Fix indices of colors read from GIMP Palette file

Tests/images/custom_gimp_palette.gpl

@@ -2,11 +2,11 @@ GIMP Palette
 Name: custompalette
 Columns: 4
 #
-  0   0   0     Index 3
- 65  38  30     Index 4
-103  62  49     Index 6
- 79  73  72     Index 7
-114 101  97     Index 8
-208 127 100     Index 9
-151 144 142     Index 10
-221 207 199     Index 11
+  0   0   0     Index 0
+ 65  38  30     Index 1
+103  62  49     Index 2
+ 79  73  72     Index 3
+114 101  97     Index 4
+208 127 100     Index 5
+151 144 142     Index 6
+221 207 199     Index 7

Tests/test_file_gimppalette.py

@@ -30,3 +30,33 @@ def test_get_palette():
 
     # Assert
     assert mode == "RGB"
+
+
+def test_palette__has_correct_color_indexes():
+    # Arrange
+    with open("Tests/images/custom_gimp_palette.gpl", "rb") as fp:
+        palette_file = GimpPaletteFile(fp)
+
+    palette, mode = palette_file.getpalette()
+
+    colors_in_test_palette = [
+        (0, 0, 0),
+        (65, 38, 30),
+        (103, 62, 49),
+        (79, 73, 72),
+        (114, 101, 97),
+        (208, 127, 100),
+        (151, 144, 142),
+        (221, 207, 199),
+    ]
+
+    for i, color in enumerate(colors_in_test_palette):
+        assert tuple(palette[i * 3: i * 3 + 3]) == color
+
+
+def test_palette_counts_number_of_colors_in_file():
+    # Arrange
+    with open("Tests/images/custom_gimp_palette.gpl", "rb") as fp:
+        palette_file = GimpPaletteFile(fp)
+
+    assert palette_file.n_colors == 8

src/PIL/GimpPaletteFile.py

@@ -18,6 +18,8 @@ import re
 
 from ._binary import o8
 
+_str_to_o8 = lambda v: o8(int(v))
+
 
 class GimpPaletteFile:
     """File handler for GIMP's palette format."""
@@ -26,17 +28,13 @@ class GimpPaletteFile:
 
     def __init__(self, fp):
 
-        self.palette = [o8(i) * 3 for i in range(256)]
+        palette = bytearray(b"".join([o8(i) * 3 for i in range(256)]))
 
         if fp.readline()[:12] != b"GIMP Palette":
             raise SyntaxError("not a GIMP palette file")
 
-        for i in range(256):
-
-            s = fp.readline()
-            if not s:
-                break
-
+        index = 0
+        for s in fp:
             # skip fields and comment lines
             if re.match(rb"\w+:|#", s):
                 continue
@@ -44,12 +42,15 @@ class GimpPaletteFile:
                 raise SyntaxError("bad palette file")
 
             v = tuple(map(int, s.split()[:3]))
-            if len(v) != 3:
+            if len(v) < 3:
                 raise ValueError("bad palette entry")
 
-            self.palette[i] = o8(v[0]) + o8(v[1]) + o8(v[2])
+            palette[index * 3: index * 3 + 3] = v
+            index += 1
+
+        self.palette = bytes(palette)
+        self.n_colors = index
 
-        self.palette = b"".join(self.palette)
 
     def getpalette(self):