reviewer and now it is saying awating core review but there are no one under reviewer section. can you plese help me

it is not saying like yours
Review has been requested on this pull request. It is not required to merge
my PR is #23166 can you please check it
thankyou

[shihai1991]

hello sir i accidently removed my reviewer and now it is saying awating core review but there are no one under reviewer section. can you plese help me

it is not saying like yours
Review has been requested on this pull request. It is not required to merge
my PR is #23166 can you please check it
thankyou

Hello, Yash.
MAYBE you can find the maintainer or experts of purges.py in https://devguide.python.org/experts/.
Or ping steve Dower in your PR directly(I saw steve is the author of purges.py in https://github.com/python/cpython/blob/master/Tools/msi/purge.py#L7)

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I rebased my PR and fixed the typo spotted by @Jongy.

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Wait, this PR doesn't make any sense.

The purpose of bpo-40170 is to move the C API towards opaque PyTypeObject structure. This PR adds PyType_SetBase() for heap types and PyType_SetBaseStatic() for static types.

PyType_SetBase() is only used in typeobject.c because it doesn't make sense to update tp_base after a type is created. So PyType_SetBase() is useless.

PyType_SetBaseStatic() doesn't solve bpo-40170: defining a static types requires that the PyTypeObject structure is not opaque. It is not possible to define a type statically if PyTypeObject is opaque. So PyType_SetBaseStatic() is useless.

This PR is useless, I close it. Moreover, PyType_FromSpecWithBases() can be used to create a heap type with a base type.

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I saw a lot of code setting tp_base on types, but I didn't notice that all these types were statically defined.