improve docs for Pad + add tests (#239)

test/test_transforms.py

@@ -136,6 +136,30 @@ class Tester(unittest.TestCase):
         assert result.size(1) == height + 2 * padding
         assert result.size(2) == width + 2 * padding
 
+    def test_pad_with_tuple_of_pad_values(self):
+        height = random.randint(10, 32) * 2
+        width = random.randint(10, 32) * 2
+        img = transforms.ToPILImage()(torch.ones(3, height, width))
+
+        padding = tuple([random.randint(1, 20) for _ in range(2)])
+        output = transforms.Pad(padding)(img)
+        assert output.size == (width + padding[0] * 2, height + padding[1] * 2)
+
+        padding = tuple([random.randint(1, 20) for _ in range(4)])
+        output = transforms.Pad(padding)(img)
+        assert output.size[0] == width + padding[0] + padding[2]
+        assert output.size[1] == height + padding[1] + padding[3]
+
+    def test_pad_raises_with_invalide_pad_sequence_len(self):
+        with self.assertRaises(ValueError):
+            transforms.Pad(())
+
+        with self.assertRaises(ValueError):
+            transforms.Pad((1, 2, 3))
+
+        with self.assertRaises(ValueError):
+            transforms.Pad((1, 2, 3, 4, 5))
+
     def test_lambda(self):
         trans = transforms.Lambda(lambda x: x.add(10))
         x = torch.randn(10)

torchvision/transforms.py

@@ -233,15 +233,22 @@ class Pad(object):
     """Pad the given PIL.Image on all sides with the given "pad" value.
 
     Args:
-        padding (int or sequence): Padding on each border. If a sequence of
-            length 4, it is used to pad left, top, right and bottom borders respectively.
-        fill: Pixel fill value. Default is 0. If a sequence of
+        padding (int or tuple): Padding on each border. If a single int is provided this
+            is used to pad all borders. If tuple of length 2 is provided this is the padding
+            on left/right and top/bottom respectively. If a tuple of length 4 is provided
+            this is the padding for the left, top, right and bottom borders
+            respectively.
+        fill: Pixel fill value. Default is 0. If a tuple of
             length 3, it is used to fill R, G, B channels respectively.
     """
 
     def __init__(self, padding, fill=0):
-        assert isinstance(padding, numbers.Number) or isinstance(padding, tuple)
-        assert isinstance(fill, numbers.Number) or isinstance(fill, str) or isinstance(fill, tuple)
+        assert isinstance(padding, (numbers.Number, tuple))
+        assert isinstance(fill, (numbers.Number, str, tuple))
+        if isinstance(padding, collections.Sequence) and len(padding) not in [2, 4]:
+            raise ValueError("Padding must be an int or a 2, or 4 element tuple, not a " +
+                             "{} element tuple".format(len(padding)))
+
         self.padding = padding
         self.fill = fill