feat: add duckdb dataframe drop_nulls

narwhals/_arrow/dataframe.py

@@ -395,10 +395,9 @@ def drop(self: Self, columns: list[str], strict: bool) -> Self:  # noqa: FBT001
         )
         return self._from_native_frame(self._native_frame.drop(to_drop))
 
-    def drop_nulls(self: Self, subset: str | list[str] | None) -> Self:
+    def drop_nulls(self: Self, subset: list[str] | None) -> Self:
         if subset is None:
             return self._from_native_frame(self._native_frame.drop_null())
-        subset = [subset] if isinstance(subset, str) else subset
         plx = self.__narwhals_namespace__()
         return self.filter(~plx.any_horizontal(plx.col(*subset).is_null()))
 

narwhals/_dask/dataframe.py

@@ -149,10 +149,9 @@ def select(
         )
         return self._from_native_frame(df)
 
-    def drop_nulls(self: Self, subset: str | list[str] | None) -> Self:
+    def drop_nulls(self: Self, subset: list[str] | None) -> Self:
         if subset is None:
             return self._from_native_frame(self._native_frame.dropna())
-        subset = [subset] if isinstance(subset, str) else subset
         plx = self.__narwhals_namespace__()
         return self.filter(~plx.any_horizontal(plx.col(*subset).is_null()))
 

narwhals/_duckdb/dataframe.py

@@ -322,17 +322,11 @@ def sort(
         )
         return self._from_native_frame(result)
 
-    def drop_nulls(self: Self, subset: str | list[str] | None) -> Self:
+    def drop_nulls(self: Self, subset: list[str] | None) -> Self:
         import duckdb
 
         rel = self._native_frame
-        subset_ = (
-            [subset]
-            if isinstance(subset, str)
-            else rel.columns
-            if subset is None
-            else subset
-        )
+        subset_ = subset if subset is not None else rel.columns
         keep_condition = " and ".join(f'"{col}" is not null' for col in subset_)
         query = f"select * from rel where {keep_condition}"  # noqa: S608
         return self._from_native_frame(duckdb.sql(query))

narwhals/_pandas_like/dataframe.py

@@ -371,10 +371,9 @@ def select(
         )
         return self._from_native_frame(df)
 
-    def drop_nulls(self, subset: str | list[str] | None) -> Self:
+    def drop_nulls(self, subset: list[str] | None) -> Self:
         if subset is None:
             return self._from_native_frame(self._native_frame.dropna(axis=0))
-        subset = [subset] if isinstance(subset, str) else subset
         plx = self.__narwhals_namespace__()
         return self.filter(~plx.any_horizontal(plx.col(*subset).is_null()))
 

narwhals/_spark_like/dataframe.py

@@ -183,7 +183,7 @@ def sort(
         sort_cols = [sort_f(col) for col, sort_f in zip(flat_by, sort_funcs)]
         return self._from_native_frame(self._native_frame.sort(*sort_cols))
 
-    def drop_nulls(self: Self, subset: str | list[str] | None) -> Self:
+    def drop_nulls(self: Self, subset: list[str] | None) -> Self:
         return self._from_native_frame(self._native_frame.dropna(subset=subset))
 
     def rename(self: Self, mapping: dict[str, str]) -> Self:

narwhals/dataframe.py

@@ -107,7 +107,8 @@ def with_row_index(self, name: str = "index") -> Self:
             self._compliant_frame.with_row_index(name),
         )
 
-    def drop_nulls(self: Self, subset: str | list[str] | None = None) -> Self:
+    def drop_nulls(self: Self, subset: str | list[str] | None) -> Self:
+        subset = [subset] if isinstance(subset, str) else subset
         return self._from_compliant_dataframe(
             self._compliant_frame.drop_nulls(subset=subset),
         )