Receive 12 elements and separate them into 3 lists

Question

I need to receive three lists generated in the shell to process in Python:

l1=4 3 2 1
l2=2 1 3 4
l3=1 2 3 4

In the shell script step this way:

python3 orth_median.py ${o_l1[*]} ${o_l2[*]} ${o_l3[*]}

Python takes 12 numbers.

In python I had to convert the 12 numbers into 3 lists to have the 3 lists again. Is this the best way or is there another way to do it? I am beginner working with shell script.

if __name__ == "__main__":
    if len(sys.argv)>11:
        size1=len(sys.argv[1:])/3 
        l1=[sys.argv[1:][0],sys.argv[1:][1],sys.argv[1:][2],sys.argv[1:][3]]
        l2=[sys.argv[1:][4],sys.argv[1:][5],sys.argv[1:][6],sys.argv[1:][7]]
        l3=[sys.argv[1:][8],sys.argv[1:][9],sys.argv[1:][10],sys.argv[1:][11]]
        A=np.array(generate_matrix(l1, int(size1)))
        B=np.array(generate_matrix(l2, int(size1)))
        C=np.array(generate_matrix(l3, int(size1)))
        M = orth_median1(A, B, C)
        print('o_m', M)
    else:
        print('No arguments provided.') 

Answer 1

You can use the syntax of slicing to take a specific excerpt from sys.argv - remembering that the first element of sys.argv (at zero position) is the name of the script, so the numbers will be at positions 1 to 12 (ie you should check if it has at least 13 elements):

if len(sys.argv) >= 13:
    l1 = sys.argv[1:5]
    l2 = sys.argv[5:9]
    l3 = sys.argv[9:13]

Thus, l1 shall have the elements of indices 1 to 4 (the final value is not included), l2 shall be indexed 5 to 8 and l3 of indices 9 to 12.

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Another option to do the same thing:

l1, l2, l3 = [ sys.argv[n:n + 4] for n in range(1, 13, 4) ]

I use a range from 1 to 12, jumping from 4 to 4 (i.e., numbers 1, 5 and 9), and for each of these numbers, I take a Slice from number to 4 positions after. I also use a comprehensilist on, which creates a list of the 3 sub-lists, and I do the multiple assignment, which assigns these sub-lists directly to l1, l2 and l3.

With this the lists will be respectively ['4', '3', '2', '1'], ['2', '1', '3', '4'] and ['1', '2', '3', '4'].

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Not directly related, but to calculate the size1, you can use the entire split operator //, that already returns an integer value (so you don’t need to call int after). And how you are using sys.argv[1:] (that is, ignoring the first element of sys.argv), it is simpler to subtract 1 of the size, instead of creating another sub-list just for that.

And it is also possible to create arrays already A, B and C at once (if not using the lists l1, l2 and l3 for nothing else):

size1 = (len(sys.argv) - 1) // 3

# criar os arrays de uma vez
A, B, C = [ np.array(generate_matrix(sys.argv[n:n + 4], size1)) for n in range(1, 13, 4) ]