In [41]: a = np.array([1, 2, 3])

    In [42]: b = np.array([4, 5, 6])

Horizontal stack:

    In [44]: np.hstack([a, b])
    Out[44]: array([1, 2, 3, 4, 5, 6])

Vertical stack:

    In [45]: np.vstack([a, b])
    Out[45]:
    array([[1, 2, 3],
           [4, 5, 6]])

Column stack:

    In [46]: np.column_stack([a, b])
    Out[46]:
    array([[1, 4],
           [2, 5],
           [3, 6]])