how is this working?

how_is_this_working.py

if args.random:
    def get_data_structures():
        # while sets do not preserve the order of insertion, they are are much faster
        return set()
    def add_entries(data_set,data):
        # adding addresses to a set which prevents duplicate entries, but does not preserve order
        data_set.add(data)
else:
    def get_data_structures():
        # from python3.6, dictionaries preserve the order of insertion
        return {}
    def add_entries(data_set,data):
        # adding items as keys to a dictionary which prevents duplicate entries and preserves order
        data_set.__setitem__(data, True)

test = get_data_structures()

with open(test_file, 'r') as f:
    [add_entries(test, line.strip()) for line in f]

print(test)

Am I not modifying a variable 'test' outside the global scope?