Add, Update and Delete Dictionary Items in Python

Add a New Key or Change a Value

To add an item to a dictionary, assign a value to a new key: ages["Lucky"] = 22. The very same line changes the value when the key already exists.

Think of it like a whiteboard of names and scores: writing a new name adds a line, and writing on an existing name rubs out the old score and puts in the new one.

12345ages = {"Sunny": 25, "Rocky": 30}
ages["Lucky"] = 22
print(ages)
ages["Sunny"] = 26
print(ages)

The rules

• d[key] = value with a new key adds a pair at the end.
• d[key] = value with a key that's already there replaces its value. The key keeps its place.
• A dictionary is changed in place, like a list with append() in chapter 9. No new dictionary is made.

Add up with +=

+= from chapter 5 works on a value too. Here, 5 more apples arrive at the shop:

123stock = {"apple": 10, "mango": 4}
stock["apple"] += 5
print(stock)

Avoid: += on a key that isn't there

+= first reads the old value. If there is no old value, Python stops:

123stock = {"apple": 10}
stock["kiwi"] += 5
print(stock)

To add a brand-new key, use =, not +=: stock["kiwi"] = 5. Or use get() with a default from the last topic, which works either way: stock["kiwi"] = stock.get("kiwi", 0) + 5.

Common beginner doubt: "How do I rename a key?" There's no rename. Add the new key with the old value, then delete the old key. You'll learn deleting two lessons from now.

d[key] = value adds the key if it's new and changes its value if it's already there.

Update a Dictionary with update()

The update() method adds the pairs of one dictionary to another in one go. Keys that are new get added; keys that already exist get the new value.

Think of it like updating your contacts from a friend's list: new names are added, and if a name is already there, its number is replaced with the newer one.

123prices = {"apple": 40, "mango": 60}
prices.update({"mango": 70, "kiwi": 25})
print(prices)

Mango already existed, so its price changed from 60 to 70. Kiwi was new, so it was added at the end.

The rules

• d.update(other) puts every pair of other into d.
• New keys are added at the end; existing keys keep their place and get the new value.
• other doesn't change.
• update() changes d in place and gives back None, just like sort() on a list in chapter 9.

One call or several lines

These two do the same thing:

12345678profile = {"name": "Sunny"}
profile.update({"age": 25, "city": "Pune"})
print(profile)

same = {"name": "Sunny"}
same["age"] = 25
same["city"] = "Pune"
print(same)

For one or two keys, d[key] = value is easiest to read. When you already have a second dictionary, like new prices from a supplier, update() is the shortcut.

Avoid: storing what update() gives back

123prices = {"apple": 40}
prices = prices.update({"kiwi": 25})
print(prices)

update() gives back None, and that line stores it in prices, so the whole dictionary is lost! Call it on its own line, without prices =.

d.update(other) adds the new keys and replaces the values of the old ones, in place.

Remove Dictionary Items with pop() and del

To remove an item from a dictionary, use pop(key) or del d[key]. Both take out the key together with its value. pop() also gives you the value it removed.

Think of it like taking a coat back from the coat check: with pop() you hand in the ticket and get the coat back in your hands. With del, the coat is simply thrown away.

pop(): remove and keep the value

1234cart = {"apple": 3, "bread": 1, "milk": 2}
gone = cart.pop("bread")
print("Removed", gone, "bread")
print(cart)

You've met pop() on lists in chapter 9. On a dictionary, you give it a key, not an index.

del: remove and forget

123cart = {"apple": 3, "bread": 1, "milk": 2}
del cart["apple"]
print(cart)

A missing key

Both stop with a KeyError when the key isn't there:

12cart = {"apple": 3}
cart.pop("cake")

pop() has a safe version, like get(): give it a default, and it gives that back instead of an error.

123cart = {"apple": 3}
print(cart.pop("cake", 0))
print(cart)

The rules

Common beginner doubt: "Should I use pop() or del?" Use pop() when you need the value you're removing, like the quantity of the item you take out of the cart. Use del when you don't. Or check with in first, then del.

pop(key) removes a pair and gives back its value; del d[key] only removes it.

Build a Dictionary from Input

You can build a dictionary step by step: start with an empty {}, then add one pair in every round of a loop. It's the same pattern you used to build a list with append() in chapter 10.

Think of it like a teacher filling in a register: the page starts empty, and each student adds one line with their name and mark.

123456marks = {}
for i in range(3):
    name = input()
    mark = int(input())
    marks[name] = mark
print(marks)

Each round reads two lines, a name and a mark, and adds them as one pair.

The pattern

How many pairs?

Often the first line of input says how many pairs follow, just like the questions in chapter 10:

123456count = int(input())
prices = {}
for i in range(count):
    item = input()
    prices[item] = int(input())
print(len(prices), "items:", prices)

Avoid: making the dictionary inside the loop

If prices = {} sits inside the loop, every round starts a new, empty dictionary, and only the last pair survives:

12345for i in range(2):
    prices = {}
    item = input()
    prices[item] = int(input())
print(prices)

No error, but tea is gone. Make the empty dictionary before the loop.

Common beginner doubt: "What if the same name is typed twice?" Then the second mark replaces the first, because keys are unique. That's often exactly what you want, like a newer price replacing an old one.

Start with {} before the loop, add d[key] = value inside it, use the dictionary after it.

Practice: Change items

Solve these to lock in what you learned in this topic. Each one runs in your browser.

1. 114.Python Program to Add or Update a Mark in a DictionaryEasySolve
2. 115.Python Program to Remove an Item from a Shopping CartMediumSolve
3. 116.Python Program to Build a Price List from InputMediumSolve