fixup! --pure-python

.github/workflows/ci-cd.yml

@@ -115,8 +115,8 @@ jobs:
       run: >-
         python -Im pip install -e .
         ${{
-          matrix.no-extensions == ''
-          && '--config-settings=--build-c-extensions='
+          matrix.no-extensions != ''
+          && '--config-settings=--pure-python='
           || ''
         }}
     - name: Run unittests

CHANGES/893.misc.rst

@@ -2,23 +2,23 @@ Replaced the packaging is replaced from an old-fashioned :file:`setup.py` to an
 in-tree :pep:`517` build backend -- by :user:`webknjaz`.
 
 Whenever the end-users or downstream packagers need to build ``yarl`` from
-source (a Git checkout or an sdist), they have to pass a ``config_settings``
-flag ``--build-c-extensions``. If this flag is not set, a pure-python
-distribution will be built.
+source (a Git checkout or an sdist), they may pass a ``config_settings``
+flag ``--pure-python``. If this flag is not set, a C-extension will be built
+and included into the distribution.
 
 Here is how this can be done with ``pip``:
 
 .. code-block:: console
 
-    $ python -m pip install . --config-settings=--build-c-extensions=
+    $ python -m pip install . --config-settings=--pure-python=
 
 This will also work with ``-e | --editable``.
 
 The same can be achieved via ``pypa/build``:
 
 .. code-block:: console
 
-    $ python -m pip build --config-settings=--build-c-extensions=
+    $ python -m pip build --config-settings=--pure-python=
 
 Adding ``-w | --wheel`` can force ``pypa/build`` produce a wheel from source
 directly, as opposed to building an ``sdist`` and then building from it.

packaging/pep517_backend/_backend.py

@@ -69,15 +69,30 @@
 )
 
 
-BUILD_C_EXT_CONFIG_SETTING = '--build-c-extensions'
-"""Config setting name toggle that is used to request C-ext in wheels."""
+PURE_PYTHON_CONFIG_SETTING = '--pure-python'
+"""Config setting name toggle that is used to opt out of making C-exts."""
 
 IS_PY3_12_PLUS = _python_version_tuple[:2] >= (3, 12)
 """A flag meaning that the current runtime is Python 3.12 or higher."""
 
 IS_CPYTHON = _system_implementation.name == "cpython"
 """A flag meaning that the current interpreter implementation is CPython."""
 
+PURE_PYTHON_MODE_CLI_FALLBACK = bool(IS_CPYTHON)
+"""A fallback for `--pure-python` is not set."""
+
+
+def _make_pure_python(config_settings: dict[str, str] | None = None) -> bool:
+    if (
+            config_settings is None or
+            PURE_PYTHON_CONFIG_SETTING not in config_settings
+    ):
+        return PURE_PYTHON_MODE_CLI_FALLBACK
+
+    truthy_values = {'', None, 'true', '1', 'on'}
+
+    return config_settings[PURE_PYTHON_CONFIG_SETTING].lower() in truthy_values
+
 
 def _get_local_cython_config():
     """Grab optional build dependencies from pyproject.toml config.
@@ -113,7 +128,7 @@ def _get_local_cython_config():
         keep-going = false
 
         [tool.local.cythonize.kwargs]
-        # This section can contain args tha have values:
+        # This section can contain args that have values:
         # * exclude=PATTERN      exclude certain file patterns from the compilation
         # * parallel=N    run builds in N parallel jobs (default: calculated per system)
         exclude = "**.py"
@@ -230,7 +245,7 @@ def _run_in_temporary_directory() -> t.Iterator[Path]:
 def maybe_prebuild_c_extensions(  # noqa: WPS210
         build_inplace: bool = False,
         config_settings: dict[str, str] | None = None,
-) -> str:
+) -> t.Generator[None, t.Any, t.Any]:
     """Pre-build C-extensions in a temporary directory, when needed.
 
     This context manager also patches metadata, setuptools and distutils.
@@ -239,10 +254,9 @@ def maybe_prebuild_c_extensions(  # noqa: WPS210
     :param config_settings: :pep:`517` config settings mapping.
 
     """
-    build_c_ext_requested = BUILD_C_EXT_CONFIG_SETTING in (
-        config_settings or {}
-    )
-    if not build_c_ext_requested:
+    is_pure_python_build = _make_pure_python(config_settings)
+
+    if is_pure_python_build:
         print("*********************", file=_standard_error_stream)
         print("* Pure Python build *", file=_standard_error_stream)
         print("*********************", file=_standard_error_stream)
@@ -255,8 +269,8 @@ def maybe_prebuild_c_extensions(  # noqa: WPS210
     if not IS_CPYTHON:
         _warn_that(
             'Building C-extensions under the runtimes other than CPython is '
-            'unsupported and will likely fail. Consider not passing the '
-            f'`{BUILD_C_EXT_CONFIG_SETTING !s}` PEP 517 config setting.',
+            'unsupported and will likely fail. Consider passing the '
+            f'`{PURE_PYTHON_CONFIG_SETTING !s}` PEP 517 config setting.',
             RuntimeWarning,
             stacklevel=999,
         )
@@ -288,9 +302,9 @@ def maybe_prebuild_c_extensions(  # noqa: WPS210
 
 
 def build_wheel(
-         wheel_directory: str,
-         config_settings: dict[str, str] | None = None,
-         metadata_directory: str | None = None,
+        wheel_directory: str,
+        config_settings: dict[str, str] | None = None,
+        metadata_directory: str | None = None,
 ) -> str:
     """Produce a built wheel.
 
@@ -313,9 +327,9 @@ def build_wheel(
 
 
 def build_editable(
-         wheel_directory: str,
-         config_settings: dict[str, str] | None = None,
-         metadata_directory: str | None = None,
+        wheel_directory: str,
+        config_settings: dict[str, str] | None = None,
+        metadata_directory: str | None = None,
 ) -> str:
     """Produce a built wheel for editable installs.
 
@@ -345,23 +359,21 @@ def get_requires_for_build_wheel(
     :param config_settings: :pep:`517` config settings mapping.
 
     """
-    build_c_ext_requested = BUILD_C_EXT_CONFIG_SETTING in (
-        config_settings or {}
-    )
+    is_pure_python_build = _make_pure_python(config_settings)
 
-    if build_c_ext_requested and not IS_CPYTHON:
+    if not is_pure_python_build and not IS_CPYTHON:
         _warn_that(
             'Building C-extensions under the runtimes other than CPython is '
-            'unsupported and will likely fail. Consider not passing the '
-            f'`{BUILD_C_EXT_CONFIG_SETTING !s}` PEP 517 config setting.',
+            'unsupported and will likely fail. Consider passing the '
+            f'`{PURE_PYTHON_CONFIG_SETTING !s}` PEP 517 config setting.',
             RuntimeWarning,
             stacklevel=999,
         )
 
-    c_ext_build_deps = [
+    c_ext_build_deps = [] if is_pure_python_build else [
         'Cython >= 3.0.0b3' if IS_PY3_12_PLUS  # Only Cython 3+ is compatible
         else 'Cython',
-    ] if build_c_ext_requested else []
+    ]
 
     return _setuptools_get_requires_for_build_wheel(
         config_settings=config_settings,