Improve efficiency: load img0 once, del reference, and don't check affine on first image.

Author: Ben Cipollini

nibabel/funcs.py

@@ -107,26 +107,27 @@ def concat_images(images, check_affines=True, axis=None):
        New image resulting from concatenating `images` across last
        dimension
     '''
+
     n_imgs = len(images)
-    img0 = images[0]
-    if not hasattr(img0, 'get_data'):
-        img0 = load(img0)
-    affine = img0.affine
-    header = img0.header
-    i0shape = img0.shape
-    del img0
-
-    if axis is None:  # collect images in output array for efficiency
-        out_shape = (n_imgs, ) + i0shape[:3]
-        out_data = np.empty(out_shape)
-    else:  # collect images in list for use with np.concatenate
-        out_data = [None] * n_imgs
+    if n_imgs == 0:
+        raise ValueError('Cannot concatenate an empty list of images.')
 
     for i, img in enumerate(images):
         if not hasattr(img, 'get_data'):
             img = load(img)
 
-        if check_affines and not np.all(img.affine == affine):
+        if i == 0:  # first image, initialize data from loaded image
+            affine = img.affine
+            header = img.header
+            klass = img.__class__
+
+            if axis is None:  # collect images in output array for efficiency
+                out_shape = (n_imgs, ) + img.shape[:3]
+                out_data = np.empty(out_shape)
+            else:  # collect images in list for use with np.concatenate
+                out_data = [None] * n_imgs
+
+        elif check_affines and not np.all(img.affine == affine):
             raise ValueError('Affines do not match')
 
         # Special case for 4D image with size[3] == 1; reshape to work!
@@ -148,7 +149,6 @@ def concat_images(images, check_affines=True, axis=None):
                         for di, data in enumerate(out_data)]
         out_data = np.concatenate(out_data, axis=axis)
 
-    klass = img0.__class__
     return klass(out_data, affine, header)
 
 

nibabel/tests/test_funcs.py

@@ -30,6 +30,8 @@ def _as_fname(img):
 
 
 def test_concat():
+    # Smoke test: concat empty list.
+    assert_raises(ValueError, concat_images, [])
 
     # Build combinations of 3D, 4D w/size[3] == 1, and 4D w/size[3] == 3
     all_shapes_3D = ((1, 2, 5), (7, 3, 1), (13, 11, 11), (0, 1, 1))
@@ -128,6 +130,7 @@ def test_concat():
                             assert_true(expect_error, ve.message)
 
 
+
 def test_closest_canonical():
     arr = np.arange(24).reshape((2,3,4,1))
     # no funky stuff, returns same thing