[WIP/rolling] test: Scope dataframe library imports to fixtures in conftest.py (#2241)

* test: Work towards making polars optional for tests

This is a batch of changes aiming at making it possible to run the test
suite without `polars` installed.  Whenever easily feasible, the use
of `pyarrow` and `pandas` was made optional as well.

Part of bug #1726

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: mgorny

tests/conftest.py

@@ -8,16 +8,14 @@
 from typing import Callable
 from typing import Sequence
 
-import pandas as pd
-import polars as pl
-import pyarrow as pa
 import pytest
 
 from narwhals.utils import generate_temporary_column_name
 from tests.utils import PANDAS_VERSION
 
 if TYPE_CHECKING:
     import duckdb
+    import polars as pl
 
     from narwhals.typing import IntoDataFrame
     from narwhals.typing import IntoFrame
@@ -72,19 +70,26 @@ def pytest_collection_modifyitems(
 
 
 def pandas_constructor(obj: dict[str, list[Any]]) -> IntoDataFrame:
+    import pandas as pd
+
     return pd.DataFrame(obj)
 
 
 def pandas_nullable_constructor(obj: dict[str, list[Any]]) -> IntoDataFrame:
+    import pandas as pd
+
     return pd.DataFrame(obj).convert_dtypes(dtype_backend="numpy_nullable")
 
 
 def pandas_pyarrow_constructor(obj: dict[str, list[Any]]) -> IntoDataFrame:
+    import pandas as pd
+
     return pd.DataFrame(obj).convert_dtypes(dtype_backend="pyarrow")
 
 
 def modin_constructor(obj: dict[str, list[Any]]) -> IntoDataFrame:  # pragma: no cover
     import modin.pandas as mpd
+    import pandas as pd
 
     return mpd.DataFrame(pd.DataFrame(obj))  # type: ignore[no-any-return]
 
@@ -93,6 +98,7 @@ def modin_pyarrow_constructor(
     obj: dict[str, list[Any]],
 ) -> IntoDataFrame:  # pragma: no cover
     import modin.pandas as mpd
+    import pandas as pd
 
     return mpd.DataFrame(pd.DataFrame(obj)).convert_dtypes(dtype_backend="pyarrow")  # type: ignore[no-any-return]
 
@@ -104,15 +110,20 @@ def cudf_constructor(obj: dict[str, list[Any]]) -> IntoDataFrame:  # pragma: no
 
 
 def polars_eager_constructor(obj: dict[str, list[Any]]) -> IntoDataFrame:
+    import polars as pl
+
     return pl.DataFrame(obj)
 
 
 def polars_lazy_constructor(obj: dict[str, list[Any]]) -> pl.LazyFrame:
+    import polars as pl
+
     return pl.LazyFrame(obj)
 
 
 def duckdb_lazy_constructor(obj: dict[str, list[Any]]) -> duckdb.DuckDBPyRelation:
     import duckdb
+    import polars as pl
 
     _df = pl.LazyFrame(obj)
     return duckdb.table("_df")
@@ -131,6 +142,8 @@ def dask_lazy_p2_constructor(obj: dict[str, list[Any]]) -> IntoFrame:  # pragma:
 
 
 def pyarrow_table_constructor(obj: dict[str, Any]) -> IntoDataFrame:
+    import pyarrow as pa
+
     return pa.table(obj)
 
 

tests/dependencies/is_into_dataframe_test.py

@@ -5,16 +5,16 @@
 from typing import Mapping
 
 import numpy as np
-import pandas as pd
-import polars as pl
-import pyarrow as pa
+import pytest
 
 import narwhals as nw
 from narwhals.stable.v1.dependencies import is_into_dataframe
 
 if TYPE_CHECKING:
     from typing_extensions import Self
 
+DATA: dict[str, Any] = {"a": [1, 2, 3], "b": [4, 5, 6]}
+
 
 class DictDataFrame:
     def __init__(self: Self, data: Mapping[str, Any]) -> None:
@@ -27,12 +27,23 @@ def __narwhals_dataframe__(self) -> Self:  # pragma: no cover
         return self
 
