Allow plugins to specify their supported modes

Tests/test_image.py

@@ -2,7 +2,10 @@
 
 from PIL import Image, TiffImagePlugin
 from PIL._util import py3
+
+from io import BytesIO
 import os
+import sys
 
 
 class TestImage(PillowTestCase):
@@ -62,8 +65,7 @@ def test_width_height(self):
 
     def test_invalid_image(self):
         if py3:
-            import io
-            im = io.BytesIO(b'')
+            im = BytesIO(b'')
         else:
             import StringIO
             im = StringIO.StringIO('')
@@ -324,8 +326,29 @@ def test_registered_extensions(self):
         for ext in ['.cur', '.icns', '.tif', '.tiff']:
             self.assertIn(ext, extensions)
 
-    def test_no_convert_mode(self):
-        self.assertTrue(not hasattr(TiffImagePlugin, '_convert_mode'))
+    def test_supported_modes(self):
+        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 = BytesIO()
+                im = Image.new(mode, (100, 100))
+                if mode in supported_modes:
+                    im.save(out, format)
+                else:
+                    self.assertRaises(Exception, im.save, out, format)
+
+    def test_no_supported_modes_method(self):
+        self.assertTrue(not hasattr(TiffImagePlugin, '_supported_modes'))
 
         temp_file = self.tempfile("temp.tiff")
 

src/PIL/GifImagePlugin.py

@@ -798,11 +798,8 @@ def write(self, data):
     return fp.data
 
 
-def _convert_mode(im):
-    return {
-        'LA':'P',
-        'CMYK':'RGB'
-    }.get(im.mode)
+def _supported_modes():
+    return ['RGB', 'RGBA', 'P', 'I', 'F', 'LA', 'L', '1']
 
 
 # --------------------------------------------------------------------

src/PIL/Image.py

@@ -1956,9 +1956,29 @@ def save(self, fp, format=None, **params):
                 fp.close()
 
     def _convert_mode(self, plugin, params):
-        if not hasattr(plugin, '_convert_mode'):
+        if not hasattr(plugin, '_supported_modes'):
             return
-        new_mode = plugin._convert_mode(self)
+        supported_modes = plugin._supported_modes()
+        if not supported_modes or self.mode in supported_modes:
+            return
+        if self.mode == 'P':
+            preferred_modes = []
+            if 'A' in self.im.getpalettemode():
+                preferred_modes.append('RGBA')
+            preferred_modes.append('RGB')
+        else:
+            preferred_modes = {
+                'RGBA': ['RGB'],
+                'LA': ['RGBA', 'P', 'L'],
+                'L': ['RGB'],
+                'I': ['L', 'RGB'],
+                'CMYK': ['RGB']
+            }.get(self.mode, [])
+        for new_mode in preferred_modes:
+            if new_mode in supported_modes:
+                break
+        else:
+            new_mode = supported_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

src/PIL/JpegImagePlugin.py

@@ -792,15 +792,8 @@ def jpeg_factory(fp=None, filename=None):
     return im
 
 
-def _convert_mode(im):
-    mode = im.mode
-    if mode == 'P':
-        return 'RGBA' if 'A' in im.im.getpalettemode() else 'RGB'
-    return {
-        'RGBA':'RGB',
-        'LA':'L',
-        'I':'L'
-    }.get(mode)
+def _supported_modes():
+    return ['RGB', 'CMYK', 'YCbCr', 'RGBX', 'L', '1']
 
 
 # -------------------------------------------------------------------q-

src/PIL/PngImagePlugin.py

@@ -859,10 +859,8 @@ def append(fp, cid, *data):
     return fp.data
 
 
-def _convert_mode(im):
-    return {
-        'CMYK':'RGB'
-    }.get(im.mode)
+def _supported_modes():
+    return ['RGB', 'RGBA', 'P', 'I', 'LA', 'L', '1']
 
 
 # --------------------------------------------------------------------

src/PIL/WebPImagePlugin.py

@@ -308,17 +308,8 @@ def _save(im, fp, filename):
     fp.write(data)
 
 
-def _convert_mode(im):
-    mode = im.mode
-    if mode == 'P':
-        return 'RGBA' if 'A' in im.im.getpalettemode() else 'RGB'
-    return {
-        # Pillow doesn't support L modes for webp for now.
-        'L':'RGB',
-        'LA':'RGBA',
-        'I':'RGB',
-        'CMYK':'RGB'
-    }.get(mode)
+def _supported_modes():
+    return ['RGB', 'RGBA', 'RGBX', 'CMYK', 'YCbCr', 'HSV', 'I', 'F', 'P', 'LA', 'L', '1']
 
 
 Image.register_open(WebPImageFile.format, WebPImageFile, _accept)