feat: dask namespace lit (#772)

narwhals/_dask/namespace.py

@@ -68,6 +68,19 @@ def col(self, *column_names: str) -> DaskExpr:
             backend_version=self._backend_version,
         )
 
+    def lit(self, value: Any, dtype: dtypes.DType | None) -> DaskExpr:
+        # TODO @FBruzzesi: cast to dtype once `reverse_translate_dtype` is implemented.
+        # It should be enough to add `.astype(reverse_translate_dtype(dtype))`
+        return DaskExpr(
+            lambda df: [df._native_dataframe.assign(lit=value).loc[:, "lit"]],
+            depth=0,
+            function_name="lit",
+            root_names=None,
+            output_names=["lit"],
+            returns_scalar=False,
+            backend_version=self._backend_version,
+        )
+
     def min(self, *column_names: str) -> DaskExpr:
         return DaskExpr.from_column_names(
             *column_names,

tests/frame/lit_test.py

@@ -20,7 +20,7 @@
 def test_lit(
     constructor: Any, dtype: DType | None, expected_lit: list[Any], request: Any
 ) -> None:
-    if "dask" in str(constructor):
+    if "dask" in str(constructor) and dtype == nw.String:
         request.applymarker(pytest.mark.xfail)
     data = {"a": [1, 3, 2], "b": [4, 4, 6], "z": [7.0, 8, 9]}
     df_raw = constructor(data)