using numpy to convert an int to an array of bits

Question

I need a way to convert 20 million 32 and 64-bit integers into corresponding bit arrays (so this has to be memory/time efficient). Under advice from a different question/answer here on SO, I'm attempting to do this by using numpy.unpackbits. While experimenting with this method I ran into unexpected results:

np.unpackbits(np.array([1], dtype=np.uint64).view(np.uint8))

produces:

array([0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
       0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
       0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], dtype=uint8)

I would expect the 1 element to be the last one, but not in the middle. So I'm obviously missing something that preserves the byte order. What am I missing?

Answer 1

Try: dtype='>i8', like so:

In [6]: np.unpackbits(np.array([1], dtype='>i8').view(np.uint8))
Out[6]: 
array([0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
       0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
       0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1], dtype=uint8)

Reference:

http://docs.scipy.org/doc/numpy/user/basics.byteswapping.html