Better documented limited MPO save feature.

At present it’s only possible to save the current frame of an MPO, not
the MPO in its entirety. Added testing verifying as much.

PIL/MpoImagePlugin.py

@@ -25,7 +25,8 @@ from PIL import Image, JpegImagePlugin
 def _accept(prefix):
     return JpegImagePlugin._accept(prefix)
 
-def _save(im, fp, filename):
+def _save(im, fp, filename, **options):
+    # Note that we can only save the current frame at present
     return JpegImagePlugin._save(im, fp, filename)
 
 ##

Tests/test_file_mpo.py

@@ -13,7 +13,8 @@ class TestFileMpo(PillowTestCase):
         if "jpeg_encoder" not in codecs or "jpeg_decoder" not in codecs:
             self.skipTest("jpeg support not available")
 
-    def roundtrip(self, im, **options):
+    def frame_roundtrip(self, im, **options):
+        # Note that for now, there is no MPO saving functionality
         out = BytesIO()
         im.save(out, "MPO", **options)
         bytes = out.tell()
@@ -106,7 +107,19 @@ class TestFileMpo(PillowTestCase):
             im02 = im.tobytes()
             self.assertEqual(im0, im02)
             self.assertNotEqual(im0, im1)
-            
+
+    def test_save(self):
+        # Note that only individual frames can be saved at present
+        for test_file in test_files:
+            im = Image.open(test_file)
+            self.assertEqual(im.tell(), 0)
+            jpg0 = self.frame_roundtrip(im)
+            self.assert_image_similar(im, jpg0, 30)
+            im.seek(1)
+            self.assertEqual(im.tell(), 1)
+            jpg1 = self.frame_roundtrip(im)
+            self.assert_image_similar(im, jpg1, 30)
+
 
 if __name__ == '__main__':
     unittest.main()