Creating a directory with strings of arrays as the name

I simply need to create a directory in python using

os.system('mkdir ' + sub_dir)

with

sub_dir = ('x_y_prior=' + str(not use_est_centroid) +
          '_randomized_x_y=' + str(randomize) +
          '_sep=' + str(separation) +
          '_num_trial_arr=' + str(num_trial_arr) + 
          '_seed_arr=' + str(seed_arr) +
          '_image_params=' + str(image_params) + 
          '_obj_a_info=' + str(obj_a) +
          '_obj_b_info=' + str(obj_a))

The variables consist of arrays. Now the problem is that when I attempt this, the command creates many directories with the individual elements of the each array as a name.

For example, if

separation = [1.2,1.4]

then I will get two directories named

1.2/ 

and

1.4/

I simply want one directory with the name of the entire string above.