feat: implement when/then/otherwise for DuckDB

narwhals/_duckdb/expr.py

@@ -31,7 +31,7 @@ class DuckDBExpr(CompliantExpr["duckdb.Expression"]):
 
     def __init__(
         self,
-        call: Callable[[DuckDBLazyFrame], list[duckdb.Expression]],
+        call: Callable[[DuckDBLazyFrame], Sequence[duckdb.Expression]],
         *,
         depth: int,
         function_name: str,

narwhals/_duckdb/namespace.py

@@ -7,6 +7,7 @@
 from typing import Any
 from typing import Literal
 from typing import Sequence
+from typing import cast
 
 from narwhals._duckdb.expr import DuckDBExpr
 from narwhals._duckdb.utils import narwhals_to_native_dtype
@@ -157,6 +158,16 @@ def func(df: DuckDBLazyFrame) -> list[duckdb.Expression]:
             kwargs={"exprs": exprs},
         )
 
+    def when(
+        self,
+        *predicates: IntoDuckDBExpr,
+    ) -> DuckDBWhen:
+        plx = self.__class__(backend_version=self._backend_version, version=self._version)
+        condition = plx.all_horizontal(*predicates)
+        return DuckDBWhen(
+            condition, self._backend_version, returns_scalar=False, version=self._version
+        )
+
     def col(self, *column_names: str) -> DuckDBExpr:
         return DuckDBExpr.from_column_names(
             *column_names, backend_version=self._backend_version, version=self._version
@@ -203,3 +214,101 @@ def func(_df: DuckDBLazyFrame) -> list[duckdb.Expression]:
             version=self._version,
             kwargs={},
         )
+
+
+class DuckDBWhen:
+    def __init__(
+        self,
+        condition: DuckDBExpr,
+        backend_version: tuple[int, ...],
+        then_value: Any = None,
+        otherwise_value: Any = None,
+        *,
+        returns_scalar: bool,
+        version: Version,
+    ) -> None:
+        self._backend_version = backend_version
+        self._condition = condition
+        self._then_value = then_value
+        self._otherwise_value = otherwise_value
+        self._returns_scalar = returns_scalar
+        self._version = version
+
+    def __call__(self, df: DuckDBLazyFrame) -> Sequence[duckdb.Expression]:
+        from duckdb import CaseExpression
+        from duckdb import ConstantExpression
+
+        from narwhals._expression_parsing import parse_into_expr
+
+        plx = df.__narwhals_namespace__()
+        condition = parse_into_expr(self._condition, namespace=plx)(df)[0]
+        condition = cast("duckdb.Expression", condition)
+
+        try:
+            value = parse_into_expr(self._then_value, namespace=plx)(df)[0]
+        except TypeError:
+            # `self._otherwise_value` is a scalar and can't be converted to an expression
+            value = ConstantExpression(self._then_value)
+        value = cast("duckdb.Expression", value)
+
+        if self._otherwise_value is None:
+            return [CaseExpression(condition=condition, value=value)]
+        try:
+            otherwise_expr = parse_into_expr(self._otherwise_value, namespace=plx)
+        except TypeError:
+            # `self._otherwise_value` is a scalar and can't be converted to an expression
+            return [
+                CaseExpression(condition=condition, value=value).otherwise(
+                    ConstantExpression(self._otherwise_value)
+                )
+            ]
+        otherwise = otherwise_expr(df)[0]
+        return [CaseExpression(condition=condition, value=value).otherwise(otherwise)]
+
+    def then(self, value: DuckDBExpr | Any) -> DuckDBThen:
+        self._then_value = value
+
+        return DuckDBThen(
+            self,
+            depth=0,
+            function_name="whenthen",
+            root_names=None,
+            output_names=None,
+            returns_scalar=self._returns_scalar,
+            backend_version=self._backend_version,
+            version=self._version,
+            kwargs={"value": value},
+        )
+
+
+class DuckDBThen(DuckDBExpr):
+    def __init__(
+        self,
+        call: DuckDBWhen,
+        *,
+        depth: int,
+        function_name: str,
+        root_names: list[str] | None,
+        output_names: list[str] | None,
+        returns_scalar: bool,
+        backend_version: tuple[int, ...],
+        version: Version,
+        kwargs: dict[str, Any],
+    ) -> None:
+        self._backend_version = backend_version
+        self._version = version
+        self._call = call
+        self._depth = depth
+        self._function_name = function_name
+        self._root_names = root_names
+        self._output_names = output_names
+        self._returns_scalar = returns_scalar
+        self._kwargs = kwargs
+
+    def otherwise(self, value: DuckDBExpr | Any) -> DuckDBExpr:
+        # type ignore because we are setting the `_call` attribute to a
+        # callable object of type `DuckDBWhen`, base class has the attribute as
+        # only a `Callable`
+        self._call._otherwise_value = value  # type: ignore[attr-defined]
+        self._function_name = "whenotherwise"
+        return self

narwhals/_duckdb/utils.py

@@ -4,6 +4,7 @@
 from functools import lru_cache
 from typing import TYPE_CHECKING
 from typing import Any
+from typing import Sequence
 
