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build: Always regenerate the Cython C source when building - #88
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Cython skips regenerating `util_cy.c` when it is newer than the `.pyx` and
its `.pxd` dependencies, judged by file timestamps. The generated C depends
on the numpy version in the build environment, but a cached numpy install
can have older timestamps than an existing C file. So after building with
one Python version, building from the same checkout for another one could
reuse C generated for a newer numpy and fail to compile. For example, after
`uv sync` on Python 3.13 (numpy 2.5),
`UV_PYTHON=3.9 uv run --isolated pytest` (numpy 2.0) failed with:
util_cy.c: error: call to undeclared function
'_PyUFuncObject_GET_ITEM_DATA'
Pass `force=True` to `cythonize`. Cython is already a build requirement, so
always regenerating only costs a second or two per build.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
kylebarron
added this pull request to stack #91
October 5, 2026 21:52
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Note
This PR was written by Claude (Claude Code), not by @kylebarron.
Makes the build always regenerate the Cython C source. This affects development builds only, so it isn't in the CHANGELOG.
Why
Cython only regenerates
util_cy.cwhen the.pyxor its dependencies are newer than the existing C file. But the generated C also depends on the numpy version in the build environment, and a cached numpy install can have older file timestamps. So after building for one Python version, building from the same checkout for another could reuse C generated for a newer numpy and fail to compile. For example, afteruv syncon Python 3.13 (numpy 2.5), this failed every time:Installs from an sdist weren't affected: the published 0.5.0 sdist builds fine on Python 3.9 and 3.10.
Fix
Pass
force=Truetocythonize. Cython is already a build requirement, so always regenerating costs only a second or two per build.There's no pytest regression test, since this is about build state; the reproduction above is the test.
🤖 Written by Claude Code