def permute_string(str):  
    if len(str) == 0:  
        return ['']  
    prev_list = permute_string(str[1:len(str)])  
    next_list = []  
    for i in range(0,len(prev_list)):  
        for j in range(0,len(str)):  
            new_str = prev_list[i][0:j]+str[0]+prev_list[i][j:len(str)-1]  
            if new_str not in next_list:  
                next_list.append(new_str)  
    return next_list  
  
print(permute_string('ABCD'));