> ## Documentation Index
> Fetch the complete documentation index at: https://python4ai.codewithsiva.dev/llms.txt
> Use this file to discover all available pages before exploring further.

# Dictionaries

> Store data with key-value pairs

## What are dictionaries?

Dictionaries store data in key-value pairs. Think of them like a real dictionary where you look up a word (key) to find its definition (value).

Real-world examples:

* Phone book: name > phone number
* Menu: dish > price
* User profile: username > user info

## Creating dictionaries

```python theme={null}
# Empty dictionary
my_dict = {}

# Dictionary with data
person = {
    "name": "Alice",
    "age": 30,
    "city": "New York"
}

# Different ways to create
scores = dict(math=95, english=87, science=92)
```

<Note>
  Dictionaries use curly braces `{}` with key-value pairs separated by colons. Keys must be unique!
</Note>

## Accessing values

```python theme={null}
person = {"name": "Alice", "age": 30, "city": "New York"}

# Get values by key
print(person["name"])       # "Alice"
print(person["age"])        # 30

# Safer with get()
print(person.get("job"))    # None (no error)
print(person.get("job", "Unknown"))  # "Unknown" (default)
```

## Changing dictionaries

```python theme={null}
person = {"name": "Alice", "age": 30}

# Add or update
person["email"] = "alice@email.com"  # Add new
person["age"] = 31                   # Update existing

# Remove items
del person["email"]              # Remove by key
age = person.pop("age")          # Remove and return
person.clear()                   # Remove all items
```

## Dictionary methods

```python theme={null}
person = {"name": "Alice", "age": 30, "city": "New York"}

# Get all keys, values, or items
print(person.keys())    # dict_keys(['name', 'age', 'city'])
print(person.values())  # dict_values(['Alice', 30, 'New York'])
print(person.items())   # dict_items([('name', 'Alice'), ...])

# Check if key exists
if "name" in person:
    print("Name found!")

# Update multiple values
person.update({"age": 31, "job": "Engineer"})
```

### Nested dictionaries

```python theme={null}
# Dictionary of dictionaries
students = {
    "alice": {"age": 20, "grade": "A"},
    "bob": {"age": 21, "grade": "B"},
    "charlie": {"age": 19, "grade": "A"}
}

# Access nested data
print(students["alice"]["grade"])  # "A"
```

## Common mistakes

<AccordionGroup>
  <Accordion title="KeyError when key doesn't exist">
    ```python theme={null}
    # Wrong
    person = {"name": "Alice"}
    print(person["age"])  # KeyError!

    # Right - use get()
    print(person.get("age", 0))  # Returns 0 if missing
    ```
  </Accordion>

  <Accordion title="Using mutable keys">
    ```python theme={null}
    # Wrong - lists can't be keys
    bad_dict = {[1, 2]: "value"}  # TypeError!

    # Right - use immutable types
    good_dict = {(1, 2): "value"}  # Tuple is OK
    good_dict = {"1,2": "value"}   # String is OK
    ```
  </Accordion>
</AccordionGroup>

## What's next?

Let's explore tuples - like lists but unchangeable!

<Card title="Tuples" icon="lock" href="/data-structures/tuples">
  Immutable sequences
</Card>