Build cells example without pip build isolation so petsc4py is found - #115
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August 3, 2026 17:52
All three test-cells-ubuntu jobs fail at the Build step: the pycells wheel build's CMake configure runs find_package(PETSc4py), whose FindPETSc4py.cmake does `import petsc4py`, and under pip build isolation that cannot see the system python3-petsc4py-real exposed through the --system-site-packages venv. Configure then fails with "PETSc4py could not be found". Build with --no-build-isolation so the venv's interpreter (which sees the system site-packages, and thus petsc4py) is used directly. That requires the build deps up front: scikit-build-core (the backend) and ninja (the CMake generator, which an isolated build would otherwise pip-fetch; it is not apt installed). Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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Pull request overview
This PR updates the Ubuntu “cells” example CI workflow to disable pip build isolation during the build step so that the virtualenv created with --system-site-packages can see the system-provided petsc4py package (needed for find_package(PETSc4py) during the scikit-build-core/CMake build).
Changes:
- Switch the
examples/cellsbuild install topip install --no-build-isolation .. - Explicitly install build requirements (
scikit-build-coreandninja) into the venv since pip will no longer fetch them in an isolated build env. - Add inline documentation in the workflow explaining why build isolation must be disabled (newer pip behavior).
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Ubuntu Cells CI fails because
import petsc4pyunder pip build isolation cannot see the systempetsc4pyvia--system-site-packages. This is because build isolation has been tightened in newer versions of pip. Use--no-build-isolationso the venv can see petsc4py.