Python Dictionary

(toc)

A dictionary in Python is an unordered collection of items, where each item is stored as a key-value pair. Dictionaries are mutable, meaning you can change them after they are created. They are widely used for storing and accessing data in a key-value format, making it easy to retrieve and manipulate data efficiently.

Key Features of Dictionaries

1. Unordered: The order of items in a dictionary is not guaranteed (before Python 3.7). In Python 3.7+, dictionaries maintain insertion order.
2. Mutable: You can modify dictionaries by adding, removing, or updating key-value pairs.
3. Unique Keys: A dictionary cannot have duplicate keys. Each key must be unique.
4. Key-Value Pairs: Each dictionary element consists of a key and an associated value.
5. Keys Must Be Immutable: The keys in a dictionary must be of an immutable data type (e.g., strings, numbers, or tuples).

Creating a Dictionary

You can create a dictionary using curly braces {} or the dict() constructor.

Examples:

# Creating a dictionary with curly braces
person = {
    "name": "Alice",
    "age": 30,
    "city": "New York"
}

# Creating a dictionary using the dict() constructor
student = dict(name="John", age=21, major="Computer Science")

print(person)  # Output: {'name': 'Alice', 'age': 30, 'city': 'New York'}
print(student)  # Output: {'name': 'John', 'age': 21, 'major': 'Computer Science'}

Accessing Dictionary Elements

You can access the value of a specific key by using the key inside square brackets [] or by using the get() method.

Examples:

person = {
    "name": "Alice",
    "age": 30,
    "city": "New York"
}

# Accessing value using key
print(person["name"])  # Output: Alice

# Using get() method
print(person.get("city"))  # Output: New York

# Using get() with default value if key does not exist
print(person.get("country", "USA"))  # Output: USA (default value since key "country" doesn't exist)

Modifying Dictionaries

You can add, update, or remove key-value pairs in a dictionary.

1. Adding or Updating
   If the key exists, its value will be updated; if the key does not exist, a new key-value pair is added.

   
   person = {
       "name": "Alice",
       "age": 30,
       "city": "New York"
   }
   
   # Adding a new key-value pair
   person["email"] = "alice@example.com"
   
   # Updating an existing key-value pair
   person["age"] = 31
   
   print(person)
   # Output: {'name': 'Alice', 'age': 31, 'city': 'New York', 'email': 'alice@example.com'}
   

2. Removing Elements
   You can use del or the pop() method to remove elements.

   • del removes a key-value pair by key.
   • pop() removes a key-value pair and returns the value.

   
   person = {
       "name": "Alice",
       "age": 30,
       "city": "New York"
   }
   
   # Using del to remove a key-value pair
   del person["age"]
   
   # Using pop() to remove a key-value pair and get the value
   city = person.pop("city")
   print(city)  # Output: New York
   
   print(person)  # Output: {'name': 'Alice'}
   

3. Removing All Elements
   To remove all items in a dictionary, use the clear() method.

   
   person.clear()
   print(person)  # Output: {}
   

Dictionary Methods

Python dictionaries have several built-in methods that help with data manipulation:

Examples:

person = {
    "name": "Alice",
    "age": 30,
    "city": "New York"
}

# Using keys(), values(), and items()
print(person.keys())    # Output: dict_keys(['name', 'age', 'city'])
print(person.values())  # Output: dict_values(['Alice', 30, 'New York'])
print(person.items())   # Output: dict_items([('name', 'Alice'), ('age', 30), ('city', 'New York')])

# Using popitem()
random_item = person.popitem()
print(random_item)      # Output: ('city', 'New York')
print(person)           # Output: {'name': 'Alice', 'age': 30}

Nested Dictionaries

A dictionary can contain another dictionary as a value. This is useful when you want to store more complex data structures.

Example:

company = {
    "employee1": {
        "name": "Alice",
        "age": 30,
        "position": "Engineer"
    },
    "employee2": {
        "name": "Bob",
        "age": 25,
        "position": "Designer"
    }
}

print(company["employee1"]["name"])  # Output: Alice

Dictionary Comprehension

You can create dictionaries using dictionary comprehensions, similar to list comprehensions.

Example:

# Creating a dictionary of squares using 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}

Use Cases of Dictionaries

1. Storing Data with Unique Identifiers
   Dictionaries are ideal for storing data that needs to be accessed by a unique key, such as storing user profiles, settings, or configurations.

2. Fast Lookups
   Dictionaries allow fast lookups of values by key, making them useful for situations where you need to quickly retrieve data based on a known identifier.

3. Counting Frequencies
   You can use dictionaries to count the frequency of items in a collection (e.g., counting words in a text).

   
   text = "apple banana apple cherry apple"
   word_count = {}
   
   for word in text.split():
       if word in word_count:
           word_count[word] += 1
       else:
           word_count[word] = 1
   
   print(word_count)  # Output: {'apple': 3, 'banana': 1, 'cherry': 1}