Python Dictionaries

A dictionary in Python is an unordered, mutable collection of key-value pairs. Each key in a dictionary is associated with a value, and you can retrieve, update, or delete a value by referencing its key. Keys in a dictionary must be unique and immutable (e.g., strings, numbers, tuples), while the values can be of any type.

Key Features of Dictionaries:

1. Unordered: Unlike lists and tuples, dictionaries do not maintain any order for their elements.
2. Mutable: You can modify the dictionary by adding, updating, or deleting key-value pairs.
3. Key-Value Pairs: Each element in the dictionary is stored as a key-value pair.
4. Keys Must Be Unique: No two keys in a dictionary can be the same.
5. Fast Lookup: Dictionaries offer fast lookups by key.

Dictionary Syntax:

Dictionaries are defined using curly braces {}, with key-value pairs separated by a colon :.

# Creating a dictionary
my_dict = {"name": "John", "age": 30, "city": "New York"}

Example: Creating a Dictionary

# A simple dictionary
person = {
    "name": "Alice",
    "age": 28,
    "profession": "Engineer"
}
print(person)
# Output: {'name': 'Alice', 'age': 28, 'profession': 'Engineer'}

Accessing Dictionary Values:

You can access a value by referencing its corresponding key.

# Accessing values using keys
print(person["name"])   # Output: Alice
print(person["age"])    # Output: 28

If you try to access a key that doesn’t exist, Python will raise a KeyError. To avoid this, you can use the .get() method, which returns None (or a default value) if the key is not found.

# Using .get() to safely access a value
print(person.get("profession"))  # Output: Engineer
print(person.get("salary"))      # Output: None

Adding and Updating Elements:

You can add new key-value pairs or update existing ones by assigning values to a key.

# Adding a new key-value pair
person["salary"] = 80000
print(person)  
# Output: {'name': 'Alice', 'age': 28, 'profession': 'Engineer', 'salary': 80000}

# Updating an existing key-value pair
person["age"] = 29
print(person)
# Output: {'name': 'Alice', 'age': 29, 'profession': 'Engineer', 'salary': 80000}

Removing Elements:

You can remove elements from a dictionary using several methods:

• pop(key): Removes the key-value pair for the specified key and returns its value.
• popitem(): Removes and returns the last inserted key-value pair (only in Python 3.7+).
• del: Removes the key-value pair by key.
• clear(): Removes all key-value pairs from the dictionary.

# Removing a specific key-value pair using pop()
person.pop("salary")
print(person)  # Output: {'name': 'Alice', 'age': 29, 'profession': 'Engineer'}

# Removing an element using del
del person["age"]
print(person)  # Output: {'name': 'Alice', 'profession': 'Engineer'}

# Clearing the entire dictionary
person.clear()
print(person)  # Output: {}

Dictionary Methods:

1. .keys(): Returns a list-like view of all the keys in the dictionary.
2. .values(): Returns a list-like view of all the values in the dictionary.
3. .items(): Returns a list-like view of all the key-value pairs in the dictionary.

person = {"name": "Alice", "age": 28, "profession": "Engineer"}

# Getting all keys
print(person.keys())  # Output: dict_keys(['name', 'age', 'profession'])

# Getting all values
print(person.values())  # Output: dict_values(['Alice', 28, 'Engineer'])

# Getting all key-value pairs
print(person.items())  # Output: dict_items([('name', 'Alice'), ('age', 28), ('profession', 'Engineer')])

Looping Through a Dictionary:

You can iterate over the keys, values, or key-value pairs in a dictionary using loops.

# Iterating through the keys
for key in person:
    print(key)

# Output:
# name
# age
# profession

# Iterating through key-value pairs
for key, value in person.items():
    print(f"{key}: {value}")

# Output:
# name: Alice
# age: 28
# profession: Engineer

Checking for Keys:

You can check whether a key exists in the dictionary using the in keyword.

# Check if a key exists in the dictionary
if "name" in person:
    print("Key 'name' exists.")
else:
    print("Key 'name' does not exist.")
# Output: Key 'name' exists.

Dictionary Comprehension:

You can create dictionaries using dictionary comprehensions, which are concise ways to build a dictionary.

# Example of dictionary comprehension
squares = {x: x**2 for x in range(1, 6)}
print(squares)  # Output: {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

Nesting Dictionaries:

Dictionaries can contain other dictionaries, creating a nested structure.

# Nested dictionary
employees = {
    "emp1": {"name": "Alice", "position": "Manager"},
    "emp2": {"name": "Bob", "position": "Engineer"},
}
print(employees["emp1"]["name"])  # Output: Alice

Example: Dictionary as a Phone Book

# Example: Dictionary as a phone book
phone_book = {
    "John": "+1-202-555-0198",
    "Alice": "+1-202-555-0123",
    "Bob": "+1-202-555-0176"
}

# Lookup a phone number by name
print(phone_book["Alice"])  # Output: +1-202-555-0123

Summary:

• A dictionary is a collection of key-value pairs.
• Keys must be unique and immutable, but values can be of any type.
• Dictionaries are useful when you need to associate and retrieve values by key, such as storing a phone book, mapping IDs to values, or building data structures like trees or graphs.