TST: add test for new numpy memmap behavior

memmap behavior is about to change with merge of
numpy/numpy#7406.

Test for behavior change so we can adapt expected mmap return values on
an image.

Author: matthew-brett

nibabel/testing/__init__.py

@@ -29,6 +29,8 @@
 data_path = abspath(pjoin(dirname(__file__), '..', 'tests', 'data'))
 
 
+from .np_features import VIRAL_MEMMAP
+
 def assert_dt_equal(a, b):
     """ Assert two numpy dtype specifiers are equal
 

nibabel/testing/np_features.py

@@ -0,0 +1,19 @@
+""" Look for changes in numpy behavior over versions
+"""
+
+import numpy as np
+
+
+def _memmap_after_ufunc():
+    """ Return True if memmap arrays always return memmap from ufuncs
+
+    This should be True for numpy < 1.12, False otherwise.
+    """
+    with open(__file__, 'rb') as fobj:
+        mm_arr = np.memmap(fobj, mode='r', shape=(10,), dtype=np.uint8)
+        mm_preserved = isinstance(mm_arr + 1, np.memmap)
+    return mm_preserved
+
+
+# True if ufunc on memmap retuns a memmap
+VIRAL_MEMMAP = _memmap_after_ufunc()