add dask null-count

narwhals/_dask/expr.py

@@ -538,6 +538,13 @@ def func(_input: Any) -> Any:
             returns_scalar=False,
         )
 
+    def null_count(self: Self) -> Self:
+        return self._from_call(
+            lambda _input: _input.isna().sum(),
+            "null_count",
+            returns_scalar=True,
+        )
+
     def tail(self: Self) -> NoReturn:
         # We can't (yet?) allow methods which modify the index
         msg = "`Expr.tail` is not supported for the Dask backend. Please use `LazyFrame.tail` instead."

tests/expr_and_series/null_count_test.py

@@ -1,7 +1,5 @@
 from typing import Any
 
-import pytest
-
 import narwhals.stable.v1 as nw
 from tests.utils import compare_dicts
 
@@ -11,9 +9,7 @@
 }
 
 
-def test_null_count_expr(constructor: Any, request: Any) -> None:
-    if "dask" in str(constructor):
-        request.applymarker(pytest.mark.xfail)
+def test_null_count_expr(constructor: Any) -> None:
     df = nw.from_native(constructor(data))
     result = df.select(nw.all().null_count())
     expected = {