feat: support std and var with ddof !=1 in pandas-like group by (#1645)

---------

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

narwhals/_arrow/expr.py

@@ -308,7 +308,7 @@ def alias(self: Self, name: str) -> Self:
             output_names=[name],
             backend_version=self._backend_version,
             version=self._version,
-            kwargs={"name": name},
+            kwargs={**self._kwargs, "name": name},
         )
 
     def null_count(self: Self) -> Self:
@@ -436,7 +436,7 @@ def func(df: ArrowDataFrame) -> list[ArrowSeries]:
             output_names=self._output_names,
             backend_version=self._backend_version,
             version=self._version,
-            kwargs={"keys": keys},
+            kwargs={**self._kwargs, "keys": keys},
         )
 
     def mode(self: Self) -> Self:
@@ -478,7 +478,7 @@ def func(df: ArrowDataFrame) -> list[ArrowSeries]:
             output_names=self._output_names,
             backend_version=self._backend_version,
             version=self._version,
-            kwargs={"function": function, "return_dtype": return_dtype},
+            kwargs={**self._kwargs, "function": function, "return_dtype": return_dtype},
         )
 
     def is_finite(self: Self) -> Self:
@@ -810,7 +810,7 @@ def keep(self: Self) -> ArrowExpr:
             output_names=root_names,
             backend_version=self._compliant_expr._backend_version,
             version=self._compliant_expr._version,
-            kwargs={},
+            kwargs=self._compliant_expr._kwargs,
         )
 
     def map(self: Self, function: Callable[[str], str]) -> ArrowExpr:
@@ -837,7 +837,7 @@ def map(self: Self, function: Callable[[str], str]) -> ArrowExpr:
             output_names=output_names,
             backend_version=self._compliant_expr._backend_version,
             version=self._compliant_expr._version,
-            kwargs={"function": function},
+            kwargs={**self._compliant_expr._kwargs, "function": function},
         )
 
     def prefix(self: Self, prefix: str) -> ArrowExpr:
@@ -862,7 +862,7 @@ def prefix(self: Self, prefix: str) -> ArrowExpr:
             output_names=output_names,
             backend_version=self._compliant_expr._backend_version,
             version=self._compliant_expr._version,
-            kwargs={"prefix": prefix},
+            kwargs={**self._compliant_expr._kwargs, "prefix": prefix},
         )
 
     def suffix(self: Self, suffix: str) -> ArrowExpr:
@@ -888,7 +888,7 @@ def suffix(self: Self, suffix: str) -> ArrowExpr:
             output_names=output_names,
             backend_version=self._compliant_expr._backend_version,
             version=self._compliant_expr._version,
-            kwargs={"suffix": suffix},
+            kwargs={**self._compliant_expr._kwargs, "suffix": suffix},
         )
 
     def to_lowercase(self: Self) -> ArrowExpr:
@@ -914,7 +914,7 @@ def to_lowercase(self: Self) -> ArrowExpr:
             output_names=output_names,
             backend_version=self._compliant_expr._backend_version,
             version=self._compliant_expr._version,
-            kwargs={},
+            kwargs=self._compliant_expr._kwargs,
         )
 
     def to_uppercase(self: Self) -> ArrowExpr:
@@ -940,7 +940,7 @@ def to_uppercase(self: Self) -> ArrowExpr:
             output_names=output_names,
             backend_version=self._compliant_expr._backend_version,
             version=self._compliant_expr._version,
-            kwargs={},
+            kwargs=self._compliant_expr._kwargs,
         )
 
 

narwhals/_arrow/namespace.py

@@ -432,7 +432,12 @@ def func(df: ArrowDataFrame) -> list[ArrowSeries]:
             function_name="concat_str",
             root_names=combine_root_names(parsed_exprs),
             output_names=reduce_output_names(parsed_exprs),
-            kwargs={"separator": separator, "ignore_nulls": ignore_nulls},
+            kwargs={
+                "exprs": exprs,
+                "more_exprs": more_exprs,
+                "separator": separator,
+                "ignore_nulls": ignore_nulls,
+            },
         )
 
