Numpy: use reshape or newaxis to add dimensions

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One reason to use numpy.newaxis over ndarray.reshape is when you have more than one “unknown” dimension to operate with. So, for example, for the following array:

>>> arr.shape
(10, 5)

This works:

>>> arr[:, np.newaxis, :].shape
(10, 1, 5)

But this does not:

>>> arr.reshape(-1, 1, -1)
...
ValueError: can only specify one unknown dimension

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