'_PyLong_NumBits': identifier not found in 3.13

[clin1234]

Bug report

Bug description:

While fixing pywin32 to let CI builds work in Python 3.13, the build process complained of not being able to find _PyLong_NumBits. All other Python versions in the CI built successfully.

BOOL PyCom_VariantFromPyObject(PyObject *obj, VARIANT *var)
{
    // ...
    if (PyLong_Check(obj)) {
        int sign = _PyLong_Sign(obj);
        size_t nbits = _PyLong_NumBits(obj);
        if (nbits == (size_t)-1 && PyErr_Occurred())
            return FALSE;
    // ...
}

The function still exists in main, apparently inaccessible via the public C API since 3.13. What's the recommended replacement?

CPython versions tested on:

3.8, 3.9, 3.10, 3.11, 3.12, 3.13

Operating systems tested on:

Windows

Linked PRs

• gh-119336: Restore _PyLong_NumBits #119418
• [3.13] gh-119336: Restore removed _PyLong_NumBits() function (GH-119418) #119970

[emmatyping]

In 5846924 Victor restored

• _PyLong_Copy()
• _PyLong_FromDigits()
• _PyLong_New()
• _PyLong_Sign()

It seems reasonable to include _PyLong_NumBits as well, since like the others there isn't a great replacement.

[clin1234]

Will it be restored in the next beta 3.13 release, or will I have to wait until the first 3.14 alpha release?

[emmatyping]

I cannot make that call, but I made #119418 against 3.14 for now (it can be backported for 3.13 if that's what people decide). Eventually it seems like a good idea to make this public.

[vstinner]

You can modify pywin32 to call the bit_length() method which returns a Python int object. Then if you need a size_t, you can call PyLong_AsSize_t.

If this function is called by one project, we should consider making it public, rather than restoring the old private function.

[clin1234]

You can modify pywin32 to call the bit_length() method which returns a Python int object. Then if you need a size_t, you can call PyLong_AsSize_t.

If this function is called by one project, we should consider making it public, rather than restoring the old private function.

How would I access the built-in bit_length() method within the C API?

[da-woods]

How would I access the built-in bit_length() method within the C API?

PyObject_CallMethod(obj, "bit_length", NULL)

(Not taking a view on whether that justifies not restoring the function though)

[vstinner]

A code search in PyPI top 7,500 projects found other projects:

• mariadb (1.1.10)
• pickle5 (0.0.12)
• zodbpickle (3.2)

pickle5 and zodbpickle contain copies of the Python source code, but MariaDB is a good usage example. So maybe it's worth it to add a function.

I created issue gh-119714: [C API] Add PyLong_GetNumBits() function.

[vstinner]

A code search in PyPI top 7,500 projects found other projects:

@encukou also identified that panda3d uses it: https://github.com/panda3d/panda3d/blob/adb92428856853019583f398d27c5d34e44a9343/panda/src/putil/bitArray_ext.cxx#L28

[vstinner]

Function restored by commit e50fac9