Fix: PY_SSIZE_T_CLEAN macro must be defined for '#' formats

[AdityaMayukhSom]

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• I am not making a trivial change, such as fixing a typo in a comment.

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Fixes #5919.

For python 3.12 and older, the macro PY_SSIZE_T_CLEAN must be defined before including Python.h to use all # variants of formats (s#, y#, etc.).

This PR defines PY_SSIZE_T_CLEAN in the first line for both AMD and NVIDIA backend C and python modules. This fixes SystemError: PY_SSIZE_T_CLEAN macro must be defined for '#' formats on python 3.12 and below when driver.c file is dynamically loaded and converted into module.

This change is necessary because as of the date of this PR, fine tuning models with unsloth and vLLM only works with python 3.12 and below.

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[Jokeren]

If you only want to fix this file, why trigger other changes?

[Jokeren]

I also don't understand

For python 3.12 and older, the macro PY_SSIZE_T_CLEAN must be defined before including Python.h to use all # variants of formats (s#, y#, etc.).

Isn't is already defined before Python.h here?

[Jokeren]

Please do clean up your PR

[AdityaMayukhSom]

I unintentionally formatted the python files by mistake, I have removed the unwanted changes.

Regarding your second question, yes it was defined before Python.h, but the error was still present (unsloth version '2025.5.7').

My understanding is that the error was due #include statements written before PY_SSIZE_T_CLEAN was defined, as per the system error message PY_SSIZE_T_CLEAN macro must be defined for '#' forma.

Defining it on top of the file removed the error for me (on python 3.12.10).

[Jokeren]

What you did is not consistent with your comment

For python 3.12 and older, the macro PY_SSIZE_T_CLEAN must be defined before including Python.h to use all # variants of formats (s#, y#, etc.).

[james-vincent]

This bug seems to still exist. The macro must also be defined in the embedded C source code defined in driver.py, in the make_lanucher function, line 229:

` params.append("&global_scratch")
src = f"""
#include "cuda.h"
#include <stdbool.h>
#include <Python.h>
#include <dlfcn.h>

`
PY_SSIZE_T_CLEAN is not defined before Python.h is included.

[debduthira]

@AdityaMayukhSom thanks for the fix. I was facing the same issue while training using SFT Trainer. Shifting #define PY_SSIZE_T_CLEAN above other #include statements fixed the issue.

[peterbell10]

This seems like the only change that is likely to make any difference. All the other cases already defined the macro before including Python.h.

[hongbo-miao]

(Came from Unsloth unslothai/unsloth#2760, met this issue for Unsloth notebook on RTX 5090. Hopefully this can fix soon, thank you! ☺️)