fix: don't error when a global build-frontend is set on pyodide - #2945
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Since pypa#2609, the pyodide platform required the 'pyodide-build' frontend, but a global build-frontend setting (TOML or CIBW_BUILD_FRONTEND) overrides the platform default, so previously-working configs like build-frontend = "build" failed with a ConfigurationError. Even "default" failed, since it was mapped to "build" before the check. Resolve "default" to the platform default, and warn and use pyodide-build when another frontend is set on pyodide, matching pre-pypa#2609 behavior where the frontend name only affected verbosity flags. Assisted-by: ClaudeCode:claude-opus-4.8
agriyakhetarpal
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Jul 23, 2026
agriyakhetarpal
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Thanks @henryiii, makes sense and sorry for the trouble! Glad we caught this before a release.
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This would cause many valid configuration files to break. Making this just a warning for now, maybe pip/uv could become an error in a future version. Maybe build / build[uv] could always just tap over with a warning.
I'd expect this will only matter if we introduce a new xbuild option eventually.
Currently blocking a patch release.
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Since #2609, the pyodide platform requires the
pyodide-buildbuild frontend. But a globalbuild-frontendsetting (in TOML orCIBW_BUILD_FRONTEND) overrides the platform default in the option cascade, so previously-working configs like a globalbuild-frontend = "build"now fail with aConfigurationErrorunless a pyodide-specific override is added. Evenbuild-frontend = "default"failed, since"default"was hard-mapped to"build"before the check.This resolves
"default"to the platform default (pyodide-buildon pyodide), and turns the mismatch error into a warning that usespyodide-buildinstead (keeping any frontendargs). That matches pre-#2609 behavior, where pyodide always ranpyodide buildand the frontend name only affected verbosity flags.Regression tests were added first and confirmed to fail without the fix.