Generating all possible combinations of characters in a string

Question

Say I have a string list:

li = ['a', 'b', 'c']

I would like to construct a new list such that each entry of the new list is a concatenation of a selection of 3 entries in the original list. Note that each entry can be chosen repeatedly:

new_li=['abc', 'acb', 'bac', 'bca', 'cab', 'cba', 'aab', 'aac',....'aaa', 'bbb', 'ccc']

The brutal force way is to construct a 3-fold nested for loop and insert each 3-combination into the new list. I was wondering if there is any Pythonic way to deal with that? Thanks.

Update: Later I will convert the new list into a set, so the order does not matter anyway.

Solution 1

This looks like a job for itertools.product.

import itertools

def foo(l):
     yield from itertools.product(*([l] * 3)) 

for x in foo('abc'):
     print(''.join(x))

aaa
aab
aac
aba
abb
abc
aca
acb
acc
baa
bab
bac
bba
bbb
bbc
bca
bcb
bcc
caa
cab
cac
cba
cbb
cbc
cca
ccb
ccc

---

yield from is available to you from python3.3 and beyond. For older version, yield within a loop:

def foo(l):
     for i in itertools.product(*([l] * 3)) :
         yield i

Solution 2

The best way to get all combinations (also called cartesian product) of a list is to use itertools.product using the len of your iterable as repeat argument (that's where it differs from the other answer):

from itertools import product
li = ['a', 'b', 'c']
for comb in product(li, repeat=len(li)):
    print(''.join(comb))

or if you want the result as list:

>>> combs = [''.join(comb) for comb in product(li, repeat=len(li))]
>>> combs
['aaa', 'aab', 'aac', 'aba', 'abb', 'abc', 'aca', 'acb', 'acc', 'baa', 
 'bab', 'bac', 'bba', 'bbb', 'bbc', 'bca', 'bcb', 'bcc', 'caa', 'cab', 
 'cac', 'cba', 'cbb', 'cbc', 'cca', 'ccb', 'ccc']

It's a bit cleaner to use the repeat argument than to multiply and unpack the list you have manually.

Solution 3

An alternate approach using list comprehension:

li = ['a', 'b', 'c']

new_li = [a+b+c for a in li for b in li for c in li]