Add convert_mode param when saving

Tests/test_file_gif.py

@@ -1411,6 +1411,18 @@ def test_save_I(tmp_path: Path) -> None:
         assert_image_equal(reloaded.convert("L"), im.convert("L"))
 
 
+def test_save_wrong_modes() -> None:
+    out = BytesIO()
+    for mode in ["CMYK"]:
+        img = Image.new(mode, (20, 20))
+        with pytest.raises(ValueError):
+            img.save(out, "GIF")
+
+    for mode in ["CMYK", "LA"]:
+        img = Image.new(mode, (20, 20))
+        img.save(out, "GIF", convert_mode=True)
+
+
 def test_getdata(monkeypatch: pytest.MonkeyPatch) -> None:
     # Test getheader/getdata against legacy values.
     # Create a 'P' image with holes in the palette.

Tests/test_file_jpeg.py

@@ -769,13 +769,24 @@ def test_save_correct_modes(self, mode: str) -> None:
         img = Image.new(mode, (20, 20))
         img.save(out, "JPEG")
 
-    @pytest.mark.parametrize("mode", ("LA", "La", "RGBA", "RGBa", "P"))
-    def test_save_wrong_modes(self, mode: str) -> None:
+    def test_save_wrong_modes(self, tmp_path: Path) -> None:
         # ref https://github.com/python-pillow/Pillow/issues/2005
         out = BytesIO()
-        img = Image.new(mode, (20, 20))
-        with pytest.raises(OSError):
-            img.save(out, "JPEG")
+        for mode in ["LA", "La", "RGBA", "RGBa", "P", "I"]:
+            img = Image.new(mode, (20, 20))
+            with pytest.raises(OSError):
+                img.save(out, "JPEG")
+
+        for mode in ["LA", "RGBA", "P", "I"]:
+            img = Image.new(mode, (20, 20))
+            img.save(out, "JPEG", convert_mode=True)
+
+        temp_file = tmp_path / "temp.jpg"
+        with Image.open("Tests/images/pil123rgba.png") as img:
+            img.save(temp_file, convert_mode=True, fill_color="red")
+
+        with Image.open(temp_file) as reloaded:
+            assert_image_similar_tofile(reloaded, "Tests/images/pil123rgba_red.jpg", 4)
 
     def test_save_tiff_with_dpi(self, tmp_path: Path) -> None:
         # Arrange

Tests/test_file_png.py

@@ -243,6 +243,14 @@ def test_load_transparent_rgb(self) -> None:
         # image has 876 transparent pixels
         assert im.getchannel("A").getcolors()[0][0] == 876
 
+    def test_save_CMYK(self) -> None:
+        out = BytesIO()
+        im = Image.new("CMYK", (20, 20))
+        with pytest.raises(IOError):
+            im.save(out, "PNG")
+
+        im.save(out, "PNG", convert_mode=True)
+
     def test_save_p_transparent_palette(self, tmp_path: Path) -> None:
         in_file = "Tests/images/pil123p.png"
         with Image.open(in_file) as im:

Tests/test_file_webp.py

@@ -94,6 +94,12 @@ def _roundtrip(
                 target = target.convert(self.rgb_mode)
             assert_image_similar(image, target, epsilon)
 
+    def test_save_convert_mode(self) -> None:
+        out = io.BytesIO()
+        for mode in ["CMYK", "I", "L", "LA", "P"]:
+            img = Image.new(mode, (20, 20))
+            img.save(out, "WEBP", convert_mode=True)
+
     def test_write_rgb(self, tmp_path: Path) -> None:
         """
         Can we write a RGB mode file to webp without error?

Tests/test_image.py

@@ -20,6 +20,7 @@
     ImageFile,
     ImagePalette,
     ImageShow,
+    TiffImagePlugin,
     UnidentifiedImageError,
     features,
 )
@@ -507,6 +508,77 @@ def test_registered_extensions(self) -> None:
         for ext in [".cur", ".icns", ".tif", ".tiff"]:
             assert ext in extensions
 
+    def test_supported_modes(self) -> None:
+        for format in Image.MIME.keys():
+            try:
+                save_handler = Image.SAVE[format]
+            except KeyError:
+                continue
+            plugin = sys.modules[save_handler.__module__]
+            if not hasattr(plugin, "_supported_modes"):
+                continue
+
+            # Check that the supported modes list is accurate
+            supported_modes = plugin._supported_modes()
+            for mode in [
+                "1",
+                "L",
+                "P",
+                "RGB",
+                "RGBA",
+                "CMYK",
+                "YCbCr",
+                "LAB",
+                "HSV",
+                "I",
+                "F",
+                "LA",
+                "La",
+                "RGBX",
+                "RGBa",
+            ]:
+                out = io.BytesIO()
+                im = Image.new(mode, (100, 100))
+                if mode in supported_modes:
+                    im.save(out, format)
+                else:
+                    with pytest.raises(Exception):
+                        im.save(out, format)
+
+    def test_no_supported_modes_method(self, tmp_path: Path) -> None:
+        assert not hasattr(TiffImagePlugin, "_supported_modes")
+
+        temp_file = tmp_path / "temp.tiff"
+
+        im = hopper()
+        im.save(temp_file, convert_mode=True)
+
+    @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
         size = (512, 512)