 
-def test_is_into_dataframe() -> None:
-    data: dict[str, Any] = {"a": [1, 2, 3], "b": [4, 5, 6]}
-    assert is_into_dataframe(pa.table(data))
-    assert is_into_dataframe(pl.DataFrame(data))
-    assert is_into_dataframe(pd.DataFrame(data))
-    assert is_into_dataframe(nw.from_native(pd.DataFrame(data)))
-    assert is_into_dataframe(DictDataFrame(data))
+def test_is_into_dataframe_pyarrow() -> None:
+    pa = pytest.importorskip("pyarrow")
+    assert is_into_dataframe(pa.table(DATA))
+
+
+def test_is_into_dataframe_polars() -> None:
+    pl = pytest.importorskip("polars")
+    assert is_into_dataframe(pl.DataFrame(DATA))
+
+
+def test_is_into_dataframe_pandas() -> None:
+    pd = pytest.importorskip("pandas")
+    assert is_into_dataframe(pd.DataFrame(DATA))
+    assert is_into_dataframe(nw.from_native(pd.DataFrame(DATA)))
+
+
+def test_is_into_dataframe_other() -> None:
+    assert is_into_dataframe(DictDataFrame(DATA))
     assert not is_into_dataframe(np.array([[1, 4], [2, 5], [3, 6]]))
-    assert not is_into_dataframe(data)
+    assert not is_into_dataframe(DATA)

tests/dependencies/is_into_series_test.py

@@ -4,9 +4,7 @@
 from typing import Any
 
 import numpy as np
-import pandas as pd
-import polars as pl
-import pyarrow as pa
+import pytest
 
 import narwhals as nw
 from narwhals.stable.v1.dependencies import is_into_series
@@ -27,11 +25,23 @@ def __narwhals_series__(self) -> Self:  # pragma: no cover
         return self
 
 
-def test_is_into_series() -> None:
+def test_is_into_series_pyarrow() -> None:
+    pa = pytest.importorskip("pyarrow")
     assert is_into_series(pa.chunked_array([["a", "b"]]))
+
+
+def test_is_into_series_polars() -> None:
+    pl = pytest.importorskip("polars")
     assert is_into_series(pl.Series([1, 2, 3]))
+
+
+def test_is_into_series_pandas() -> None:
+    pd = pytest.importorskip("pandas")
     assert is_into_series(pd.Series([1, 2, 3]))
     assert is_into_series(nw.from_native(pd.Series([1, 2, 3]), series_only=True))
+
+
+def test_is_into_series() -> None:
     assert is_into_series(ListBackedSeries("a", [1, 4, 2]))
     assert not is_into_series(np.array([1, 2, 3]))
     assert not is_into_series([1, 2, 3])

tests/dependencies/is_pandas_dataframe_test.py

@@ -1,11 +1,15 @@
 from __future__ import annotations
 
 import pandas as pd
-import polars as pl
+import pytest
 
 from narwhals.stable.v1.dependencies import is_pandas_dataframe
 
 
 def test_is_pandas_dataframe() -> None:
     assert is_pandas_dataframe(pd.DataFrame())
+
+
+def test_not_is_pandas_dataframe() -> None:
+    pl = pytest.importorskip("polars")
     assert not is_pandas_dataframe(pl.DataFrame())

tests/dtypes_test.py

@@ -8,7 +8,6 @@
 
 import numpy as np
 import pandas as pd
-import polars as pl
 import pyarrow as pa
 import pytest
 
@@ -222,6 +221,7 @@ def test_pandas_fixed_offset_1302() -> None:
 
 def test_huge_int() -> None:
     duckdb = pytest.importorskip("duckdb")
+    pl = pytest.importorskip("polars")
     df = pl.DataFrame({"a": [1, 2, 3]})
 
     if POLARS_VERSION >= (1, 18):
@@ -252,6 +252,7 @@ def test_huge_int() -> None:
 @pytest.mark.skipif(PANDAS_VERSION < (1, 5), reason="too old for pyarrow")
 def test_decimal() -> None:
     duckdb = pytest.importorskip("duckdb")
+    pl = pytest.importorskip("polars")
     df = pl.DataFrame({"a": [1]}, schema={"a": pl.Decimal})
     result = nw.from_native(df).schema
     assert result["a"] == nw.Decimal
@@ -325,6 +326,7 @@ def test_dtype_is_x() -> None:
 @pytest.mark.skipif(POLARS_VERSION < (1, 18), reason="too old for Int128")
 def test_huge_int_to_native() -> None:
     duckdb = pytest.importorskip("duckdb")
+    pl = pytest.importorskip("polars")
     df = pl.DataFrame({"a": [1, 2, 3]})
     df_casted = (
         nw.from_native(df).with_columns(a_int=nw.col("a").cast(nw.Int128())).to_native()
@@ -350,6 +352,7 @@ def test_huge_int_to_native() -> None:
 
