? Python Add Dictionary Items

⚡ Quick Overview

In Python, dictionaries are mutable collections of key–value pairs. You can easily add new items to a dictionary using:

Direct assignment for a single key–value pair.
update() to add multiple items or merge dictionaries.

? Key Concepts

A dictionary stores data as key: value pairs.
Adding to a dictionary does not change its identity (it is mutable).
Assigning a value to a new key automatically adds that key to the dictionary.
update() can add multiple key–value pairs at once.
If a key already exists, both assignment and update() will overwrite its value.

? Syntax & Theory

1) Add a single item by assignment

dictionary[new_key] = value

2) Add multiple items using update()

dictionary.update({key1: value1, key2: value2})

➕ Adding a Single Item

You can add a new key–value pair to a dictionary by assigning a value to a key that does not exist yet.

? View Code Example – Single Item

person = {"name": "Alice", "age": 25}
person["city"] = "New York"

print(person)
# {'name': 'Alice', 'age': 25, 'city': 'New York'}

? Adding Multiple Items with update()

The update() method allows you to add multiple key–value pairs at once from another dictionary or iterable of pairs.

? View Code Example – Multiple Items

person = {"name": "Alice", "age": 25, "city": "New York"}

person.update({
    "email": "alice@example.com",
    "phone": "123-456-7890"
})

print(person)
# {'name': 'Alice', 'age': 25, 'city': 'New York', 'email': 'alice@example.com', 'phone': '123-456-7890'}

? Merging Dictionaries with update()

When the same key appears in both dictionaries, the value from the dictionary passed to update() wins.

? View Code Example – Merging

defaults = {"theme": "light", "font": "Arial", "size": 12}
user_settings = {"theme": "dark", "size": 14}

defaults.update(user_settings)

print(defaults)
# {'theme': 'dark', 'font': 'Arial', 'size': 14}

? Live Output & Explanation

For the single-item example:

Start: {"name": "Alice", "age": 25}
After person["city"] = "New York", the key "city" is added.

Printed dictionary:

{'name': 'Alice', 'age': 25, 'city': 'New York'}

For the update() example:

The call to person.update({...}) adds both "email" and "phone" in one step. If any of those keys already existed, their values would be replaced with the new ones.

For the merging example:

"theme" changes from "light" to "dark".
"size" changes from 12 to 14.
"font" remains "Arial" because it is not present in user_settings.

✅ Tips & Best Practices

Use direct assignment when adding a single new key–value pair.
Use update() when adding multiple items or merging two dictionaries.
Remember that dictionaries are mutable, so changes happen in place.
Be aware that assigning to an existing key or using update() on it will overwrite its value.
When merging settings or configuration, decide which dictionary should have the final say on conflicting keys.

? Try It Yourself

Create a dictionary called student with keys like "name" and "roll_no". Add a new key "grade" using assignment.
Use update() to add "email" and "phone" to the same student dictionary.
Make two dictionaries, defaults and custom, then merge them with defaults.update(custom) and observe which values are kept.
Experiment by calling update() with a list of tuples, e.g. person.update([("country", "USA")]).