Allow putpalette to accept 1024 integers to include alpha values

Tests/test_image_putpalette.py

@@ -1,8 +1,8 @@
 import pytest
 
-from PIL import ImagePalette
+from PIL import Image, ImagePalette
 
-from .helper import hopper
+from .helper import assert_image_equal, hopper
 
 
 def test_putpalette():
@@ -39,3 +39,20 @@ def test_imagepalette():
     im.putpalette(ImagePalette.random())
     im.putpalette(ImagePalette.sepia())
     im.putpalette(ImagePalette.wedge())
+
+
+def test_putpalette_with_alpha_values():
+    with Image.open("Tests/images/transparent.gif") as im:
+        expected = im.convert("RGBA")
+
+        palette = im.getpalette()
+        transparency = im.info.pop("transparency")
+
+        palette_with_alpha_values = []
+        for i in range(256):
+            color = palette[i * 3 : i * 3 + 3]
+            alpha = 0 if i == transparency else 255
+            palette_with_alpha_values += color + [alpha]
+        im.putpalette(palette_with_alpha_values, "RGBA")
+
+        assert_image_equal(im.convert("RGBA"), expected)

src/PIL/Image.py

@@ -815,9 +815,15 @@ class Image:
         """
         if self.im and self.palette and self.palette.dirty:
             # realize palette
-            self.im.putpalette(*self.palette.getdata())
+            mode, arr = self.palette.getdata()
+            if mode == "RGBA":
+                mode = "RGB"
+                self.info["transparency"] = arr[3::4]
+                arr = bytes(
+                    value for (index, value) in enumerate(arr) if index % 4 != 3
+                )
+            self.im.putpalette(mode, arr)
             self.palette.dirty = 0
-            self.palette.mode = "RGB"
             self.palette.rawmode = None
             if "transparency" in self.info:
                 if isinstance(self.info["transparency"], int):
@@ -825,6 +831,8 @@ class Image:
                 else:
                     self.im.putpalettealphas(self.info["transparency"])
                 self.palette.mode = "RGBA"
+            else:
+                self.palette.mode = "RGB"
 
         if self.im:
             if cffi and USE_CFFI_ACCESS:
@@ -1672,12 +1680,14 @@ class Image:
 
     def putpalette(self, data, rawmode="RGB"):
         """
-        Attaches a palette to this image.  The image must be a "P",
-        "PA", "L" or "LA" image, and the palette sequence must contain
-        768 integer values, where each group of three values represent
-        the red, green, and blue values for the corresponding pixel
-        index. Instead of an integer sequence, you can use an 8-bit
-        string.
+        Attaches a palette to this image.  The image must be a "P", "PA", "L"
+        or "LA" image.
+
+        The palette sequence must contain either 768 integer values, or 1024
+        integer values if alpha is included. Each group of values represents
+        the red, green, blue (and alpha if included) values for the
+        corresponding pixel index. Instead of an integer sequence, you can use
+        an 8-bit string.
 
         :param data: A palette sequence (either a list or a string).
         :param rawmode: The raw mode of the palette.