 

narwhals/_arrow/selectors.py

@@ -124,7 +124,7 @@ def call(df: ArrowDataFrame) -> list[ArrowSeries]:
                 output_names=None,
                 backend_version=self._backend_version,
                 version=self._version,
-                kwargs={"other": other},
+                kwargs={**self._kwargs, "other": other},
             )
         else:
             return self._to_expr() - other
@@ -145,7 +145,7 @@ def call(df: ArrowDataFrame) -> Sequence[ArrowSeries]:
                 output_names=None,
                 backend_version=self._backend_version,
                 version=self._version,
-                kwargs={"other": other},
+                kwargs={**self._kwargs, "other": other},
             )
         else:
             return self._to_expr() | other
@@ -166,7 +166,7 @@ def call(df: ArrowDataFrame) -> list[ArrowSeries]:
                 output_names=None,
                 backend_version=self._backend_version,
                 version=self._version,
-                kwargs={"other": other},
+                kwargs={**self._kwargs, "other": other},
             )
         else:
             return self._to_expr() & other

narwhals/_dask/expr.py

@@ -177,7 +177,7 @@ def func(df: DaskLazyFrame) -> list[dask_expr.Series]:
             returns_scalar=self._returns_scalar or returns_scalar,
             backend_version=self._backend_version,
             version=self._version,
-            kwargs=kwargs,
+            kwargs={**self._kwargs, **kwargs},
         )
 
     def alias(self, name: str) -> Self:
@@ -194,7 +194,7 @@ def func(df: DaskLazyFrame) -> list[dask_expr.Series]:
             returns_scalar=self._returns_scalar,
             backend_version=self._backend_version,
             version=self._version,
-            kwargs={"name": name},
+            kwargs={**self._kwargs, "name": name},
         )
 
     def __add__(self, other: Any) -> Self:
@@ -847,7 +847,7 @@ def func(df: DaskLazyFrame) -> list[Any]:
             returns_scalar=False,
             backend_version=self._backend_version,
             version=self._version,
-            kwargs={"keys": keys},
+            kwargs={**self._kwargs, "keys": keys},
         )
 
     def mode(self: Self) -> Self:
@@ -1334,7 +1334,7 @@ def keep(self: Self) -> DaskExpr:
             returns_scalar=self._compliant_expr._returns_scalar,
             backend_version=self._compliant_expr._backend_version,
             version=self._compliant_expr._version,
-            kwargs={},
+            kwargs=self._compliant_expr._kwargs,
         )
 
     def map(self: Self, function: Callable[[str], str]) -> DaskExpr:
@@ -1362,7 +1362,7 @@ def map(self: Self, function: Callable[[str], str]) -> DaskExpr:
             returns_scalar=self._compliant_expr._returns_scalar,
             backend_version=self._compliant_expr._backend_version,
             version=self._compliant_expr._version,
-            kwargs={"function": function},
+            kwargs={**self._compliant_expr._kwargs, "function": function},
         )
 
     def prefix(self: Self, prefix: str) -> DaskExpr:
@@ -1388,7 +1388,7 @@ def prefix(self: Self, prefix: str) -> DaskExpr:
             returns_scalar=self._compliant_expr._returns_scalar,
             backend_version=self._compliant_expr._backend_version,
             version=self._compliant_expr._version,
-            kwargs={"prefix": prefix},
+            kwargs={**self._compliant_expr._kwargs, "prefix": prefix},
         )
 
     def suffix(self: Self, suffix: str) -> DaskExpr:
@@ -1415,7 +1415,7 @@ def suffix(self: Self, suffix: str) -> DaskExpr:
             returns_scalar=self._compliant_expr._returns_scalar,
             backend_version=self._compliant_expr._backend_version,
             version=self._compliant_expr._version,
-            kwargs={"suffix": suffix},
+            kwargs={**self._compliant_expr._kwargs, "suffix": suffix},
         )
 
     def to_lowercase(self: Self) -> DaskExpr:
@@ -1442,7 +1442,7 @@ def to_lowercase(self: Self) -> DaskExpr:
             returns_scalar=self._compliant_expr._returns_scalar,
             backend_version=self._compliant_expr._backend_version,
             version=self._compliant_expr._version,
-            kwargs={},
+            kwargs=self._compliant_expr._kwargs,
         )
 
