gh-111138: Add PyList_Extend() and PyList_Clear() functions

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• Split list_extend() into two sub-functions: list_extend_fast() and list_extend_iter().
• list_inplace_concat() no longer has to call Py_DECREF() on the list_extend() result, since list_extend() now returns an int.

• Issue: C API: Consider adding a public PyList_Extend() function #111138

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📚 Documentation preview 📚: https://cpython-previews--111862.org.readthedocs.build/

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This change keeps the internal C API:

Include/internal/pycore_list.h: extern PyObject* _PyList_Extend(PyListObject *, PyObject *);

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Oh, these functions should be added to the limited C API.

[serhiy-storchaka]

I suggest to remove _PyList_Extend and inline list_extend_method.

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I prefer to not remove _PyList_Extend() in the same PR, since remove private functions seems to be a controversial topic. If it's removed, I prefer to remove it in a separated PR. PyList_Extend() returns an int, whereas _PyList_Extend() returns an object.

I modified the code to remove list_extend_method().

[eendebakpt]

Suggested change

	``PyList_SetSlice(L, 0, PY_SSIZE_T_MAX, NULL)``.
	``PyList_SetSlice(list, 0, PY_SSIZE_T_MAX, NULL)``.

What does it mean to be similar? Does it have exactly the same effect? If not, what is the difference

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PyList_Clear(list) is the same as PyList_SetSlice(list, 0, PY_SSIZE_T_MAX, NULL). Internally, both end up calling list_clear(): same code path.

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@serhiy-storchaka@eendebakpt: I addressed your review. Please review updated PR.

I also added a note about the corner case of finalizer modifying the list, and I even added tests on this corner case.

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I removed notes form the documentation about finalizers which can modify the list while PyList_Clear() and PyList_SetSlice().

[serhiy-storchaka]

I suggest to change exceptions to SystemError. It is easier to change from SystemError to other type if needed than in the other direction.

[vstinner]

I suggest to change exceptions to SystemError.

Ok, done.

[vstinner]

Oh, these functions should be added to the limited C API.

Done: I added the functions to the limited C API version 3.13.

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@encukou: I added these 2 "new" functions to the limited C API.

[serhiy-storchaka]

I think there was a value in the note about possible non-emptiness of the list after clear(), but it can be added later, and perhaps in different form. The false assumption that the list is empty after clear() can lead to more severe bugs in C than in Python. It is not only about modifying the list in the item's destructor, it is mainly about releasing the GIL and modifying the list in other thread.

I left some suggestions for the documentation. I also think that it is better to move the documentation for new functions just past the documentation for PyList_SetSlice. The order here is not lexicographical, but semantical.

The rest LGTM.

[serhiy-storchaka]

Suggested change

	Remove all items from *list*. Similar to:
	``PyList_SetSlice(list, 0, PY_SSIZE_T_MAX, NULL)``.
	Remove all items from *list*.
	This is the same as ``PyList_SetSlice(list, 0, PY_SSIZE_T_MAX, NULL)`` and
	analogous to ``list.clear()`` or ``del list[:]``.

[serhiy-storchaka]

Suggested change

	Extend *list* with the contents of *iterable*. Similar to:
	``PyList_SetSlice(list, PY_SSIZE_T_MAX, PY_SSIZE_T_MAX, iterable)``.
	Extend *list* with the contents of *iterable*.
	This is the same as ``PyList_SetSlice(list, PY_SSIZE_T_MAX, PY_SSIZE_T_MAX, iterable)``
	and analogous to ``list.extend(iterable)`` or ``list += iterable``.

[vstinner]

I think there was a value in the note about possible non-emptiness of the list after clear()

The problem is that Python API is also affected. Other mutable and mapping types are affected. Should we add note in all Python and C functions which might be affected by the issue?

I added the note since it exists in the C implementation. But I concur with Antoine, it's not worth it to confuse users about this corner case.

[vstinner]

I left some suggestions for the documentation.

I applied your suggestions.

I also think that it is better to move the documentation for new functions just past the documentation for PyList_SetSlice. The order here is not lexicographical, but semantical.

Right, it's semantical. I moved PyList_Clear(), but left Extend after Append. For me, they go together :-) Maybe functions should just be sorted :-)

[serhiy-storchaka]

You moved PyList_Clear() after PyList_SET_ITEM(), not after PyList_SetSlice().

The reason of moving PyList_Clear() and PyList_Extend() after PyList_SetSlice() is that functions that work with a single item go first, followed by functions that work with multiple items, and that both PyList_Clear() and PyList_Extend() are convenient functions for particular use cases of PyList_SetSlice().

[vstinner]

Ok, I moved the functions after PyList_SetSlice() in the doc.

[serhiy-storchaka]

LGTM.

[encukou]

@encukou: I added these 2 "new" functions to the limited C API.

Please wait until we have a better process for adding to the limited API -- e.g. until the WG is formed and can discuss this.

[vstinner]

@encukou:

Please wait until we have a better process for adding to the limited API -- e.g. until the WG is formed and can discuss this.

I modified the PR just before you added this comment to not add these functions to the stable ABI / limited C API :-)

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I modified the PR just before you added this comment to not add these functions to the stable ABI / limited C API :-)

I also moved the definition to Include/cpython/.