tests: Add backcompat for time_unit

[dangotbanned]

What type of PR is this? (check all applicable)

• 💾 Refactor
• ✨ Feature
• 🐛 Bug Fix
• 🔧 Optimization
• 📝 Documentation
• ✅ Test
• 🐳 Other

Related issues

• Completes feat: Warn on incompatible TimeUnit in pandas<2 #3007 (comment)
• Unblocks feat: Warn on incompatible TimeUnit in pandas<2 #3007

Checklist

• Code follows style guide (ruff)
• Tests added
• Documented the changes

If you have comments or can explain your changes, please do so below

Will avoid introducing 22 new warnings in the other PR

[FBruzzesi]

Thanks @dangotbanned, I left a couple of non-blocking comments

[FBruzzesi]

I am having a brain fart - what's happening here?
IIUC it's a way to avoid using this function outside a test that has a constructor. I must say that the "hint" was not very clear to me 🙈

[dangotbanned]

I am having a brain fart - what's happening here?

😂

So pretty recently I added a constructor_pandas_like fixture and learned a bit more pytest 😄

So the first check is to make sure the test function which request: pytest.FixtureRequest came from has got one of these 3 as a parameter

narwhals/tests/utils.py

Lines 55 to 57 in 3b3b481

	_CONSTRUCTOR_FIXTURE_NAMES = frozenset[str](
	("constructor_eager", "constructor", "constructor_pandas_like")
	)

If it does, then request.node.callspec.id ...

narwhals/tests/utils.py

Lines 240 to 241 in 3b3b481

	request_id = request.node.callspec.id
	if "duckdb" in request_id:

... will be this string ...

timestamp_test.py::test_timestamp_datetimes_tz_aware[duckdb-ms-ms-expected8]'
#                                                    ^^^^^^^^^^^^^^^^^^^^^^

... which gets the name from here

narwhals/tests/conftest.py

Lines 217 to 234 in 3b3b481

	EAGER_CONSTRUCTORS: dict[str, ConstructorEager] = {
	"pandas": pandas_constructor,
	"pandas[nullable]": pandas_nullable_constructor,
	"pandas[pyarrow]": pandas_pyarrow_constructor,
	"pyarrow": pyarrow_table_constructor,
	"modin": modin_constructor,
	"modin[pyarrow]": modin_pyarrow_constructor,
	"cudf": cudf_constructor,
	"polars[eager]": polars_eager_constructor,
	}
	LAZY_CONSTRUCTORS: dict[str, ConstructorLazy] = {
	"dask": dask_lazy_p2_constructor,
	"polars[lazy]": polars_lazy_constructor,
	"duckdb": duckdb_lazy_constructor,
	"pyspark": pyspark_lazy_constructor, # type: ignore[dict-item]
	"sqlframe": sqlframe_pyspark_lazy_constructor,
	"ibis": ibis_lazy_constructor,
	}

But why?

I don't know about you, but I've been bitten a few times by getting this pattern wrong

narwhals/tests/expr_and_series/dt/timestamp_test.py

Lines 158 to 174 in 3b3b481

	if "dask" in str(constructor):
	pytest.skip(reason="https://github.com/narwhals-dev/narwhals/issues/2808")
	if any(x in str(constructor) for x in ("duckdb", "pyspark", "ibis")):
	request.applymarker(
	pytest.mark.xfail(reason="Backend timestamp conversion not yet implemented")
	)
	unsupported_backends = (
	"pandas_constructor",
	"pandas_nullable_constructor",
	"cudf",
	"modin_constructor",
	)
	if any(x in str(constructor) for x in unsupported_backends):
	pytest.skip("Backend does not support date type")
	dates = {"a": [datetime(2001, 1, 1), None, datetime(2001, 1, 3)]}
	if "dask" in str(constructor):

Mainly with

• "pyarrow" unintentionally matching pandas-like
• always forgetting how to select every/a subset of pandas 😞

[dangotbanned]

Bringing this all back ...

time_unit_compat should just require that you've written a test like any of these:

def test_concat_diagonal(
    constructor: Constructor, request: pytest.FixtureRequest
) -> None:

def test_array_dunder(
    request: pytest.FixtureRequest, constructor_eager: ConstructorEager
) -> None:

Then once you're in the test, you can do this without even knowing about constructor 🤯

def test_array_dunder(
    request: pytest.FixtureRequest, constructor_eager: ConstructorEager
) -> None:
    replaced = time_unit_compat("us", request)

[dangotbanned]

Then once you're in the test, you can do this without even knowing about constructor 🤯

It was specifically that part I was thinking might lead to someone hitting that error message

[FBruzzesi]

Thanks @dangotbanned, feel free to merge if you are happy with it