src/PIL/GifImagePlugin.py

@@ -1199,6 +1199,10 @@ def write(self, data: Buffer) -> int:
     return fp.data
 
 
+def _supported_modes() -> list[str]:
+    return ["RGB", "RGBA", "P", "I", "F", "LA", "L", "1"]
+
+
 # --------------------------------------------------------------------
 # Registry
 

src/PIL/Image.py

@@ -2557,6 +2557,14 @@ def save(
         else:
             save_handler = SAVE[format.upper()]
 
+        if params.get("convert_mode"):
+            plugin = sys.modules[save_handler.__module__]
+            if hasattr(plugin, "_supported_modes"):
+                modes = plugin._supported_modes()
+                converted_im = self._convert_mode(modes, params)
+                if converted_im:
+                    return converted_im.save(fp, format, **params)
+
         created = False
         if open_fp:
             created = not os.path.exists(filename)
@@ -2590,6 +2598,66 @@ def _attach_default_encoderinfo(self, im: Image) -> dict[str, Any]:
         self.encoderinfo = {**im._default_encoderinfo, **encoderinfo}
         return encoderinfo
 
+    def _convert_mode(
+        self, modes: list[str], params: dict[str, Any] = {}
+    ) -> Image | None:
+        if not modes or self.mode in modes:
+            return None
+        if self.mode == "P":
+            preferred_modes = []
+            if "A" in self.im.getpalettemode():
+                preferred_modes.append("RGBA")
+            preferred_modes.append("RGB")
+        else:
+            preferred_modes = {
+                "CMYK": ["RGB"],
+                "RGB": ["CMYK"],
+                "RGBX": ["RGB"],
+                "RGBa": ["RGBA", "RGB"],
+                "RGBA": ["RGB"],
+                "LA": ["RGBA", "P", "L"],
+                "La": ["LA", "L"],
+                "L": ["RGB"],
+                "F": ["I"],
+                "I": ["L", "RGB"],
+                "1": ["L"],
+                "YCbCr": ["RGB"],
+                "LAB": ["RGB"],
+                "HSV": ["RGB"],
+            }.get(self.mode, [])
+        for new_mode in preferred_modes:
+            if new_mode in modes:
+                break
+        else:
+            new_mode = modes[0]
+        if self.mode == "LA" and new_mode == "P":
+            alpha = self.getchannel("A")
+            # Convert the image into P mode but only use 255 colors
+            # in the palette out of 256.
+            im = self.convert("L").convert("P", palette=Palette.ADAPTIVE, colors=255)
+            # Set all pixel values below 128 to 255, and the rest to 0.
+            mask = eval(alpha, lambda px: 255 if px < 128 else 0)
+            # Paste the color of index 255 and use alpha as a mask.
+            im.paste(255, mask)
+            # The transparency index is 255.
+            im.info["transparency"] = 255
+            return im
+
+        elif self.mode == "I":
+            im = self.point([i // 256 for i in range(65536)], "L")
+            return im.convert(new_mode) if new_mode != "L" else im
+
+        elif self.mode in ("RGBA", "LA") and new_mode in ("RGB", "L"):
+            fill_color = params.get("fill_color", "white")
+            background = new(new_mode, self.size, fill_color)
+            background.paste(self, self.getchannel("A"))
+            return background
+
+        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

@@ -875,6 +875,10 @@ def jpeg_factory(
     return im
 
 
+def _supported_modes() -> list[str]:
+    return ["RGB", "CMYK", "YCbCr", "RGBX", "L", "1"]
+
+
 # ---------------------------------------------------------------------
 # Registry stuff
 

src/PIL/PngImagePlugin.py

@@ -1541,6 +1541,10 @@ def append(fp: IO[bytes], cid: bytes, *data: bytes) -> None:
     return chunks
 
 
+def _supported_modes() -> list[str]:
+    return ["RGB", "RGBA", "P", "I", "LA", "L", "1"]
+
+
 # --------------------------------------------------------------------
 # Registry
 

src/PIL/WebPImagePlugin.py

@@ -314,6 +314,25 @@ def _save(im: Image.Image, fp: IO[bytes], filename: str | bytes) -> None:
     fp.write(data)
 
 
+def _supported_modes() -> list[str]:
+    return [
+        "RGB",
+        "RGBA",
+        "RGBa",
+        "RGBX",
+        "CMYK",
+        "YCbCr",
+        "HSV",
+        "I",
+        "F",
+        "P",
+        "LA",
+        "LAB",
+        "L",
+        "1",
+    ]
+
+
 Image.register_open(WebPImageFile.format, WebPImageFile, _accept)
 if SUPPORTED:
     Image.register_save(WebPImageFile.format, _save)