gh-115754: Use Py_GetConstant(Py_CONSTANT_EMPTY_BYTES)

[vstinner]

Replace PyBytes_FromStringAndSize(NULL, 0)
with Py_GetConstant(Py_CONSTANT_EMPTY_BYTES).

• Issue: C API: Implement singletons as functions calls in the stable ABI 3.13 #115754

[serhiy-storchaka]

Please wait.

[serhiy-storchaka]

I am not comfortable with this change.

1. There is nothing wrong in using PyBytes_FromStringAndSize(NULL, 0). It works, it worked, and it will always work. This is not inefficient or private API.
2. PyBytes_FromStringAndSize() returns a new reference, while Py_GetConstant() returns a borrowed reference. Even if an empty bytes object is immutable, there is a value of always using balanced incref/decref. It makes the intention clearer and the code more error-proof. When there are several exit points and some return a new reference and other return borrowed reference, you can immediately say that there is a bug. If it is not a bug, you will waste time and mental efforts in vain on analyzing the code.
3. This creates a tiny obstacle for future backportings.

In summary, this is a cosmetic change which makes the code more difficult to maintain without significant gain.

[vstinner]

PyBytes_FromStringAndSize() returns a new reference, while Py_GetConstant() returns a borrowed reference.

Py_GetConstant() returns a new reference: https://docs.python.org/dev/c-api/object.html#c.Py_GetConstant

[serhiy-storchaka]

Oh, nice. That eliminates most of my objections. There is still an issue with reasoning such change. This look like change for the sake of change.

[vstinner]

I would like to get rid of PyBytes_FromStringAndSize(NULL, size) calls which create an incomplete bytes object, but PyBytes_FromStringAndSize(NULL, 0) doesn't have this issue.

Py_GetConstant(Py_CONSTANT_EMPTY_BYTES) is simpler and a little bit faster than PyBytes_FromStringAndSize(NULL, 0).

In general, I would prefer to make Py_GetConstant(Py_CONSTANT_EMPTY_BYTES) the reference function to get an empty byte string.

[vstinner]

See also https://discuss.python.org/t/what-do-you-think-of-deprecating-c-apis-to-modify-immutable-strings/67057