add selector support to "to_dummies" frame method

py-polars/polars/dataframe/frame.py

@@ -7949,7 +7949,7 @@ def quantile(
 
     def to_dummies(
         self,
-        columns: str | Sequence[str] | None = None,
+        columns: str | Sequence[str] | SelectorType | None = None,
         *,
         separator: str = "_",
         drop_first: bool = False,
@@ -7988,8 +7988,8 @@ def to_dummies(
         └───────┴───────┴───────┴───────┴───────┴───────┘
 
         """
-        if isinstance(columns, str):
-            columns = [columns]
+        if columns is not None:
+            columns = expand_selectors(self, columns)
         return self._from_pydf(self._df.to_dummies(columns, separator, drop_first))
 
     def unique(

py-polars/tests/unit/dataframe/test_df.py

@@ -805,7 +805,10 @@ def test_to_dummies() -> None:
     assert_frame_equal(res, expected)
 
     df = pl.DataFrame(
-        {"i": [1, 2, 3], "category": ["dog", "cat", "cat"]},
+        {
+            "i": [1, 2, 3],
+            "category": ["dog", "cat", "cat"],
+        },
         schema={"i": pl.Int32, "category": pl.Categorical},
     )
     expected = pl.DataFrame(
@@ -816,8 +819,9 @@ def test_to_dummies() -> None:
         },
         schema={"i": pl.Int32, "category|cat": pl.UInt8, "category|dog": pl.UInt8},
     )
-    result = df.to_dummies(columns=["category"], separator="|")
-    assert_frame_equal(result, expected)
+    for _cols in ("category", cs.string()):
+        result = df.to_dummies(columns=["category"], separator="|")
+        assert_frame_equal(result, expected)
 
     # test sorted fast path
     assert pl.DataFrame({"x": pl.arange(0, 3, eager=True)}).to_dummies("x").to_dict(

py-polars/tests/unit/datatypes/test_struct.py

@@ -115,7 +115,7 @@ def test_struct_unnesting() -> None:
         }
     )
     for cols in ("foo", cs.ends_with("oo")):
-        out = df.unnest(cols)
+        out = df.unnest(cols)  # type: ignore[arg-type]
         assert_frame_equal(out, expected)
 
     out = (