build array list

[Gribouillis]

Here is how I would do with pathlib

from pathlib import Path

def find_case_insensitive(dirname, extensions):
    sufset = set(x.lower() for x in extensions)
    return [f for f in Path(dirname).iterdir()
            if f.suffix.lower() in sufset]

if __name__ == '__main__':
    working_dir = Path('..')/'2018-01'
    print(working_dir)
    print(find_case_insensitive(working_dir, ['.py', '.csv']))

Actually, it would be more python3ish to return an iterable from find_case_insensitive()
instead of a list.