Fix Python 3.12 compatibility by updating Cython template function signatures

[Copilot]

This PR fixes compilation failures on Python 3.12 by updating the Cython template to use modern function signatures compatible with newer Cython versions.

Problem

The current build fails on Python 3.12 with errors like:

Cannot assign type 'int (const_void *, const_void *) except? -1 nogil' to 'int (*)(const_void *, const_void *) noexcept nogil'. Exception values are incompatible. Suggest adding 'noexcept' to the type of 'int_compare'.

This happens because modern Cython versions (3.1+) are stricter about exception handling in nogil functions and generate Python 3.12-compatible C code that expects noexcept qualifiers.

Solution

Updated three comparison functions in lightfm/_lightfm_fast.pyx.template to include the noexcept qualifier:

• reverse_pair_compare
• int_compare
• flt_compare

These functions are used with qsort() and bsearch() from the C standard library, which expect non-throwing comparison functions.

Changes

-cdef int reverse_pair_compare(const_void *a, const_void *b) nogil:
+cdef int reverse_pair_compare(const_void *a, const_void *b) noexcept nogil:

-cdef int int_compare(const_void *a, const_void *b) nogil:
+cdef int int_compare(const_void *a, const_void *b) noexcept nogil:

-cdef int flt_compare(const_void *a, const_void *b) nogil:
+cdef int flt_compare(const_void *a, const_void *b) noexcept nogil:

Testing

• ✅ Compiles successfully with Python 3.12.3 and Cython 3.1.4
• ✅ All core functionality works (import, model creation)
• ✅ Maintains backward compatibility with earlier Python versions
• ✅ Generated wheel installs and runs correctly

Impact

This minimal change enables LightFM to build and run on Python 3.12 environments, including modern Databricks clusters, without requiring users to downgrade their Python runtime.

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[Copilot]

Pull Request Overview

This PR fixes Python 3.12 compatibility issues by adding noexcept qualifiers to Cython function signatures. The changes address compilation failures caused by stricter exception handling requirements in modern Cython versions when generating Python 3.12-compatible code.

• Updates three comparison function signatures to include noexcept qualifier
• Maintains backward compatibility while enabling builds on Python 3.12
• Resolves Cython compilation errors related to exception value incompatibilities

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