return implicit RGBA to JPEG save, raise warning

PIL/JpegImagePlugin.py

@@ -534,6 +534,7 @@ RAWMODE = {
     "1": "L",
     "L": "L",
     "RGB": "RGB",
+    "RGBA": "RGB",
     "RGBX": "RGB",
     "CMYK": "CMYK;I",  # assume adobe conventions
     "YCbCr": "YCbCr",
@@ -582,6 +583,14 @@ def _save(im, fp, filename):
     except KeyError:
         raise IOError("cannot write mode %s as JPEG" % im.mode)
 
+    if im.mode == 'RGBA':
+        warnings.warn(
+            'You are saving RGBA image as JPEG. The alpha channel will be '
+            'discarded. This conversion is deprecated and will be disabled '
+            'in Pillow 3.7. Please, convert the image to RGB explicitly.',
+            DeprecationWarning
+        )
+
     info = im.encoderinfo
 
     dpi = info.get("dpi", (0, 0))

Tests/helper.py

@@ -99,7 +99,7 @@ class PillowTestCase(unittest.TestCase):
             " average pixel value difference %.4f > epsilon %.4f" % (
                 ave_diff, epsilon))
 
-    def assert_warning(self, warn_class, func):
+    def assert_warning(self, warn_class, func, *args, **kwargs):
         import warnings
 
         result = None
@@ -108,7 +108,7 @@ class PillowTestCase(unittest.TestCase):
             warnings.simplefilter("always")
 
             # Hopefully trigger a warning.
-            result = func()
+            result = func(*args, **kwargs)
 
             # Verify some things.
             self.assertGreaterEqual(len(w), 1)

Tests/test_file_jpeg.py

@@ -457,12 +457,18 @@ class TestFileJpeg(PillowTestCase):
             img.save(out, "JPEG")
 
     def test_save_wrong_modes(self):
-        # ref https://github.com/python-pillow/Pillow/issues/2005
         out = BytesIO()
-        for mode in ['LA', 'La', 'RGBA', 'RGBa', 'P']:
+        for mode in ['LA', 'La', 'RGBa', 'P']:
             img = Image.new(mode, (20, 20))
             self.assertRaises(IOError, img.save, out, "JPEG")
 
+    def test_save_modes_with_warnings(self):
+        # ref https://github.com/python-pillow/Pillow/issues/2005
+        out = BytesIO()
+        for mode in ['RGBA']:
+            img = Image.new(mode, (20, 20))
+            self.assert_warning(DeprecationWarning, img.save, out, "JPEG")
+
 
 if __name__ == '__main__':
     unittest.main()