How to get all combinations of several letters in Python? -

how can combinations of several letters in python 3 without using itertools? example, have a,b,c, how possible combinations including a,b,c,ab,ba,ac,ca,bc,cb,abc,acb,bac,bca,cab,cba?

itertools.combinations equal following functions pyhtoinic way go calculating combinations :

def combinations(iterable, r):     # combinations('abcd', 2) --> ab ac ad bc bd cd     # combinations(range(4), 3) --> 012 013 023 123     pool = tuple(iterable)     n = len(pool)     if r > n:         return     indices = list(range(r))     yield tuple(pool[i] in indices)     while true:         in reversed(range(r)):             if indices[i] != + n - r:                 break         else:             return         indices[i] += 1         j in range(i+1, r):             indices[j] = indices[j-1] + 1         yield tuple(pool[i] in indices) 

if don't want import itertools module can use function.