Is this the best way to add an extra dimension to a numpy array in one line of code?

It's easier like this:

k.reshape(k.shape + (1,))

But if all you want is to add an empty dimension at the end, you should use numpy.newaxis:

import numpy as np
k = k[..., np.newaxis]

or

k = k[..., None]

(See the documentation on slicing).

You can use numpy.expand_dims

In [4]: import numpy as np                                                          

In [5]: a = [1,2,3,4]                                                                              

In [6]: np.expand_dims(a, axis=1)                                                                  
Out[6]: 
array([[1],
       [2],
       [3],
       [4]])