Key-Value Pairs - Dictionary in Python

In Python, a dictionary is an unordered collection of key-value pairs. Dictionaries are one of the most versatile data structures in Python and are used to store and retrieve data efficiently. This article explains the concept of dictionaries, their properties, and how to use them with examples.

1. What is a Dictionary?

A dictionary in Python is a collection of items stored as key-value pairs. Each key in a dictionary is unique, and it maps to a specific value. The key-value structure allows you to quickly retrieve values based on their corresponding keys.

Example: Defining a Dictionary

    # Defining a dictionary
    person = {
        "name": "John",
        "age": 30,
        "profession": "Engineer"
    }
    print(person)  # Outputs: {'name': 'John', 'age': 30, 'profession': 'Engineer'}
        

2. Accessing Values in a Dictionary

You can access the values in a dictionary by using their corresponding keys. If the key does not exist, Python raises a KeyError.

Example: Accessing Values

    # Accessing values using keys
    person = {"name": "John", "age": 30, "profession": "Engineer"}
    print(person["name"])  # Outputs: John
    print(person["age"])   # Outputs: 30
        

Example: Handling KeyError

    # Handling a KeyError
    person = {"name": "John", "age": 30}

    try:
        print(person["address"])  # This will raise a KeyError
    except KeyError as e:
        print(f"Error: {e}")  # Outputs: Error: 'address'
        

3. Adding and Modifying Key-Value Pairs

You can add new key-value pairs to a dictionary or modify the value of an existing key by simply assigning a new value to the key.

Example: Adding and Modifying Key-Value Pairs

    # Adding a new key-value pair
    person = {"name": "John", "age": 30}
    person["profession"] = "Engineer"  # Adds the new key 'profession' with value 'Engineer'
    print(person)  # Outputs: {'name': 'John', 'age': 30, 'profession': 'Engineer'}

    # Modifying an existing key-value pair
    person["age"] = 31  # Updates the value of the 'age' key
    print(person)  # Outputs: {'name': 'John', 'age': 31, 'profession': 'Engineer'}
        

4. Removing Key-Value Pairs

To remove a key-value pair from a dictionary, you can use the del statement or the pop() method.

Example: Using del to Remove a Key-Value Pair

    # Using del to remove a key-value pair
    person = {"name": "John", "age": 30, "profession": "Engineer"}
    del person["age"]  # Removes the 'age' key-value pair
    print(person)  # Outputs: {'name': 'John', 'profession': 'Engineer'}
        

Example: Using pop() to Remove a Key-Value Pair

    # Using pop to remove a key-value pair and return the value
    person = {"name": "John", "age": 30, "profession": "Engineer"}
    age = person.pop("age")  # Removes 'age' and returns its value
    print(age)  # Outputs: 30
    print(person)  # Outputs: {'name': 'John', 'profession': 'Engineer'}
        

5. Looping Through a Dictionary

You can loop through a dictionary to access its keys, values, or both.

Example: Looping Through Keys and Values

    # Looping through keys and values
    person = {"name": "John", "age": 30, "profession": "Engineer"}
    for key, value in person.items():
        print(f"{key}: {value}")
        # Outputs:
        # name: John
        # age: 30
        # profession: Engineer
        

6. Dictionary Methods

Python dictionaries come with various built-in methods for performing common tasks.

Example: Using get() Method

    # Using the get() method
    person = {"name": "John", "age": 30}
    print(person.get("name"))  # Outputs: John
    print(person.get("address", "Not Available"))  # Outputs: Not Available (default value if key does not exist)
        

Example: Using keys(), values(), and items()

    # Using keys(), values(), and items() methods
    person = {"name": "John", "age": 30, "profession": "Engineer"}
    print(person.keys())   # Outputs: dict_keys(['name', 'age', 'profession'])
    print(person.values()) # Outputs: dict_values(['John', 30, 'Engineer'])
    print(person.items())  # Outputs: dict_items([('name', 'John'), ('age', 30), ('profession', 'Engineer')])
        

7. Conclusion

Dictionaries are an essential data structure in Python that allow for efficient storage and retrieval of data in key-value pairs. Understanding how to use and manipulate dictionaries is crucial for handling complex data and solving real-world problems in Python.