The reason why you get your error is because a "1 by n" matrix is different from an array of length n.
I recommend using hstack() and vstack() instead.
Like this:
import numpy as np
a = np.arange(32).reshape(4,8) # 4 rows 8 columns matrix.
b = a[:,-1:] # last column of that matrix.
result = np.hstack((a,b)) # stack them horizontally like this:
#array([[ 0, 1, 2, 3, 4, 5, 6, 7, 7],
# [ 8, 9, 10, 11, 12, 13, 14, 15, 15],
# [16, 17, 18, 19, 20, 21, 22, 23, 23],
# [24, 25, 26, 27, 28, 29, 30, 31, 31]])
Notice the repeated "7, 15, 23, 31" column.
Also, notice that I used a[:,-1:] instead of a[:,-1]. My version generates a column:
array([[7],
[15],
[23],
[31]])
Instead of a row array([7,15,23,31])
Edit: append() is much slower. Read this answer.