Python numpy array of numpy arrays

Question

I have a requirement. I need to create a numpy array of numpy arrays and for that I need a loop. It should be something like this

a=np.array([])
while(...):
   ...
   b= //a numpy array generated
   a=np.append(a,b)
   ...

My output:

[1,5,3,9,10,1,... 4,8,6]

Desired output:

[[1,5,3], [9,10,1], ..., [4,8,6]]

What am I doing wrong? Is it even possible?