Add Key-Value Pairs to a Dictionary in Python

In Python, you can add key-value pairs to a dictionary using the following methods:

(1) Using square brackets []

You can add a key-value pair to a dictionary by assigning a value to a new key using square brackets.

For example:

# Creating an empty dictionary
person = {}

# Adding key-value pairs
person["name"] = "James"
person["age"] = 35

print(person) # Output: {'name': 'James', 'age': 35}

If the key already exists, its value will be updated.

person = {"name": "James", "age": 35}

person["age"] = 50

print(person) # Output: {'name': 'James', 'age': 50}

(2) Using update() method

The update() method allows you to add multiple key-value pairs to a dictionary at once by passing another dictionary or an iterable of key-value pairs.

For example:

person = {"name": "James"}

person.update({"age": 35, "city": "New York"})

print(person) # Output: {'name': 'James', 'age': 35, 'city': 'New York'}

If a key already exists, its value is updated; otherwise, a new key-value pair is added to the dictionary.

For example:

person = {"name": "James", "age": 35}

person.update({"age": 50, "city": "New York"})

print(person) # Output: {'name': 'James', 'age': 50, 'city': 'New York'}

(3) Using setdefault() method

The setdefault() method adds a key with a default value only if the key doesn’t already exist, and returns that value. If the key already exists, it returns the current value without modifying the dictionary.

For example:

person = {"name": "James", "age": 35}

age = person.setdefault("age", 50)
print(age) # Output: 35

city = person.setdefault("city", "New York")
print(city) # Output: New York

print(person) # Output: {'name': 'James', 'age': 35, 'city': 'New York'}

Choosing the right method

Use [] for adding a single key-value pair.

Use update() method for adding multiple key-value pairs at once.

Use setdefault() method to add a key-value pair only if the key doesn’t exist.