have the correct shape.

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

>>> np.append([[1, 2, 3], [4, 5, 6]], [7, 8, 9], axis=0)
Traceback (most recent call last):
    ...
ValueError: all the input arrays must have same number of dimensions, but
the array at index 0 has 2 dimension(s) and the array at index 1 has 1
dimension(s)

>>> a = np.array([1, 2], dtype=int)
>>> c = np.append(a, [])
>>> c
array([1., 2.])
>>> c.dtype
float64

Default dtype for empty ndarrays is `float64` thus making the output of dtype
`float64` when appended with dtype `int64`

rg