Calculate subsets of elements in an array

subsets.py

import copy
import string

alphabet_string = list(string.ascii_lowercase)


def subsets(arr):
    """
    Calculate subsets of elements in an array ..
    arr = ['a', 'b']
    output = ['', 'a', 'b', 'ab',]
    """
    expected = 2**len(arr)
    print(f'result of N integer is  2**N ==> {expected}')

    data_box = [arr[0]]
    prev_box = []

    for i in arr[1:]:
        prev_box = copy.deepcopy(data_box)
        for j in data_box:
            prev_box.append(i + "," +j)
        prev_box.append(i)
        data_box, prev_box = prev_box, []

    print(data_box)
    print(f'{len(data_box)}==== {expected}')


subsets(alphabet_string)