How to Add a Key to a Dictionary in Python

If you want to add a new key to a Python dictionary, the simplest way is to assign a value with square brackets.

This page shows you how to:

Add one new key-value pair to a dictionary
Update a key if it already exists
Choose between bracket assignment and update()
Understand what happens when the key is already present

Quick answer

person = {"name": "Ana"}
person["age"] = 25

print(person)
# {'name': 'Ana', 'age': 25}

Use square brackets with a new key name. If the key already exists, the value is replaced.

What this page helps you do

Add one new key-value pair to a dictionary
Update a key if it already exists
Choose between bracket assignment and update()
Understand what happens when the key is already present

If you are new to dictionaries, see Python dictionaries explained first.

Add a key with square brackets

The most direct way to add a key is:

my_dict[key] = value

If the key does not exist, Python creates it.

If the key already exists, Python updates its value.

Example:

student = {"name": "Liam"}
student["grade"] = "A"

print(student)
# {'name': 'Liam', 'grade': 'A'}

In this example:

"grade" is the new key
"A" is the value stored for that key

You can also update an existing key the same way:

student = {"name": "Liam", "grade": "A"}
student["grade"] = "B"

print(student)
# {'name': 'Liam', 'grade': 'B'}

This does not create a second "grade" key. It replaces the old value.

If you need a refresher on creating dictionaries, see creating a dictionary in Python.

Add multiple keys with `update()`

Use update() when you want to add several items at once.

Example:

person = {"name": "Ana"}
person.update({"age": 25, "city": "Madrid"})

print(person)
# {'name': 'Ana', 'age': 25, 'city': 'Madrid'}

Here, update() takes another dictionary and adds its key-value pairs.

It is useful when:

You want to add several keys in one step
You already have another dictionary
You want a short way to merge small sets of values

Example with an existing key:

person = {"name": "Ana", "age": 20}
person.update({"age": 25, "city": "Madrid"})

print(person)
# {'name': 'Ana', 'age': 25, 'city': 'Madrid'}

Notice that "age" was overwritten.

If you want more detail, see the Python dictionary update() method.

What happens if the key already exists

A Python dictionary cannot store duplicate keys.

If you assign the same key again, Python replaces the old value.

Example:

product = {"name": "Book", "price": 10}
product["price"] = 12

print(product)
# {'name': 'Book', 'price': 12}

Important points:

Dictionaries keep one value per key
Reusing a key updates the value
Python does not warn you when a value is overwritten

This is a common source of bugs for beginners, especially when the key name is typed incorrectly or reused by mistake.

Add a key only if it is missing

Sometimes you want to add a key only if it does not already exist.

A simple beginner-friendly pattern is:

if key not in my_dict:
    my_dict[key] = value

Example:

user = {"name": "Mina"}

if "age" not in user:
    user["age"] = 30

print(user)
# {'name': 'Mina', 'age': 30}

This helps you avoid overwriting existing data.

Another example:

user = {"name": "Mina", "age": 22}

if "age" not in user:
    user["age"] = 30

print(user)
# {'name': 'Mina', 'age': 22}

Because "age" is already present, the value stays the same.

If you want to learn this check in more detail, read how to check if a key exists in a dictionary in Python.

Common beginner mistakes

Here are some common problems when adding keys to dictionaries.

Using `append()` on a dictionary

append() is a list method, not a dictionary method.

This is wrong:

data = {"name": "Ana"}
# data.append("age", 25)

Use bracket assignment instead:

data = {"name": "Ana"}
data["age"] = 25

print(data)
# {'name': 'Ana', 'age': 25}

Forgetting that keys must be unique

This code does not create two "age" keys:

person = {"age": 20}
person["age"] = 25

print(person)
# {'age': 25}

The old value is replaced.

Expecting `update()` to keep old values for the same key

update() also overwrites matching keys:

settings = {"theme": "light"}
settings.update({"theme": "dark"})

print(settings)
# {'theme': 'dark'}

Using a variable name as a string by mistake

This mistake is easy to make:

key_name = "age"
person = {"name": "Ana"}

person[key_name] = 25
print(person)
# {'name': 'Ana', 'age': 25}

That uses the value inside key_name, which is "age".

But this is different:

key_name = "age"
person = {"name": "Ana"}

person["key_name"] = 25
print(person)
# {'name': 'Ana', 'key_name': 25}

Now the actual key is the text "key_name".

If your dictionary is not changing the way you expect, these quick checks can help:

print(my_dict)
print(my_dict.keys())
print("age" in my_dict)
print(type(my_dict))

These checks help you:

See the current dictionary contents
See all keys
Check whether a key exists
Confirm that the object is really a dictionary

If you run into missing-key problems while reading values, see how to access values in a dictionary in Python and how to fix KeyError when accessing dictionary values.

FAQ

How do I add a new key to a dictionary in Python?

Use square bracket assignment:

my_dict["new_key"] = value

What if the key already exists?

Python updates the value for that key instead of creating a second copy.

How do I add many keys at once?

Use update() with another dictionary:

my_dict.update({"a": 1, "b": 2})

Can a Python dictionary have duplicate keys?

No. Each key must be unique. Reusing a key replaces the old value.

How do I add a key only if it does not exist?

Check first, then assign:

if key not in my_dict:
    my_dict[key] = value