RF: add AffineError

Change append_diag to use AffineError

Author: matthew-brett

nibabel/affines.py

@@ -8,6 +8,13 @@
 from .externals.six.moves import reduce
 
 
+class AffineError(ValueError):
+    """ Errors in calculating or using affines """
+    # Inherits from ValueError to keep compatibility with ValueError previously
+    # raised in append_diag
+    pass
+
+
 def apply_affine(aff, pts):
     """ Apply affine matrix `aff` to points `pts`
 
@@ -213,7 +220,7 @@ def append_diag(aff, steps, starts=()):
     if len(starts) == 0:
         starts = np.zeros(n_steps, dtype=steps.dtype)
     elif len(starts) != n_steps:
-        raise ValueError('Steps should have same length as starts')
+        raise AffineError('Steps should have same length as starts')
     old_n_out, old_n_in = aff.shape[0] - 1, aff.shape[1] - 1
     # make new affine
     aff_plus = np.zeros((old_n_out + n_steps + 1,

nibabel/tests/test_affines.py

@@ -3,8 +3,8 @@
 
 import numpy as np
 
-from ..affines import (apply_affine, append_diag, to_matvec, from_matvec,
-                       dot_reduce)
+from ..affines import (AffineError, apply_affine, append_diag, to_matvec,
+                       from_matvec, dot_reduce)
 
 
 from nose.tools import assert_equal, assert_raises
@@ -123,7 +123,7 @@ def test_append_diag():
                         [0, 0, 0, 5, 9],
                         [0, 0, 0, 0, 1]])
     # Length of starts has to match length of steps
-    assert_raises(ValueError, append_diag, aff, [5, 6], [9])
+    assert_raises(AffineError, append_diag, aff, [5, 6], [9])
 
 
 def test_dot_reduce():