fix: don't error when a global build-frontend is set on pyodide (#2945)

Since #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-#2609 behavior where the frontend name only affected verbosity
flags.

Assisted-by: ClaudeCode:claude-opus-4.8

cibuildwheel/options.py

@@ -43,7 +43,7 @@ from packaging.specifiers import SpecifierSet
 from cibuildwheel import errors
 from cibuildwheel.architecture import Architecture
 from cibuildwheel.environment import EnvironmentParseError, ParsedEnvironment, parse_environment
-from cibuildwheel.frontend import BuildFrontendConfig
+from cibuildwheel.frontend import BuildFrontendConfig, BuildFrontendName
 from cibuildwheel.logger import log
 from cibuildwheel.oci_container import OCIContainerEngineConfig
 from cibuildwheel.projectfiles import get_requires_python_str, resolve_dependency_groups
@@ -850,8 +850,11 @@ class Options:
                 env_plat=False,
                 option_format=ShlexTableFormat(sep="; ", pair_sep=":", allow_merge=False),
             )
+            default_frontend: BuildFrontendName = (
+                "pyodide-build" if self.platform == "pyodide" else "build"
+            )
             if not build_frontend_str or build_frontend_str == "default":
-                build_frontend = BuildFrontendConfig("build")
+                build_frontend = BuildFrontendConfig(default_frontend)
             else:
                 try:
                     build_frontend = BuildFrontendConfig.from_config_string(build_frontend_str)
@@ -860,8 +863,13 @@ class Options:
                     raise errors.ConfigurationError(msg) from e
 
             if self.platform == "pyodide" and build_frontend.name != "pyodide-build":
-                msg = "The pyodide platform requires the 'pyodide-build' build frontend"
+                # pip and uv could become an error, eventually. build -> pyodide-build is
-                raise errors.ConfigurationError(msg)
+                # probably fine to keep as a warning
+                log.warning(
+                    f"The pyodide platform ignores the {build_frontend.name!r} build "
+                    "frontend; using 'pyodide-build' instead"
+                )
+                build_frontend = BuildFrontendConfig("pyodide-build", build_frontend.args)
             if self.platform != "pyodide" and build_frontend.name == "pyodide-build":
                 msg = "The 'pyodide-build' build frontend is only supported on the pyodide platform"
                 raise errors.ConfigurationError(msg)

docs/options.md

@@ -483,9 +483,9 @@ files (from workspaces) are not supported.
 
 On Android, the "pip" frontend is not supported.
 
-On Pyodide, `build-frontend` must be `pyodide-build`, which is the default for that
+On Pyodide, `pyodide-build` is the default and only supported frontend; setting
-platform. See [Pyodide build frontend support](platforms.md#pyodide-build-frontend) for
+another frontend produces a warning and uses `pyodide-build` instead. See
-details.
+[Pyodide build frontend support](platforms.md#pyodide-build-frontend) for details.
 
 You can specify extra arguments to pass to the build frontend using the
 optional `args` option.

docs/platforms.md

@@ -190,7 +190,7 @@ The `--libdir` option specifies the directory containing cross-compiled shared l
 
 ### Build frontend support {: #pyodide-build-frontend}
 
-The pyodide platform builds wheels by shelling out to `pyodide build`, via the `pyodide-build` [`build-frontend`](options.md#build-frontend), which itself is a meta build frontend and passes through commands to pypa/build with specialised handling. This is the only supported frontend for this platform, and is used by default.
+The pyodide platform builds wheels by shelling out to `pyodide build`, via the `pyodide-build` [`build-frontend`](options.md#build-frontend), which itself is a meta build frontend and passes through commands to pypa/build with specialised handling. This is the only supported frontend for this platform, and is used by default. A global [`build-frontend`](options.md#build-frontend) setting naming another frontend is ignored with a warning on this platform.
 
 [`build-verbosity`](options.md#build-verbosity) is passed through to `pyodide build` as `-v`/`-vv`. It is capped at `-vv`. `pyodide build` has no `-vvv` flag.
 

unit_test/options_test.py

@@ -515,13 +515,33 @@ def test_pyodide_build_frontend_args(tmp_path: Path) -> None:
     assert build_frontend.args == ["--exports=whole_archive"]
 
 
+@pytest.mark.parametrize(
+    "build_frontend_str",
+    ["default", "pip", "build", "build[uv]", "uv"],
+)
+def test_pyodide_global_build_frontend_coerced(tmp_path: Path, build_frontend_str: str) -> None:
+    """A global non-pyodide frontend setting is coerced to pyodide-build, not an error."""
+    args = CommandLineArguments.defaults()
+    args.package_dir = tmp_path
+
+    tmp_path.joinpath("pyproject.toml").write_text(
+        textwrap.dedent(
+            f"""\
+            [tool.cibuildwheel]
+            build-frontend = "{build_frontend_str}"
+            """
+        )
+    )
+
+    options = Options(platform="pyodide", command_line_arguments=args, env={})
+    build_frontend = options.build_options(identifier=None).build_frontend
+
+    assert build_frontend.name == "pyodide-build"
+
+
 @pytest.mark.parametrize(
     ("platform", "build_frontend_str"),
     [
-        ("pyodide", "pip"),
-        ("pyodide", "build"),
-        ("pyodide", "build[uv]"),
-        ("pyodide", "uv"),
         ("linux", "pyodide-build"),
         ("macos", "pyodide-build"),
         ("windows", "pyodide-build"),