How can I print the number of unique elements in a list?

[AnOddGirl]

Using the dictionary below, I'm supposed to create a function that returns the number of unique items in the "items" list.

shopping_cart = {
    "tax": .08,
    "items": [
        {
            "title": "orange juice",
            "price": 3.99,
            "quantity": 1
        },
        {
            "title": "rice",
            "price": 1.99,
            "quantity": 3
        },
        {
            "title": "beans",
            "price": 0.99,
            "quantity": 3
        },
        {
            "title": "chili sauce",
            "price": 2.99,
            "quantity": 1
        },
        {
            "title": "chocolate",
            "price": 0.75,
            "quantity": 9
        }
    ]
}

I've created the code below which outputs the number of items (5), but I need the code to only count unique items. For example, if two items in this list were identical, it should return 4 (since only 4 items would be unique in that case). How can I accomplish this?

def get_tax_rate(d):
    print (d['tax'])

Output:
5