BF+TST - ensure and test for affine, hdr isolation

Spatial images accept affines and headers as input, but they should copy
or otherwise isolate them from external changes to the affine, header
objects they have been passed.

Author: matthew-brett

nibabel/spatialimages.py

@@ -301,7 +301,8 @@ def __init__(self, data, affine, header=None,
             # Check that affine is array-like 4,4.  Maybe this is too strict at
             # this abstract level, but so far I think all image formats we know
             # do need 4,4.
-            affine = np.asarray(affine)
+            # Copy affine to  isolate from environment
+            affine = np.array(affine, copy=True)
             if not affine.shape == (4,4):
                 raise ValueError('Affine should be shape 4,4')
         self._affine = affine

nibabel/tests/test_spatialimages.py

@@ -177,6 +177,23 @@ class TestSpatialImage(TestCase):
     # class for testing images
     image_class = SpatialImage
 
+    def test_isolation(self):
+        # Test image isolated from external changes to header and affine
+        img_klass = self.image_class
+        arr = np.arange(3, dtype=np.int16)
+        aff = np.eye(4)
+        img = img_klass(arr, aff)
+        assert_array_equal(img.get_affine(), aff)
+        aff[0,0] = 99
+        assert_false(np.all(img.get_affine() == aff))
+        # header, created by image creation
+        ihdr = img.get_header()
+        # Pass it back in
+        img = img_klass(arr, aff, ihdr)
+        # Check modifying header outside does not modify image
+        ihdr.set_zooms((4,))
+        assert_not_equal(img.get_header(), ihdr)
+
     def test_images(self):
         # Assumes all possible images support int16
         # See https://github.com/nipy/nibabel/issues/58