Enumerating k-mers Lexicographically solution for ROSALIND - https://rosalind.info/problems/lexf/

 f = open("out.txt", "w")  
 input_file = open("rosalind_lexf.txt", "r")  
 lines = input_file.readlines()  
 char_array = lines[0].replace('\n', '').split(' ')  
 seq_len = int(lines[1])  
 char_array.sort()  
 char_array_len = len(char_array)  
 rep_len = []  
 rep_inc_counter = []  
 rep_char_index = []  
 rep_len_i = 1  
 for i in range(seq_len):  
   rep_inc_counter.append(0)  
   rep_char_index.append(0)  
   rep_len.append(rep_len_i)  
   rep_len_i = rep_len_i * char_array_len  
 num_combination = char_array_len ** seq_len  
 C = []  
 for i in range(num_combination):  
   s = ""  
   for l in range(seq_len):   
     s = char_array[rep_char_index[l]] + s  
     rep_inc_counter[l] = rep_inc_counter[l] + 1  
     if rep_inc_counter[l] == rep_len[l]:  
       rep_char_index[l] = rep_char_index[l] + 1  
       rep_inc_counter[l] = 0   
     if rep_char_index[l]==char_array_len:  
       rep_char_index[l] = 0  
   C.append(s)  
 for s in C:  
   print(s)  
   f.write(s+'\n')  
 f.close()