 def test_cast_decimal_to_native() -> None:
     duckdb = pytest.importorskip("duckdb")
+    pl = pytest.importorskip("polars")
     data = {"a": [1, 2, 3]}
 
     df = pl.DataFrame(data)

tests/expr_and_series/cast_test.py

@@ -4,10 +4,8 @@
 from datetime import time
 from datetime import timedelta
 from datetime import timezone
-from typing import Any
 
 import pandas as pd
-import polars as pl
 import pytest
 
 import narwhals.stable.v1 as nw
@@ -312,11 +310,13 @@ def test_cast_struct(request: pytest.FixtureRequest, constructor: Constructor) -
     assert result.schema == {"a": dtype}
 
 
-@pytest.mark.parametrize("dtype", [pl.String, pl.String()])
-def test_raise_if_polars_dtype(constructor: Constructor, dtype: Any) -> None:
-    df = nw.from_native(constructor({"a": [1, 2, 3], "b": [4, 5, 6]}))
-    with pytest.raises(TypeError, match="Expected Narwhals dtype, got:"):
-        df.select(nw.col("a").cast(dtype))
+def test_raise_if_polars_dtype(constructor: Constructor) -> None:
+    pl = pytest.importorskip("polars")
+
+    for dtype in [pl.String, pl.String()]:
+        df = nw.from_native(constructor({"a": [1, 2, 3], "b": [4, 5, 6]}))
+        with pytest.raises(TypeError, match="Expected Narwhals dtype, got:"):
+            df.select(nw.col("a").cast(dtype))
 
 
 def test_cast_time(request: pytest.FixtureRequest, constructor: Constructor) -> None:

tests/expr_and_series/nth_test.py

@@ -3,7 +3,6 @@
 from typing import Any
 from typing import Mapping
 
-import polars as pl
 import pytest
 
 import narwhals.stable.v1 as nw
@@ -40,6 +39,7 @@ def test_nth(
     reason="1.0.0",
 )
 def test_nth_not_supported() -> None:  # pragma: no cover
+    pl = pytest.importorskip("polars")
     df = nw.from_native(pl.DataFrame(data))
     with pytest.raises(
         AttributeError, match="`nth` is only supported for Polars>=1.0.0."

tests/frame/arrow_c_stream_test.py

@@ -1,14 +1,15 @@
 from __future__ import annotations
 
-import polars as pl
-import pyarrow as pa
-import pyarrow.compute as pc
 import pytest
 
 import narwhals.stable.v1 as nw
 from tests.utils import POLARS_VERSION
 from tests.utils import PYARROW_VERSION
 
+pl = pytest.importorskip("polars")
+pa = pytest.importorskip("pyarrow")
+pc = pytest.importorskip("pyarrow.compute")
+
 
 @pytest.mark.skipif(POLARS_VERSION < (1, 3), reason="too old for pycapsule in Polars")
 @pytest.mark.skipif(

tests/frame/getitem_test.py

@@ -6,8 +6,6 @@
 
 import numpy as np
 import pandas as pd
-import polars as pl
-import pyarrow as pa
 import pytest
 
 import narwhals.stable.v1 as nw
@@ -47,6 +45,7 @@ def test_slice_rows_with_step(
 
 
 def test_slice_rows_with_step_pyarrow() -> None:
+    pa = pytest.importorskip("pyarrow")
     with pytest.raises(
         NotImplementedError,
         match="Slicing with step is not supported on PyArrow tables",
@@ -55,6 +54,7 @@ def test_slice_rows_with_step_pyarrow() -> None:
 
 
 def test_slice_lazy_fails() -> None:
+    pl = pytest.importorskip("polars")
     with pytest.raises(TypeError, match="Slicing is not supported on LazyFrame"):
         _ = nw.from_native(pl.LazyFrame(data))[1:]
 

tests/frame/interchange_native_namespace_test.py

@@ -3,11 +3,12 @@
 from typing import Any
 from typing import Mapping
 
-import polars as pl
 import pytest
 
 import narwhals.stable.v1 as nw
 
+pl = pytest.importorskip("polars")
+
 data: Mapping[str, Any] = {"a": [1, 2, 3], "b": [4.5, 6.7, 8.9], "z": ["x", "y", "w"]}