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# Example 1: Creating a dictionary
user = {
    "name": "Alice",
    "age": 28,
    "city": "Chicago"
}

# Accessing values using keys
print("Name:", user["name"])      
print("Age:", user["age"])   

print('=' * 40)

# Example 2: Adding a new key-value pair
user["email"] = "alice@example.com"
print("Updated user:", user)

print('=' * 40)

# Example 3: Updating an existing value
user["age"] = 29
print("Updated age:", user["age"])

print('=' * 40)

# Example 4: Checking if a key exists
if "city" in user:
    print("City found:", user["city"])

# Dictionaries: Keys and Values

In Python, a *dictionary* is a collection of key-value pairs.

In the example below, the keys are the fruits and the values are the numbers.

```python title="Creating a Dictionary"
my_dict = {
  "apple": 1,
  "banana": 2,
  "cherry": 3
}
```

Dictionaries are useful when you need to store and retrieve data by a specific key.

<br/>

## Why Use Dictionaries?

Unlike lists, where you access items by position, dictionaries let you access data by **key**.

This makes your code easier to understand, especially when dealing with labeled data.

For example, if you want to store the information of a student, you can use a dictionary like this:

```python title="Student Dictionary"
student = {
  "name": "Emily",
  "age": 22,
  "major": "Biology"
}
```

In the example above, to retrieve the student's name, you would use `student["name"]`.

<br/>

## Key Concepts

* Each item in a dictionary is a *pair* of a key and a value.
* Keys must be *unique* and *immutable*.
* Values can be any type: numbers, strings, lists, or even other dictionaries.
* Use `dict[key]` to retrieve or update a value.

<br/>

## When Are Dictionaries Useful?

Dictionaries are especially useful when your data has labels, such as user profiles, settings, or grouped information.

They are also essential for working with JSON-like data structures and APIs.

Keys in a Python dictionary must be unique to avoid ambiguity when accessing values; they function like identifiers. Additionally, they must be immutable, meaning they can't change, to ensure that the dictionary remains stable and reliable during operations. This allows for consistent data retrieval using the same keys.

### In Python dictionaries, keys must be unique and immutable.