Question

Am I doing something wrong here?

>>> import numpy as np
>>> dt = np.dtype({"names": ["First"], 
                   "formats": [np.uint32], 
                   "offsets": [3], 
                   "itemsize": 8})
>>> dt.itemsize
7

Answer 1

It looks like a bug to me. But if you just need n bytes of padding at the end a composite type may help:

import numpy as np
dt = np.dtype({"names": ["First", "_"], 
               "formats": [np.uint32, (np.void,1)], 
               "offsets": [3,7],
})
arr = np.empty( (5,), dtype=dt )