 from narwhals.dtypes import DType
 from narwhals.exceptions import InvalidIntoExprError
@@ -76,7 +77,7 @@ def parse_exprs_and_named_exprs(
 
 def _columns_from_expr(
     df: DuckDBLazyFrame, expr: IntoDuckDBExpr
-) -> list[duckdb.Expression]:
+) -> Sequence[duckdb.Expression]:
     if isinstance(expr, str):  # pragma: no cover
         from duckdb import ColumnExpression
 

tests/expr_and_series/when_test.py

@@ -17,9 +17,7 @@
 }
 
 
-def test_when(constructor: Constructor, request: pytest.FixtureRequest) -> None:
-    if "duckdb" in str(constructor):
-        request.applymarker(pytest.mark.xfail)
+def test_when(constructor: Constructor) -> None:
     df = nw.from_native(constructor(data))
     result = df.select(nw.when(nw.col("a") == 1).then(value=3).alias("a_when"))
     expected = {
@@ -28,9 +26,7 @@ def test_when(constructor: Constructor, request: pytest.FixtureRequest) -> None:
     assert_equal_data(result, expected)
 
 
-def test_when_otherwise(constructor: Constructor, request: pytest.FixtureRequest) -> None:
-    if "duckdb" in str(constructor):
-        request.applymarker(pytest.mark.xfail)
+def test_when_otherwise(constructor: Constructor) -> None:
     df = nw.from_native(constructor(data))
     result = df.select(nw.when(nw.col("a") == 1).then(3).otherwise(6).alias("a_when"))
     expected = {
@@ -39,11 +35,7 @@ def test_when_otherwise(constructor: Constructor, request: pytest.FixtureRequest
     assert_equal_data(result, expected)
 
 
-def test_multiple_conditions(
-    constructor: Constructor, request: pytest.FixtureRequest
-) -> None:
-    if "duckdb" in str(constructor):
-        request.applymarker(pytest.mark.xfail)
+def test_multiple_conditions(constructor: Constructor) -> None:
     df = nw.from_native(constructor(data))
     result = df.select(
         nw.when(nw.col("a") < 3, nw.col("c") < 5.0).then(3).alias("a_when")
@@ -85,11 +77,7 @@ def test_value_series(constructor_eager: ConstructorEager) -> None:
     assert_equal_data(result, expected)
 
 
-def test_value_expression(
-    constructor: Constructor, request: pytest.FixtureRequest
-) -> None:
-    if "duckdb" in str(constructor):
-        request.applymarker(pytest.mark.xfail)
+def test_value_expression(constructor: Constructor) -> None:
     df = nw.from_native(constructor(data))
     result = df.select(nw.when(nw.col("a") == 1).then(nw.col("a") + 9).alias("a_when"))
     expected = {
@@ -122,11 +110,7 @@ def test_otherwise_series(constructor_eager: ConstructorEager) -> None:
     assert_equal_data(result, expected)
 
 
-def test_otherwise_expression(
-    constructor: Constructor, request: pytest.FixtureRequest
-) -> None:
-    if "duckdb" in str(constructor):
-        request.applymarker(pytest.mark.xfail)
+def test_otherwise_expression(constructor: Constructor) -> None:
     df = nw.from_native(constructor(data))
     result = df.select(
         nw.when(nw.col("a") == 1).then(-1).otherwise(nw.col("a") + 7).alias("a_when")

tpch/execute.py

@@ -5,6 +5,7 @@
 from pathlib import Path
 
 import dask.dataframe as dd
+import duckdb
 import pandas as pd
 import polars as pl
 import pyarrow as pa
@@ -29,14 +30,18 @@
     "pandas[pyarrow]": (pd, {"engine": "pyarrow", "dtype_backend": "pyarrow"}),
     "polars[lazy]": (pl, {}),
     "pyarrow": (pa, {}),
+    "duckdb": (duckdb, {}),
     "dask": (dd, {"engine": "pyarrow", "dtype_backend": "pyarrow"}),
 }
 
 BACKEND_COLLECT_FUNC_MAP = {
     "polars[lazy]": lambda x: x.collect(),
+    "duckdb": lambda x: x.pl(),
     "dask": lambda x: x.compute(),
 }
 
+DUCKDB_XFAILS = ["q11", "q14", "q15", "q16", "q18", "q22"]
+
 QUERY_DATA_PATH_MAP = {
     "q1": (LINEITEM_PATH,),
     "q2": (REGION_PATH, NATION_PATH, SUPPLIER_PATH, PART_PATH, PARTSUPP_PATH),
@@ -90,6 +95,10 @@ def execute_query(query_id: str) -> None:
     data_paths = QUERY_DATA_PATH_MAP[query_id]
 
     for backend, (native_namespace, kwargs) in BACKEND_NAMESPACE_KWARGS_MAP.items():
+        if backend == "duckdb" and query_id in DUCKDB_XFAILS:
+            print(f"\nSkipping {query_id} for DuckDB")  # noqa: T201
+            continue
+
         print(f"\nRunning {query_id} with {backend=}")  # noqa: T201
         result = query_module.query(
             *(