Combining two lists and removing duplicates, without removing duplicates in original list

Question

I have two lists that i need to combine where the second list has any duplicates of the first list ignored. .. A bit hard to explain, so let me show an example of what the c

Answer 1

You need to append to the first list those elements of the second list that aren't in the first - sets are the easiest way of determining which elements they are, like this:

first_list = [1, 2, 2, 5]
second_list = [2, 5, 7, 9]

in_first = set(first_list)
in_second = set(second_list)

in_second_but_not_in_first = in_second - in_first

result = first_list + list(in_second_but_not_in_first)
print(result)  # Prints [1, 2, 2, 5, 9, 7]

Or if you prefer one-liners 8-)

print(first_list + list(set(second_list) - set(first_list)))