feat: added dask Expr.__sub__ (#643)

* dask dunder sub

* added sub test cases in test_with_columns

* resolving

narwhals/_dask/expr.py

@@ -137,6 +137,13 @@ def __add__(self, other: Any) -> Self:
             other,
         )
 
+    def __sub__(self, other: Any) -> Self:
+        return self._from_call(
+            lambda _input, other: _input.__sub__(other),
+            "__sub__",
+            other,
+        )
+
     def __mul__(self, other: Any) -> Self:
         return self._from_call(
             lambda _input, other: _input.__mul__(other),

narwhals/dependencies.py

@@ -4,6 +4,7 @@
 from __future__ import annotations
 
 import sys
+import warnings
 from typing import TYPE_CHECKING
 from typing import Any
 
@@ -72,7 +73,17 @@ def get_dask() -> Any:
 
 def get_dask_dataframe() -> Any:
     """Get dask.dataframe module (if already imported - else return None)."""
-    return sys.modules.get("dask.dataframe", None)
+    if "dask" in sys.modules:
+        with warnings.catch_warnings():
+            warnings.filterwarnings(
+                "ignore",
+                message="\nDask dataframe query planning.*",
+                category=FutureWarning,
+            )
+            import dask.dataframe as dd
+
+        return dd
+    return None  # pragma: no cover
 
 
 def get_dask_expr() -> Any:

tests/dask_test.py

@@ -42,6 +42,8 @@ def test_with_columns() -> None:
         (nw.col("a") + nw.col("b").mean()).alias("c"),
         d=nw.col("a"),
         e=nw.col("a") + nw.col("b"),
+        f=nw.col("b") - 1,
+        g=nw.col("a") - nw.col("b"),
         h=nw.col("a") * 3,
         i=nw.col("a") * nw.col("b"),
     )
@@ -55,6 +57,8 @@ def test_with_columns() -> None:
             "c": [6.0, 7.0, 8.0],
             "d": [1, 2, 3],
             "e": [5, 7, 9],
+            "f": [3, 4, 5],
+            "g": [-3, -3, -3],
             "h": [3, 6, 9],
             "i": [4, 10, 18],
         },

tests/expr_and_series/gather_every_test.py

@@ -21,8 +21,8 @@ def test_gather_every_expr(constructor_with_lazy: Any, n: int, offset: int) -> N
 
 @pytest.mark.parametrize("n", [1, 2, 3])
 @pytest.mark.parametrize("offset", [1, 2, 3])
-def test_gather_every_series(constructor: Any, n: int, offset: int) -> None:
-    series = nw.from_native(constructor(data), eager_only=True)["a"]
+def test_gather_every_series(constructor_series: Any, n: int, offset: int) -> None:
+    series = nw.from_native(constructor_series(data["a"]), series_only=True)
 
     result = series.gather_every(n=n, offset=offset)
     expected = data["a"][offset::n]