Ref: scipy/scipy#10569

The s parameter for nd-fft functions is documented as being a "sequence of ints", however it is also possible to supply None in any of the elements and the corresponding default value is used. e.g.

import numpy as np
x = np.arange(10)
assert np.fft.fftn(x, s=(None,)).shape == 10

This is a consequence of numpy using the 1d fft internally

Is this behaviour intentional? If so then it doesn't seem to be documented anywhere that I could see.