Remove PixelGroup dependence on pic and key attrs

skimage/novice/_novice.py

@@ -439,8 +439,6 @@ class PixelGroup(Picture):
 
     """
     def __init__(self, pic, key):
-        self._pic = pic
-
         # Flip y axis
         y_slice = key[1]
         start = y_slice.start if y_slice.start is not None else 0
@@ -451,21 +449,16 @@ class PixelGroup(Picture):
 
         key = (key[0], slice(stop, start + 1, y_slice.step))
 
-        # array dimensions are row, column (i.e. y, x)
-        self._key = (key[1], key[0])
-
-        self._array = pic._array[self._key]
+        self.array = pic._array[(key[1], key[0])]
 
     def __iter__(self):
         """Iterates through all pixels in the pixel group.
 
         """
-        x_idx = range(self._pic.width)[self._key[0]]
-        y_idx = range(self._pic.height)[self._key[1]]
-
-        for x in x_idx:
-            for y in y_idx:
-                yield self._pic._makepixel(x, y)
+        height, width, channels = self.array.shape
+        for x in range(width):
+            for y in range(height):
+                yield self._makepixel(x, y)
 
     def __repr__(self):
         return "PixelGroup ({0} pixels)".format(self.size[0] * self.size[1])

skimage/novice/tests/test_novice.py

@@ -11,6 +11,10 @@ IMAGE_PATH = os.path.join(data_dir, "elephant.png")
 SMALL_IMAGE_PATH = os.path.join(data_dir, "block.png")
 
 
+def _array_2d_to_RGB(array):
+    return np.tile(array[:, :, np.newaxis], (1, 1, 3))
+
+
 def test_pic_info():
     pic = novice.open(IMAGE_PATH)
     assert_equal(pic.format, "png")
@@ -144,32 +148,35 @@ def test_indexing():
 
 
 def test_pixel_group():
-    array = np.tile(np.arange(0, 10)[np.newaxis, :, np.newaxis], (1, 1, 3))
+    array = _array_2d_to_RGB(np.arange(0, 10)[np.newaxis, :])
     pic = novice.Picture(array=array)
-    index = (slice(None), slice(3, 8))
-    pixel_group = pic[index[::-1]]
-    assert_allclose(pixel_group.array, array[index])
+
+    x_slice = slice(3, 8)
+    pixel_group = pic[:, x_slice]
+    assert_allclose(pixel_group.array, array[x_slice, :])
 
 
 def test_slicing():
-    cut = 40
-    pic = novice.open(IMAGE_PATH)
+    h, w = 3, 12
+    array = _array_2d_to_RGB(np.linspace(0, 255, h * w).reshape(h, w))
+    array = array.astype(np.uint8)
+
+    pic = novice.Picture(array=array)
+    pic_orig = novice.Picture(array=array.copy())
+
+    # Move left cut of image to the right side.
+    cut = 5
     rest = pic.width - cut
     temp = pic[:cut, :]
+    temp.array = temp.array.copy()
     pic[:rest, :] = pic[cut:, :]
     pic[rest:, :] = temp
 
-    pic_orig = novice.open(IMAGE_PATH)
+    for pixel, pixel_orig in zip(pic[rest:, :], pic_orig[:cut, :]):
+        assert pixel.rgb == pixel_orig.rgb
 
-    # Check center line
-    half_height = int(pic.height/2)
-    for p1 in pic_orig[rest:, half_height]:
-        for p2 in pic[:cut, half_height]:
-            assert p1.rgb == p2.rgb
-
-    for p1 in pic_orig[:cut, half_height]:
-        for p2 in pic[rest:, half_height]:
-            assert p1.rgb == p2.rgb
+    for pixel, pixel_orig in zip(pic[:rest, :], pic_orig[cut:, :]):
+        assert pixel.rgb == pixel_orig.rgb
 
 
 @raises(IndexError)