Update Narwhals Testing

Narwhals testing for cudf-polars and cudf

This rule describes the procedure for updating Narwhals unit testing for cudf-polars and cudf in CI. A successful update involves verifying locally that the variations of Narwhals unit test run in ci/test_narwhals.sh all pass.

Prerequisites

1. Prompt the user for a suitable development environment that contains both cudf and cudf-polars.
2. Prompt the user for the target Narwhals version for the upgrade.

Notes on the development environment:

• The environment must also have pytest-env installed (it is a CI-only dependency and is often missing from local dev environments). Without it, the runs fail with ImportError: Error importing plugin "env".
• The environment may contain extra packages that the test_python_narwhals CI job does not install (for example dask). Such packages can cause spurious local-only failures (for example tests/tpch_q1_test.py::test_q1[dask], which pytest.importorskips away in CI). Do not add these to a plugin; confirm a failure is reproducible in CI's dependency set before recording it.

Step 1. Bump Narwhals in dependencies.yaml

In the depends_on_narwhals section in dependencies.yaml, change the narwhals== version pinning to the target Narwhals version

Step 2. Install Narwhals from a local repository

Identify a directory that contains the Narwhals repository to install it into the development environment. If it is not provided by the user or one cannot be identified, clone it to a temporary location.

Ensure the Narwhals Git source tree is checked-out to the target upgrade version before installing.

Recent Narwhals versions use the uv_build build backend, so the editable install requires uv and uv-build in the environment and must be run with --no-build-isolation.

For example, to clone Narwhals and install it in a fictional, conda development environment named "cudf-dev"

NARWHALS_VERSION="v2.24.0"
git clone https://github.com/narwhals-dev/narwhals.git --single-branch --branch $NARWHALS_VERSION /tmp/narwhals
cd /tmp/narwhals
conda run -n cudf-dev pip install uv uv-build
conda run -n cudf-dev pip install -U -e . --no-build-isolation

Step 3. Iterate and classify test failures from pytest invocations in ci/test_narwhals.sh

Next, iterate through the various Narwhals unit test invocations in ci/test_narwhals.sh via pytest until there are no test failures.

Currently 3 variations of Narwhals unit tests are run:

1. With cudf through setting --constructors=cudf
2. With cudf-polars through setting the NARWHALS_POLARS_GPU=1 environment variable and --constructors=polars[lazy]
3. With cudf.pandas through setting the NARWHALS_DEFAULT_CONSTRUCTORS=pandas environment variable and -p cudf.pandas

Each of these variations has an accompanying pytest plugin to mark tests to skip or as expected to fail before the tests are run.

1. The cudf test run uses ci/narwhals_cudf_test_plugin.py via -p narwhals_cudf_test_plugin
2. The cudf.polars test run uses ci/narwhals_cudf_polars_test_plugin.py via -p narwhals_cudf_polars_test_plugin
3. The cudf.pandas test run uses ci/narwhals_cudf_pandas_test_plugin.py via -p narwhals_cudf_pandas_test_plugin

The TESTS_TO_SKIP and EXPECTED_FAILURES dictionaries in each file contain keys that are valid pytest test identifiers and values that are the reasons for skipping or failure of the test respectively.

Before running Narwhals unit tests, review each TESTS_TO_SKIP and EXPECTED_FAILURES in the plugin files and remove entries where the pytest test identifier no longer points to a test in Narwhals, as tests may have been removed in a newer Narwhals version.

Next, for each variation of Narwhals unit tests:

1. Run the exact pytest command found in ci/test_narwhals.sh for the test variation and pipe the output to a file. Run it from within the cloned Narwhals repository (the CI script does pushd narwhals first) and set PYTHONPATH to the ci/ directory so the -p narwhals_*_test_plugin plugins can be imported, e.g. PYTHONPATH="<repo>/ci" .... Node ids in this working directory are of the form tests/...py::<test>[...], which is exactly the dictionary key format.
2. Review the test output file and collect the failing tests.
3. For each failing test:
   • If a test is failing because it exercises behavior that cannot be supported in cudf or cudf-polars, add a key, value pair to the TESTS_TO_SKIP dictionary where the key is in the format <path>::<pytest_node_id> and the value is a short reason for skipping.
   • If a test is failing because of a bug in cudf or cudf-polars, or a bug or bad assumption in Narwhals, add a key, value pair to the EXPECTED_FAILURES dictionary where the key is in the format <path>::<pytest_node_id> and the value is a short reason for failure.
   • If a test reports [XPASS(strict)] (Narwhals marks it xfail(strict=True) for the constructor but it now passes), it must go in TESTS_TO_SKIP, not EXPECTED_FAILURES. Adding an xfail marker does not override Narwhals' own strict marker, so the strict XPASS would still fail; skipping is the equivalent of the old deselection.
4. Repeat the process until the test output file reports no failures.

Importantly when evaluating failing tests, do not attempt to fix cudf or cudf-polars to solve the failures. This is out of scope.

Once all variations of Narwhals unit tests pass, report the new test failures for each variation.