Concatenate a NumPy array to another NumPy array

Well, the error message says it all: NumPy arrays do not have an append() method. There's a free function numpy.append() however:

numpy.append(M, a)

This will create a new array instead of mutating M in place. Note that using numpy.append() involves copying both arrays. You will get better performing code if you use fixed-sized NumPy arrays.

In [1]: import numpy as np

In [2]: a = np.array([[1, 2, 3], [4, 5, 6]])

In [3]: b = np.array([[9, 8, 7], [6, 5, 4]])

In [4]: np.concatenate((a, b))
Out[4]: 
array([[1, 2, 3],
       [4, 5, 6],
       [9, 8, 7],
       [6, 5, 4]])

or this:

In [1]: a = np.array([1, 2, 3])

In [2]: b = np.array([4, 5, 6])

In [3]: np.vstack((a, b))
Out[3]: 
array([[1, 2, 3],
       [4, 5, 6]])