GH1226 Add Styler.map to the stubs

pandas-stubs/io/formats/style.pyi

@@ -53,6 +53,11 @@ class _DataFrameFunc(Protocol):
         self, series: DataFrame, /, *args: Any, **kwargs: Any
     ) -> npt.NDArray | DataFrame: ...
 
+class _MapCallable(Protocol):
+    def __call__(
+        self, first_arg: Scalar, /, *args: Any, **kwargs: Any
+    ) -> str | None: ...
+
 class Styler(StylerRenderer):
     def __init__(
         self,
@@ -71,6 +76,19 @@ class Styler(StylerRenderer):
         formatter: ExtFormatter | None = ...,
     ) -> None: ...
     def concat(self, other: Styler) -> Styler: ...
+    @overload
+    def map(
+        self,
+        func: Callable[[Scalar], str | None],
+        subset: Subset | None = ...,
+    ) -> Styler: ...
+    @overload
+    def map(
+        self,
+        func: _MapCallable,
+        subset: Subset | None = ...,
+        **kwargs: Any,
+    ) -> Styler: ...
     def set_tooltips(
         self,
         ttips: DataFrame,

tests/test_styler.py

@@ -30,7 +30,6 @@
 
 DF = DataFrame({"a": [1, 2, 3], "b": [3.14, 2.72, 1.61]})
 
-
 PWD = pathlib.Path(os.path.split(os.path.abspath(__file__))[0])
 
 if TYPE_CHECKING:
@@ -233,3 +232,30 @@ def test_styler_columns_and_index() -> None:
     styler = DF.style
     check(assert_type(styler.columns, Index), Index)
     check(assert_type(styler.index, Index), Index)
+
+
+def test_styler_map() -> None:
+    """Test type returned with Styler.map GH1226."""
+    df = DataFrame(data={"col1": [1, -2], "col2": [-3, 4]})
+    check(
+        assert_type(
+            df.style.map(
+                lambda v: "color: red;" if isinstance(v, float) and v < 0 else None
+            ),
+            Styler,
+        ),
+        Styler,
+    )
+
+    def color_negative(v: Scalar, /, color: str) -> str | None:
+        return f"color: {color};" if isinstance(v, float) and v < 0 else None
+
+    df = DataFrame(np.random.randn(5, 2), columns=["A", "B"])
+
+    check(
+        assert_type(
+            df.style.map(color_negative, color="red"),
+            Styler,
+        ),
+        Styler,
+    )