Making list empty after return in function

[DeaD_EyE]

If you have only a nested list, not a deep nested list, you can use itertools.chain.from_iterable

from itertools import chain


flatten = chain.from_iterable
nested = [('a', 'b', 'r', 'a'),('c', 'a', 'd', 'a', 'b', 'r', 'a')]
result = list(flatten(nested))

print(result)

Output:
['a', 'b', 'r', 'a', 'c', 'a', 'd', 'a', 'b', 'r', 'a']

chain.from_iterable returns an iterator. You have to consume this iterator to result by result.
If you always want to have a list as result, you can make a function for it.

from itertools import chain


def flatten_to_list(iterable):
    return list(chain.from_iterable(iterable))


nested = [('a', 'b', 'r', 'a'),('c', 'a', 'd', 'a', 'b', 'r', 'a')]

print(flatten_to_list(nested))

Output:
['a', 'b', 'r', 'a', 'c', 'a', 'd', 'a', 'b', 'r', 'a']

Reference from Python documentation: itertools itertools-recipes

Do you like it? You should read the whole description of this module. It's very powerful. Mostly all parts of Python language have implemented iteration. The benefit is also, that you can work with "big data" or data-streams. The iterator protocol allows this. The function flatten_to_list removes this ability, because the list is constructed in memory until the iteration ends. It's not possible with infinite streams, except if you use itertools.islice(iterable, start, end).

In addition, you get back, what you want. If you want to have a set, then use set(chain.from_iterable(nested_something)).
This can be done also with tuple, list, dict.fromkeys and other types which consumes iterables.


Bonus: You can do it also with str.join

result1 = str.join('-', flatten(nested))
# or
result2 = '-'.join(flatten(nested))

print(result1)
print(result2)

Output:
a-b-r-a-c-a-d-a-b-r-a
a-b-r-a-c-a-d-a-b-r-a