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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 that maps one item (the key) to another (the value).

In the example below, the keys represent fruits, and the values represent their corresponding 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, which access items by position, dictionaries retrieve data using **keys**.

This makes your code easier to read and maintain, especially when working with labeled information.

For instance, if you want to store details about 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 particularly useful when working with labeled data, such as user profiles, configuration settings, or grouped information.

They are also essential for handling JSON-like data and API responses.

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.