How to insert list of ndarray lists to a newndarray in python [duplicate]

I have a numpy feature_matrix as below.

feature_matrix = [[0.1,0.3,0.5], [0.7,0.8,0.1]]

Now I want to add 0, 1, 2, so on at the beginning of this feature matrix as below.

altered_feature_matrix = [[0, 0.1,0.3,0.5], [1, 0.7,0.8,0.1]]

I tried to do it using np.insert. However, then I get a flat list.

for l in write_fw_feature_matrix:)
    l=np.insert(l,[0],pos)
    altered_feature_matrix = np.insert(altered_feature_matrix, l)
    #print(l)
    pos = pos+1
print(altered_feature_matrix)

Please help me to resolve my problem.