Python Dictionary - A Comprehensive Guide

Overview

A dictionary in Python is a collection of key-value pairs that are unordered, mutable, and indexed. It is one of the most powerful and commonly used data structures in Python, allowing fast lookups, insertion, and deletion.

Key Characteristics

• Key-Value Pairs: Each item in a dictionary is stored as a key-value pair.
• Keys Are Unique: Keys must be unique and immutable (e.g., strings, numbers, tuples).
• Mutable: Dictionaries can be modified after creation (e.g., adding, updating, or deleting items).
• Dynamic: The size of a dictionary can grow or shrink dynamically.

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Creating Dictionaries

Dictionaries can be created using curly braces {} or the dict() constructor.

# Using curly braces
empty_dict = {}
student = {"name": "Alice", "age": 22, "major": "Computer Science"}

# Using the dict() constructor
employee = dict(name="Bob", age=30, department="HR")

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Accessing Dictionary Elements

By Key

You can access the value associated with a key using square brackets [] or the get() method.

student = {"name": "Alice", "age": 22, "major": "Computer Science"}

# Accessing elements
print(student["name"])  # Output: Alice
print(student.get("age"))  # Output: 22

Iterating Through a Dictionary

# Iterating through keys
for key in student:
    print(key, student[key])

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

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Modifying Dictionaries

Adding or Updating Elements

student = {"name": "Alice", "age": 22}
student["major"] = "Computer Science"  # Adding a new key-value pair
student["age"] = 23  # Updating an existing key-value pair
print(student)
# Output: {'name': 'Alice', 'age': 23, 'major': 'Computer Science'}

Removing Elements

• pop(key): Removes the key-value pair for the specified key.
• popitem(): Removes and returns the last inserted key-value pair.
• del: Deletes a specific key or the entire dictionary.
• clear(): Removes all items from the dictionary.

student = {"name": "Alice", "age": 22, "major": "Computer Science"}

# Removing an item
student.pop("age")
print(student)  # Output: {'name': 'Alice', 'major': 'Computer Science'}

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

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Dictionary Methods

Common Methods

• keys(): Returns a view object of all keys.
• values(): Returns a view object of all values.
• items(): Returns a view object of all key-value pairs.
• update(other_dict): Updates the dictionary with another dictionary.

student = {"name": "Alice", "age": 22}
print(student.keys())   # Output: dict_keys(['name', 'age'])
print(student.values()) # Output: dict_values(['Alice', 22])
print(student.items())  # Output: dict_items([('name', 'Alice'), ('age', 22)])

# Updating the dictionary
student.update({"major": "Computer Science"})
print(student)
# Output: {'name': 'Alice', 'age': 22, 'major': 'Computer Science'}

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Difference: Tuple vs List vs Set vs Dictionary

Feature	Tuple	List	Set	Dictionary
Ordered	Yes	Yes	No	No
Mutable	No	Yes	Yes	Yes (keys immutable)
Duplicates Allowed	Yes	Yes	No	Keys: No, Values: Yes
Access by Index	Yes	Yes	No	No
Syntax	(1, 2)	[1, 2]	{1, 2}	{"key": "value"}

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Difference: Dictionary vs JSON

Feature	Dictionary	JSON
Format	Python-specific	Language-neutral
Syntax	{} for key-value pairs	Text-based (string)
Keys	Must be immutable and hashable	Must be strings
Boolean Values	True/False	true/false
Null Representation	None	null
Data Types	Python objects	Strings, numbers, arrays
Use Case	In-memory operations	Data exchange

Syntax and Code Examples

Dictionary Example

data = {"name": "Alice", "age": 22, "is_student": True}
print(data["name"])  # Output: Alice

JSON Example

import json

# Convert dictionary to JSON
data = {"name": "Alice", "age": 22, "is_student": True}
data_json = json.dumps(data)
print(data_json)  # Output: '{"name": "Alice", "age": 22, "is_student": true}'

# Convert JSON to dictionary
data_dict = json.loads(data_json)
print(data_dict["name"])  # Output: Alice