Merge pull request #3859 from radarhere/error

Updated resampling filter error messages

Tests/test_image_transform.py

@@ -160,6 +160,15 @@ class TestImageTransform(PillowTestCase):
         im = hopper()
         self.assertRaises(ValueError, im.transform, (100, 100), None)
 
+    def test_unknown_resampling_filter(self):
+        im = hopper()
+        (w, h) = im.size
+        for resample in (Image.BOX, "unknown"):
+            self.assertRaises(ValueError, im.transform, (100, 100), Image.EXTENT,
+                              (0, 0,
+                               w, h),
+                              resample)
+
 
 class TestImageTransformAffine(PillowTestCase):
     transform = Image.AFFINE

src/PIL/Image.py

@@ -1794,7 +1794,18 @@ class Image(object):
         if resample not in (
                 NEAREST, BILINEAR, BICUBIC, LANCZOS, BOX, HAMMING,
         ):
-            raise ValueError("unknown resampling filter")
+            message = "Unknown resampling filter ({}).".format(resample)
+
+            filters = ["{} ({})".format(filter[1], filter[0]) for filter in (
+                (NEAREST, "Image.NEAREST"),
+                (LANCZOS, "Image.LANCZOS"),
+                (BILINEAR, "Image.BILINEAR"),
+                (BICUBIC, "Image.BICUBIC"),
+                (BOX, "Image.BOX"),
+                (HAMMING, "Image.HAMMING")
+            )]
+            raise ValueError(
+                message+" Use "+", ".join(filters[:-1])+" or "+filters[-1])
 
         size = tuple(size)
 
@@ -2263,7 +2274,22 @@ class Image(object):
             raise ValueError("unknown transformation method")
 
         if resample not in (NEAREST, BILINEAR, BICUBIC):
-            raise ValueError("unknown resampling filter")
+            if resample in (BOX, HAMMING, LANCZOS):
+                message = {
+                    BOX: "Image.BOX",
+                    HAMMING: "Image.HAMMING",
+                    LANCZOS: "Image.LANCZOS/Image.ANTIALIAS"
+                }[resample]+" ({}) cannot be used.".format(resample)
+            else:
+                message = "Unknown resampling filter ({}).".format(resample)
+
+            filters = ["{} ({})".format(filter[1], filter[0]) for filter in (
+                (NEAREST, "Image.NEAREST"),
+                (BILINEAR, "Image.BILINEAR"),
+                (BICUBIC, "Image.BICUBIC")
+            )]
+            raise ValueError(
+                message+" Use "+", ".join(filters[:-1])+" or "+filters[-1])
 
         image.load()