Added has_transparency_data

Tests/test_file_webp.py

@@ -234,3 +234,13 @@ def test_duration(self, tmp_path):
 
         with Image.open(out_webp) as reloaded:
             assert reloaded.info["duration"] == 1000
+
+    def test_roundtrip_rgba_palette(self, tmp_path):
+        temp_file = str(tmp_path / "temp.webp")
+        im = Image.new("RGBA", (1, 1)).convert("P")
+        assert im.mode == "P"
+        assert im.palette.mode == "RGBA"
+        im.save(temp_file)
+
+        with Image.open(temp_file) as im:
+            assert im.getpixel((0, 0)) == (0, 0, 0, 0)

Tests/test_image.py

@@ -906,6 +906,31 @@ def test_zero_tobytes(self, size):
         im = Image.new("RGB", size)
         assert im.tobytes() == b""
 
+    def test_has_transparency_data(self):
+        for mode in ("1", "L", "P", "RGB"):
+            im = Image.new(mode, (1, 1))
+            assert not im.has_transparency_data
+
+        for mode in ("LA", "La", "PA", "RGBA", "RGBa"):
+            im = Image.new(mode, (1, 1))
+            assert im.has_transparency_data
+
+        # P mode with "transparency" info
+        with Image.open("Tests/images/first_frame_transparency.gif") as im:
+            assert "transparency" in im.info
+            assert im.has_transparency_data
+
+        # RGB mode with "transparency" info
+        with Image.open("Tests/images/rgb_trns.png") as im:
+            assert "transparency" in im.info
+            assert im.has_transparency_data
+
+        # P mode with RGBA palette
+        im = Image.new("RGBA", (1, 1)).convert("P")
+        assert im.mode == "P"
+        assert im.palette.mode == "RGBA"
+        assert im.has_transparency_data
+
     def test_apply_transparency(self):
         im = Image.new("P", (1, 1))
         im.putpalette((0, 0, 0, 1, 1, 1))

docs/reference/Image.rst

@@ -195,6 +195,7 @@ This helps to get the bounding box coordinates of the input image::
 .. automethod:: PIL.Image.Image.getpalette
 .. automethod:: PIL.Image.Image.getpixel
 .. automethod:: PIL.Image.Image.getprojection
+.. autoproperty:: PIL.Image.Image.has_transparency_data
 .. automethod:: PIL.Image.Image.histogram
 .. automethod:: PIL.Image.Image.paste
 .. automethod:: PIL.Image.Image.point

src/PIL/Image.py

@@ -915,7 +915,7 @@ def convert(
 
         self.load()
 
-        has_transparency = self.info.get("transparency") is not None
+        has_transparency = "transparency" in self.info
         if not mode and self.mode == "P":
             # determine default mode
             if self.palette:
@@ -1531,6 +1531,24 @@ def getpalette(self, rawmode="RGB"):
             rawmode = mode
         return list(self.im.getpalette(mode, rawmode))
 
+    @property
+    def has_transparency_data(self) -> bool:
+        """
+        Determine if an image has transparency data, whether in the form of an
+        alpha channel, a palette with an alpha channel, or a "transparency" key
+        in the info dictionary.
+
+        Note the image might still appear solid, if all of the values shown
+        within are opaque.
+
+        :returns: A boolean.
+        """
+        return (
+            self.mode in ("LA", "La", "PA", "RGBA", "RGBa")
+            or (self.mode == "P" and self.palette.mode.endswith("A"))
+            or "transparency" in self.info
+        )
+
     def apply_transparency(self):
         """
         If a P mode image has a "transparency" key in the info dictionary,

src/PIL/WebPImagePlugin.py

@@ -332,12 +332,7 @@ def _save(im, fp, filename):
     exact = 1 if im.encoderinfo.get("exact") else 0
 
     if im.mode not in _VALID_WEBP_LEGACY_MODES:
-        alpha = (
-            "A" in im.mode
-            or "a" in im.mode
-            or (im.mode == "P" and "transparency" in im.info)
-        )
-        im = im.convert("RGBA" if alpha else "RGB")
+        im = im.convert("RGBA" if im.has_transparency_data else "RGB")
 
     data = _webp.WebPEncode(
         im.tobytes(),