feat: DuckDB dt.to_string (#1794)

---------

Co-authored-by: Marco Edward Gorelli <[email protected]>

narwhals/_duckdb/expr_dt.py

@@ -94,6 +94,18 @@ def nanosecond(self) -> DuckDBExpr:
             returns_scalar=self._compliant_expr._returns_scalar,
         )
 
+    def to_string(self, format: str) -> DuckDBExpr:  # noqa: A002
+        from duckdb import ConstantExpression
+        from duckdb import FunctionExpression
+
+        return self._compliant_expr._from_call(
+            lambda _input: FunctionExpression(
+                "strftime", _input, ConstantExpression(format)
+            ),
+            "to_string",
+            returns_scalar=self._compliant_expr._returns_scalar,
+        )
+
     def weekday(self) -> DuckDBExpr:
         from duckdb import FunctionExpression
 

tests/expr_and_series/dt/to_string_test.py

@@ -62,7 +62,7 @@ def test_dt_to_string_series(constructor_eager: ConstructorEager, fmt: str) -> N
 def test_dt_to_string_expr(
     constructor: Constructor, fmt: str, request: pytest.FixtureRequest
 ) -> None:
-    if ("pyspark" in str(constructor)) or "duckdb" in str(constructor):
+    if "pyspark" in str(constructor):
         request.applymarker(pytest.mark.xfail)
     input_frame = nw.from_native(constructor(data))
 
@@ -180,7 +180,7 @@ def test_dt_to_string_iso_local_date_expr(
     expected: str,
     request: pytest.FixtureRequest,
 ) -> None:
-    if ("pyspark" in str(constructor)) or "duckdb" in str(constructor):
+    if "pyspark" in str(constructor):
         request.applymarker(pytest.mark.xfail)
     df = constructor({"a": [data]})
     result = nw.from_native(df).with_columns(