Deprecate ImagePalette size parameter

Tests/test_imagepalette.py

@@ -10,8 +10,9 @@ def test_sanity():
     palette = ImagePalette.ImagePalette("RGB", list(range(256)) * 3)
     assert len(palette.colors) == 256
 
-    with pytest.raises(ValueError):
-        ImagePalette.ImagePalette("RGB", list(range(256)) * 3, 10)
+    with pytest.warns(DeprecationWarning):
+        with pytest.raises(ValueError):
+            ImagePalette.ImagePalette("RGB", list(range(256)) * 3, 10)
 
 
 def test_reload():

docs/deprecations.rst

@@ -92,6 +92,17 @@ dictionary. The :py:attr:`~PIL.JpegImagePlugin.convert_dict_qtables` method no l
 performs any operations on the data given to it, has been deprecated and will be
 removed in Pillow 10.0.0 (2023-01-02).
 
+ImagePalette size parameter
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. deprecated:: 8.4.0
+
+The ``size`` parameter will be removed in Pillow 10.0.0 (2023-01-02).
+
+Before Pillow 8.3.0, ImagePalette required palette data of particular lengths by
+default, and the size parameter could be used to override that. Pillow 8.3.0 removed
+the default required length, also removing the need for the size parameter.
+
 Removed features
 ----------------
 

src/PIL/ImagePalette.py

@@ -17,6 +17,7 @@
 #
 
 import array
+import warnings
 
 from . import GimpGradientFile, GimpPaletteFile, ImageColor, PaletteFile
 
@@ -40,8 +41,14 @@ class ImagePalette:
         self.rawmode = None  # if set, palette contains raw data
         self.palette = palette or bytearray()
         self.dirty = None
-        if size != 0 and size != len(self.palette):
-            raise ValueError("wrong palette size")
+        if size != 0:
+            warnings.warn(
+                "The size parameter is deprecated and will be removed in Pillow 10 "
+                "(2023-01-02).",
+                DeprecationWarning,
+            )
+            if size != len(self.palette):
+                raise ValueError("wrong palette size")
 
     @property
     def palette(self):