     def to_uppercase(self: Self) -> DaskExpr:
@@ -1469,5 +1469,5 @@ def to_uppercase(self: Self) -> DaskExpr:
             returns_scalar=self._compliant_expr._returns_scalar,
             backend_version=self._compliant_expr._backend_version,
             version=self._compliant_expr._version,
-            kwargs={},
+            kwargs=self._compliant_expr._kwargs,
         )

narwhals/_dask/namespace.py

@@ -159,7 +159,7 @@ def func(df: DaskLazyFrame) -> list[dask_expr.Series]:
             returns_scalar=False,
             backend_version=self._backend_version,
             version=self._version,
-            kwargs={},
+            kwargs={"exprs": exprs},
         )
 
     def any_horizontal(self, *exprs: IntoDaskExpr) -> DaskExpr:
@@ -178,7 +178,7 @@ def func(df: DaskLazyFrame) -> list[dask_expr.Series]:
             returns_scalar=False,
             backend_version=self._backend_version,
             version=self._version,
-            kwargs={},
+            kwargs={"exprs": exprs},
         )
 
     def sum_horizontal(self, *exprs: IntoDaskExpr) -> DaskExpr:
@@ -197,7 +197,7 @@ def func(df: DaskLazyFrame) -> list[dask_expr.Series]:
             returns_scalar=False,
             backend_version=self._backend_version,
             version=self._version,
-            kwargs={},
+            kwargs={"exprs": exprs},
         )
 
     def concat(
@@ -279,7 +279,7 @@ def func(df: DaskLazyFrame) -> list[dask_expr.Series]:
             returns_scalar=False,
             backend_version=self._backend_version,
             version=self._version,
-            kwargs={},
+            kwargs={"exprs": exprs},
         )
 
     def min_horizontal(self, *exprs: IntoDaskExpr) -> DaskExpr:
@@ -301,7 +301,7 @@ def func(df: DaskLazyFrame) -> list[dask_expr.Series]:
             returns_scalar=False,
             backend_version=self._backend_version,
             version=self._version,
-            kwargs={},
+            kwargs={"exprs": exprs},
         )
 
     def max_horizontal(self, *exprs: IntoDaskExpr) -> DaskExpr:
@@ -323,7 +323,7 @@ def func(df: DaskLazyFrame) -> list[dask_expr.Series]:
             returns_scalar=False,
             backend_version=self._backend_version,
             version=self._version,
-            kwargs={},
+            kwargs={"exprs": exprs},
         )
 
     def when(
@@ -388,7 +388,12 @@ def func(df: DaskLazyFrame) -> list[dask_expr.Series]:
             returns_scalar=False,
             backend_version=self._backend_version,
             version=self._version,
-            kwargs={},
+            kwargs={
+                "exprs": exprs,
+                "more_exprs": more_exprs,
+                "separator": separator,
+                "ignore_nulls": ignore_nulls,
+            },
         )
 
 
@@ -451,7 +456,7 @@ def then(self, value: DaskExpr | Any) -> DaskThen:
             returns_scalar=self._returns_scalar,
             backend_version=self._backend_version,
             version=self._version,
-            kwargs={},
+            kwargs={"value": value},
         )
 
 

narwhals/_expression_parsing.py

@@ -225,14 +225,13 @@ def func(df: CompliantDataFrame) -> Sequence[CompliantSeries]:
     ):  # pragma: no cover
         msg = "Safety assertion failed, please report a bug to https://github.com/narwhals-dev/narwhals/issues"
         raise AssertionError(msg)
-
     return plx._create_expr_from_callable(  # type: ignore[return-value]
         func,  # type: ignore[arg-type]
         depth=expr._depth + 1,
         function_name=f"{expr._function_name}->{attr}",
         root_names=root_names,
         output_names=output_names,
-        kwargs=kwargs,
+        kwargs={**expr._kwargs, **kwargs},
     )
 
 
@@ -272,7 +271,7 @@ def reuse_series_namespace_implementation(
         function_name=f"{expr._function_name}->{series_namespace}.{attr}",
         root_names=expr._root_names,
         output_names=expr._output_names,
-        kwargs=kwargs,
+        kwargs={**expr._kwargs, **kwargs},
     )