Comparing items of case-insensitive list

Question

So I have two lists, with items in varying cases. Some of the items are the same but are lowercase/uppercase. How can I create a loop to run through all the items and do something with each item, that also ignores what case the item may have?

fruits = ['Apple','banana','Kiwi','melon']
fruits_add = ['apple','Banana','KIWI','strawberry']

I want to create a loop that goes through each item in fruits_add and adds it to fruits , if the item is not already in fruits . However an item like 'apple' and 'Apple' need to count as the same item.

I understand how to convert individual items to different cases and how to check if one specific item is identical to another (disregarding case). I don't know how to create a loop that just does this for all items.

Found answers to similar questions for other languages, but not for Python 3.

My attempt:

for fruit.lower() in fruits_add:
    if fruit in fruits:
        print("already in list")

This gives me an error:

SyntaxError: can't assign to function call

I've also tried converting every item in each list to lowercase before comparing the lists, but that doesn't work either.

Answer 1

fruit.lower() in the for loop won't work as the error message implies, you can't assign to a function call..

What you could do is create an auxiliary structure ( set here) that holds the lowercase items of the existing fruits in fruits , and, append to fruits if a fruit.lower() in fruit_add isn't in the t set (containing the lowercase fruits from fruits ):

t = {i.lower() for i in fruits}
for fruit in fruits_add:
    if fruit.lower() not in t:
        fruits.append(fruit)

With fruits now being:

print(fruits)
['Apple', 'banana', 'Kiwi', 'melon', 'strawberry']

Answer 2

I wouldn't use the lower() function there. Use lower like this:

for fruit in fruits_add:
    if fruit.lower() in fruits:
        print("already in list")