Python: 3rd level nested list comprehension not acting as expected

Question

I am trying to get all possible permutations of length 1, 2 and 3 for the charactes "a", "b" and "c"

from itertools import permutations

a = ['a','b', 'c']
perm1 = permutations(a, 1)
perm2 = permutations(a, 2)
perm3 = permutations(a, 3)
p_list = []
p_list.extend(list(perm1))    
p_list.extend(list(perm2))
p_list.extend(list(perm3))

str = [[j for j in i] for i in p_list]

print(str[3])

At this point things are expected

When I modify the str = line to this str = [[[''.join(k) for k in j] for j in i] for i in p_list] I would expect to get a list of strings where each permutation is a string with no commas. eg ["abc", "a", "b", "c"] etc. but instead I get a list of lists.

Answer 1

First thing: please don't set str as variable.

Below I copied your code, commenting what p_list looks like.

from itertools import permutations

a = ['a','b', 'c']
perm1 = permutations(a, 1)
perm2 = permutations(a, 2)
perm3 = permutations(a, 3)
p_list = []
p_list.extend(list(perm1))    
p_list.extend(list(perm2))
p_list.extend(list(perm3))

# p_list = [('a',), ('b',), ('c',), ('a', 'b'), ('a', 'c'), ('b', 'a'), ('b', 'c'), ('c', 'a'), ('c', 'b'), ('a', 'b', 'c'), ('a', 'c', 'b'), ('b', 'a', 'c'), ('b', 'c', 'a'), ('c', 'a', 'b'), ('c', 'b', 'a')]

As you can see, your data structure is not 3rd level: it is only a list of tuples. Now I think it becomes clear that your list comprehension is just iterating over each element inside the tuples, not joining them like:

result = [''.join(n) for n in p_list]
# result = ['a', 'b', 'c', 'ab', 'ac', 'ba', 'bc', 'ca', 'cb', 'abc', 'acb', 'bac', 'bca', 'cab', 'cba']