? Python Dictionaries

? Quick Overview

A dictionary in Python is a collection of key–value pairs. Each key must be unique and is used to access its corresponding value. Dictionaries are:

Unordered – items have no fixed position (in older Python versions).
Mutable – you can change, add, or remove items after creation.
Dynamic – you can grow or shrink the dictionary at runtime.

⚙️ Key Concepts

A dictionary is defined using curly braces {} with key: value pairs.
Keys must be immutable types (for example: strings, numbers, tuples).
Values can be of any type (strings, numbers, lists, other dictionaries, etc.).
Keys are unique – if you repeat a key, the last value assigned will overwrite earlier ones.
You access values using the key in square brackets or the get() method.

? Syntax & Structure

Basic syntax to create a dictionary:

? View Dictionary Syntax

# General form
dictionary_name = {
    key1: value1,
    key2: value2,
    key3: value3
}

# Example
person = {"name": "Alice", "age": 25, "city": "New York"}

Dictionaries can also be created using the dict() constructor, but using curly braces is the most common and readable approach.

? Code Examples

? Creating a Dictionary

You can create a dictionary using curly braces with key–value pairs.

? View Code Example

person = {"name": "Alice", "age": 25, "city": "New York"}
print(person)
# {'name': 'Alice', 'age': 25, 'city': 'New York'}

? Accessing Values

Access dictionary values using their keys. Use square brackets when you are sure the key exists, or get() when the key might be missing.

? View Code Example

print(person["name"])   # Alice
print(person.get("age"))  # 25

➕ Adding or Updating Items

You can add a new key–value pair or update an existing key simply by assigning to that key.

? View Code Example

person["email"] = "alice@example.com"  # Add new
person["age"] = 26                     # Update existing
print(person)
# {'name': 'Alice', 'age': 26, 'city': 'New York', 'email': 'alice@example.com'}

? Removing Items

Remove items using pop(), popitem(), or del.

? View Code Example

person.pop("city")   # Remove by key
person.popitem()     # Remove last inserted item
del person["age"]    # Delete by key
print(person)
# {'name': 'Alice'}

? Live Output & Explanation

? How the Dictionary Changes

Starting with:

# Initial dictionary
person = {"name": "Alice", "age": 25, "city": "New York"}

After adding email and updating age:

person["email"] = "alice@example.com" # Add a new key-value pair
person["age"] = 26 # Update the existing 'age' key
print(person) # Print the updated dictionary

Output:

# Updated dictionary after changes
{'name': 'Alice', 'age': 26, 'city': 'New York', 'email': 'alice@example.com'}

Notice how the value of "age" changed from 25 to 26, and a new key "email" was added. This shows that dictionaries are both mutable and dynamic.

? Use Cases

Use dictionaries when you need:

Fast lookups by a meaningful key (like username, roll number, or product ID).
To group related data together (like a record of a person, product, or configuration).
To count or map items (for example, word counts, frequency tables).

? Tips & Best Practices

Use get() to safely access keys that might not exist, e.g. person.get("phone").
Remember that dictionaries are mutable, so changes affect the original object.
Keep your keys simple and consistent, usually strings like "name", "age", etc.
Avoid using duplicate keys – the last value for a key will overwrite earlier ones.
Do not use mutable types (like lists) as keys; only immutable types are allowed.
Handle missing keys properly to avoid KeyError (use get() or in checks).

? Try It Yourself

Create a dictionary of your favorite movies and their release years.
Update one of the movies with a new release year.
Access a value safely using get() for a key that might not exist.
Remove a movie from the dictionary using pop() and print the updated dictionary.