Added type hints

Tests/test_image.py

@@ -514,21 +514,31 @@ def test_no_supported_modes_method(self, tmp_path: Path) -> None:
         im = hopper()
         im.save(temp_file, convert_mode=True)
 
-    def test_convert_mode(self) -> None:
-        for mode, modes in [["P", []], ["P", ["P"]]]:  # no modes, same mode
-            im = Image.new(mode, (100, 100))
-            assert im._convert_mode(modes) is None
-
-        for mode, modes in [
-            ["P", ["RGB"]],
-            ["P", ["L"]],  # converting to a non-preferred mode
-            ["LA", ["P"]],
-            ["I", ["L"]],
-            ["RGB", ["L"]],
-            ["RGB", ["CMYK"]],
-        ]:
-            im = Image.new(mode, (100, 100))
-            assert im._convert_mode(modes) is not None
+    @pytest.mark.parametrize(
+        "mode, modes",
+        (
+            ("P", ["RGB"]),
+            ("P", ["L"]),  # converting to a non-preferred mode
+            ("LA", ["P"]),
+            ("I", ["L"]),
+            ("RGB", ["L"]),
+            ("RGB", ["CMYK"]),
+        ),
+    )
+    def test_convert_mode(self, mode: str, modes: list[str]) -> None:
+        im = Image.new(mode, (100, 100))
+        assert im._convert_mode(modes) is not None
+
+    @pytest.mark.parametrize(
+        "mode, modes",
+        (
+            ("P", []),  # no mode
+            ("P", ["P"]),  # same mode
+        ),
+    )
+    def test_convert_mode_noop(self, mode: str, modes: list[str]) -> None:
+        im = Image.new(mode, (100, 100))
+        assert im._convert_mode(modes) is None
 
     def test_effect_mandelbrot(self) -> None:
         # Arrange

src/PIL/GifImagePlugin.py

@@ -1181,7 +1181,7 @@ def write(self, data: Buffer) -> int:
     return fp.data
 
 
-def _supported_modes():
+def _supported_modes() -> list[str]:
     return ["RGB", "RGBA", "P", "I", "F", "LA", "L", "1"]
 
 

src/PIL/Image.py

@@ -2618,9 +2618,11 @@ def save(
         if open_fp:
             fp.close()
 
-    def _convert_mode(self, modes, params={}):
+    def _convert_mode(
+        self, modes: list[str], params: dict[str, Any] = {}
+    ) -> Image | None:
         if not modes or self.mode in modes:
-            return
+            return None
         if self.mode == "P":
             preferred_modes = []
             if "A" in self.im.getpalettemode():
@@ -2674,6 +2676,8 @@ def _convert_mode(self, modes, params={}):
         elif new_mode:
             return self.convert(new_mode)
 
+        return None
+
     def seek(self, frame: int) -> None:
         """
         Seeks to the given frame in this sequence file. If you seek

src/PIL/JpegImagePlugin.py

@@ -891,7 +891,7 @@ def jpeg_factory(
     return im
 
 
-def _supported_modes():
+def _supported_modes() -> list[str]:
     return ["RGB", "CMYK", "YCbCr", "RGBX", "L", "1"]
 
 

src/PIL/PngImagePlugin.py

@@ -1532,7 +1532,7 @@ def append(fp: IO[bytes], cid: bytes, *data: bytes) -> None:
     return chunks
 
 
-def _supported_modes():
+def _supported_modes() -> list[str]:
     return ["RGB", "RGBA", "P", "I", "LA", "L", "1"]
 
 

src/PIL/WebPImagePlugin.py

@@ -314,7 +314,7 @@ def _save(im: Image.Image, fp: IO[bytes], filename: str | bytes) -> None:
     fp.write(data)
 
 
-def _supported_modes():
+def _supported_modes() -> list[str]:
     return [
         "RGB",
         "RGBA",