feat: add DuckDB list.len method (#1850)

* feat: add DuckDB `list.len` method

* change back constructors append

narwhals/_duckdb/expr.py

@@ -13,6 +13,7 @@
 from duckdb import FunctionExpression
 
 from narwhals._duckdb.expr_dt import DuckDBExprDateTimeNamespace
+from narwhals._duckdb.expr_list import DuckDBExprListNamespace
 from narwhals._duckdb.expr_name import DuckDBExprNameNamespace
 from narwhals._duckdb.expr_str import DuckDBExprStringNamespace
 from narwhals._duckdb.utils import binary_operation_returns_scalar
@@ -571,3 +572,7 @@ def dt(self: Self) -> DuckDBExprDateTimeNamespace:
     @property
     def name(self: Self) -> DuckDBExprNameNamespace:
         return DuckDBExprNameNamespace(self)
+
+    @property
+    def list(self: Self) -> DuckDBExprListNamespace:
+        return DuckDBExprListNamespace(self)

narwhals/_duckdb/expr_list.py

@@ -0,0 +1,20 @@
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+from duckdb import FunctionExpression
+
+if TYPE_CHECKING:
+    from narwhals._duckdb.expr import DuckDBExpr
+
+
+class DuckDBExprListNamespace:
+    def __init__(self, expr: DuckDBExpr) -> None:
+        self._compliant_expr = expr
+
+    def len(self) -> DuckDBExpr:
+        return self._compliant_expr._from_call(
+            lambda _input: FunctionExpression("len", _input),
+            "len",
+            returns_scalar=self._compliant_expr._returns_scalar,
+        )

narwhals/_duckdb/utils.py

@@ -195,8 +195,8 @@ def narwhals_to_native_dtype(dtype: DType | type[DType], version: Version) -> st
     if isinstance_or_issubclass(dtype, dtypes.Date):  # pragma: no cover
         return "DATE"
     if isinstance_or_issubclass(dtype, dtypes.List):
-        msg = "todo"
-        raise NotImplementedError(msg)
+        inner = narwhals_to_native_dtype(dtype.inner, version)  # type: ignore[union-attr]
+        return f"{inner}[]"
     if isinstance_or_issubclass(dtype, dtypes.Struct):  # pragma: no cover
         msg = "todo"
         raise NotImplementedError(msg)

tests/conftest.py

@@ -242,13 +242,9 @@ def pytest_generate_tests(metafunc: pytest.Metafunc) -> None:
         )
     elif "constructor" in metafunc.fixturenames:
         if (
-            any(
-                x in str(metafunc.module)
-                for x in ("list", "unpivot", "from_dict", "from_numpy")
-            )
+            any(x in str(metafunc.module) for x in ("unpivot", "from_dict", "from_numpy"))
             and LAZY_CONSTRUCTORS["duckdb"] in constructors
         ):
-            # TODO(unassigned): list and name namespaces still need implementing for duckdb
             constructors.remove(LAZY_CONSTRUCTORS["duckdb"])
             constructors_ids.remove("duckdb")
         metafunc.parametrize("constructor", constructors, ids=constructors_ids)