Shift elements in a numpy array

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Not numpy but scipy provides exactly the shift functionality you want,

import numpy as np
from scipy.ndimage.interpolation import shift

xs = np.array([ 0.,  1.,  2.,  3.,  4.,  5.,  6.,  7.,  8.,  9.])

shift(xs, 3, cval=np.NaN)

where default is to bring in a constant value from outside the array with value cval, set here to nan. This gives the desired output,

array([ nan, nan, nan, 0., 1., 2., 3., 4., 5., 6.])

and the negative shift works similarly,

shift(xs, -3, cval=np.NaN)

Provides output 

array([  3.,   4.,   5.,   6.,   7.,   8.,   9.,  nan,  nan,  nan])

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