Skip tests for unavailable backends

[frenzymadness]

I'd like to package narwhals for Fedora Linux and I'd like to run testsuite during the build of the RPM package. Unfortunately, the testsuite expects all supported backends to be installed but in Fedora, only pandas and pyarrow are available.

Would it make sense to add a piece of logic that skips all tests using backends that cannot be imported? I saw some constructors in the conftest module, but I'm not sure whether this is the right place.

[FBruzzesi]

Hey @frenzymadness thanks for your request! If you are in control of what to run and what not, you can add the following flag --constructors pandas,pyarrow when using pytest to run only pandas and pyarrow backends. Namely:

pytest tests --constructors pandas,pyarrow

FYI, we also have pandas constructors for nullable dtype backends:

pytest tests --constructors pandas,pandas[nullable],pandas[pyarrow],pyarrow

Would this work for you?

[frenzymadness]

It works, thank you!

Would it make sense to make the tests for missing constructors skipped by default?

[dangotbanned]

Would it make sense to make the tests for missing constructors skipped by default?

Yeah I also think that'd be nice to have

The only concern might be silently skipping a subset of the requested --constructors ...

Maybe an alternative would be changing the options to be --include, --exclude

narwhals/tests/conftest.py

Lines 36 to 66 in bf5bede

	# When testing cudf.pandas in Kaggle, we get an error if we try to run
	# python -m cudf.pandas -m pytest --constructors=pandas. This gives us
	# a way to run `python -m cudf.pandas -m pytest` and control which constructors
	# get tested.
	if default_constructors := os.environ.get(
	"NARWHALS_DEFAULT_CONSTRUCTORS", None
	): # pragma: no cover
	DEFAULT_CONSTRUCTORS = default_constructors
	else:
	DEFAULT_CONSTRUCTORS = (
	"pandas,pandas[pyarrow],polars[eager],pyarrow,duckdb,sqlframe,ibis"
	)
	def pytest_addoption(parser: pytest.Parser) -> None:
	parser.addoption(
	"--runslow", action="store_true", default=False, help="run slow tests"
	)
	parser.addoption(
	"--all-cpu-constructors",
	action="store_true",
	default=False,
	help="run tests with all cpu constructors",
	)
	parser.addoption(
	"--constructors",
	action="store",
	default=DEFAULT_CONSTRUCTORS,
	type=str,
	help="libraries to test",
	)

Possibly expand that to have groups with names like:

narwhals/narwhals/_namespace.py

Lines 71 to 88 in bf5bede

	Polars: TypeAlias = Literal[_Polars, Implementation.POLARS]
	Arrow: TypeAlias = Literal[_Arrow, Implementation.PYARROW]
	Dask: TypeAlias = Literal[_Dask, Implementation.DASK]
	DuckDB: TypeAlias = Literal[_DuckDB, Implementation.DUCKDB]
	Ibis: TypeAlias = Literal[_Ibis, Implementation.IBIS]
	PandasLike: TypeAlias = Literal[
	_PandasLike, Implementation.PANDAS, Implementation.CUDF, Implementation.MODIN
	]
	SparkLike: TypeAlias = Literal[
	_SparkLike,
	Implementation.PYSPARK,
	Implementation.SQLFRAME,
	Implementation.PYSPARK_CONNECT,
	]
	EagerOnly: TypeAlias = "PandasLike | Arrow"
	EagerAllowed: TypeAlias = "EagerOnly | Polars"
	LazyOnly: TypeAlias = "SparkLike | Dask | DuckDB | Ibis"
	LazyAllowed: TypeAlias = "LazyOnly | Polars"