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What is a dictionary in Python and how to use it?

A dictionary in Python is an unordered collection of key-value pairs, where each key is unique. Dictionaries are implemented as hash maps and are optimized for fast access to values given a key. You can think of a dictionary as a map or a table that relates keys to values.

To create a dictionary in Python, you can use curly braces **{}** and specify key-value pairs separated by a colon **:**, like this:

person = {'name': 'John Doe', 'age': 30, 'gender': 'male'}

You can access the value of a key in a dictionary using square brackets **[]** and the key itself, like this:

print(person['name'])  # Output: John Doe

You can add a new key-value pair to a dictionary using square brackets **[]** and a new key, like this:

person['country'] = 'United States'

You can remove a key-value pair from a dictionary using the **del** keyword and the key, like this:

del person['age']

You can also check if a key exists in a dictionary using the **in** operator, like this: 

if 'age' in person:
    print('Age is present.')
else:
    print('Age is not present.')

The **keys** method returns a list of all keys in the dictionary:

print(person.keys())  # Output: ['name', 'gender', 'country']

The **values** method returns a list of all values in the dictionary:

print(person.values())  # Output: ['John Doe', 'male', 'United States']

The **items** method returns a list of tuples, where each tuple contains a key-value pair:

print(person.items())  # Output: [('name', 'John Doe'), ('gender', 'male'), ('country', 'United States')]

Dictionaries are very useful in Python when you need to store key-value pairs, such as configuration settings, user information, or data that can be looked up by a key. They are flexible, fast, and easy to use, making them a staple of the Python standard library.

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