<pre> >>> import numpy as np >>> a = np.ones((10, 10)) >>> np.save('a.npy', a) >>> b = np.load('a.npy', mmap_mode='r') >>> c = b[3:] >>> c.offset 80 >>> b.offset 80 </pre> The offset of c is wrong: it does not take in account the fact that the slicing has changed the offset.