build failure if Cython is not available

[Kiskae]

Describe the bug

If cython isn't available, then the import fails but gets suppressed:

yarl/packaging/pep517_backend/_backend.py

Lines 43 to 48 in 0ca5599

	with suppress(ImportError):
	# NOTE: Only available for wheel builds that bundle C-extensions. Declared
	# NOTE: by `get_requires_for_build_wheel()` and
	# NOTE: `get_requires_for_build_editable()`, when `--pure-python`
	# NOTE: is not passed.
	from Cython.Build.Cythonize import main as _cythonize_cli_cmd

However the imported function is always invoked during the build:

yarl/packaging/pep517_backend/_backend.py

Line 303 in 0ca5599

_cythonize_cli_cmd(cythonize_args)

Since there is no fallback, this causes a NameError and a subsequent build failure

To Reproduce

1. Set up build environment without Cython
2. python -m build

Expected behavior

Succesful compilation without cython support.

Logs/tracebacks

Running command Building wheel for yarl (pyproject.toml)
  **********************
  * Accelerated build *
  **********************
  Traceback (most recent call last):
    File "/nix/store/slhj69him45n2pck7wkknnffi4ay0jy0-python3.11-pip-23.2.1/lib/python3.11/site-packages/pip/_vendor/pyproject_hooks/_in_process/_in_process.py", line 353, in <module>
      main()
    File "/nix/store/slhj69him45n2pck7wkknnffi4ay0jy0-python3.11-pip-23.2.1/lib/python3.11/site-packages/pip/_vendor/pyproject_hooks/_in_process/_in_process.py", line 335, in main
      json_out['return_val'] = hook(**hook_input['kwargs'])
                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    File "/nix/store/slhj69him45n2pck7wkknnffi4ay0jy0-python3.11-pip-23.2.1/lib/python3.11/site-packages/pip/_vendor/pyproject_hooks/_in_process/_in_process.py", line 251, in build_wheel
      return _build_backend().build_wheel(wheel_directory, config_settings,
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    File "/nix/store/qp5zys77biz7imbk6yy85q5pdv7qk84j-python3-3.11.6/lib/python3.11/contextlib.py", line 81, in inner
      return func(*args, **kwds)
             ^^^^^^^^^^^^^^^^^^^
    File "/build/yarl-1.9.3/packaging/pep517_backend/_backend.py", line 325, in build_wheel
      with maybe_prebuild_c_extensions(
    File "/nix/store/qp5zys77biz7imbk6yy85q5pdv7qk84j-python3-3.11.6/lib/python3.11/contextlib.py", line 137, in __enter__
      return next(self.gen)
             ^^^^^^^^^^^^^^
    File "/build/yarl-1.9.3/packaging/pep517_backend/_backend.py", line 304, in maybe_prebuild_c_extensions
      _cythonize_cli_cmd(cythonize_args)
      ^^^^^^^^^^^^^^^^^^
  NameError: name '_cythonize_cli_cmd' is not defined

Python Version

3.11.6

multidict Version

6.0.4

yarl Version

1.9.3

OS

NixOS 23.05

Additional context

No response

Code of Conduct

• I agree to follow the aio-libs Code of Conduct

[webknjaz]

If you want a pure Python build, you must request it explicitly. If you want C-extensions, provision the build env properly. Cython is injected dynamically, only when building wheels is requested (including editable installs), through the corresponding PEP 517 hook, just like wheel.
Building with pypa/build in its default mode works because it follows the PEP and calls the corresponding hooks to determine all the deps, not just the statically listed ones.

I think that there's a CLI flag for build for verifying that the current env contains all the deps. Try using that as a self-check.