Python Dictionary

What is a Dictionary?

• A Dictionary is a Python data structure used to store data in key-value pairs.

student = {
    "name": "Punitha",
    "age": 22,
    "course": "Data Analytics"
}
print(student)

Output
{'name': 'Punitha', 'age': 22, 'course': 'Data Analytics'}

Why Do We Use Dictionary?

• Student information
• Employee information
• Product details
• Customer details
• JSON data
• API data
• Configuration data

Creating a Dictionary

• We normally create a Dictionary using {}.

student = {
    "name": "Punitha",
    "age": 22,
    "course": "Python"
}
print(student)

Output
{'name': 'Punitha', 'age': 22, 'course': 'Python'}

Empty Dictionary

• We can create an empty Dictionary using {}.

student = {}
student = dict()
print(student)

Output
{}

Accessing Dictionary Values

• We can access a value using its key.

student = {
    "name": "Punitha",
    "age": 22,
    "course": "Python"
}
print(student["name"])
print(student["age"])
print(student["course"])

Output:
Punitha 22 Python

Using get()

• We can also use get() to access a value.

student = {
    "name": "Punitha",
    "age": 22
}
print(student.get("name"))

Output:
Punitha

Adding New Items

• We can add a new key-value pair.

student = {
    "name": "Punitha",
    "age": 22
}
student["city"] = "Chennai"
print(student)

Output:
{'name': 'Punitha', 'age': 22, 'city': 'Chennai'}

Updating Existing Values

• If the key already exists, assigning a new value will update it.

student = {
    "name": "Punitha",
    "age": 22
}
student["age"] = 23
print(student)

Output
{'name': 'Punitha', 'age': 23}

Removing Items

• There are different ways to remove dictionary items.

pop()

student = {
    "name": "Punitha",
    "age": 22,
    "city": "Chennai"
}
student.pop("age")
print(student)

Output:
{'name': 'Punitha', 'city': 'Chennai'}

• pop() removes the specified key-value pair.

popitem()

• popitem() removes the last inserted key-value pair.

student = {
    "name": "Punitha",
    "age": 22,
    "city": "Chennai"
}
student.popitem()
print(student)

Output:
{'name': 'Punitha', 'age': 22}

del

• We can use del to remove a specific item.

student = {
    "name": "Punitha",
    "age": 22,
    "city": "Chennai"
}
del student["age"]
print(student)

Output:
{'name': 'Punitha', 'city': 'Chennai'}

clear()

• clear() removes all items.

student = {
    "name": "Punitha",
    "age": 22
}
student.clear()
print(student)

Output:
{}

Dictionary Length

• We can use len() to find the number of key-value pairs.

student = {
    "name": "Punitha",
    "age": 22,
    "city": "Chennai"
}
print(len(student))

Output:
3

Checking Keys

• We can use in to check whether a key exists.

student = {
    "name": "Punitha",
    "age": 22
}
print("name" in student)
print("city" in student)

Output:
True
False

Important Dictionary Methods

Method                       Purpose

keys()                       Returns all keys
values()                 Returns all values
items()                      Returns key-value pairs
get()                        Gets a value using a key
update()                 Adds or updates items
pop()                        Removes a specified item
popitem()                Removes the last item
clear()                      Removes all items
copy()                       Creates a copy

Nested Dictionary

• A dictionary can contain another dictionary.

students = {
    "student1": {
        "name": "Punitha",
        "age": 22
    },
    "student2": {
        "name": "Anu",
        "age": 21
    }
}
print(students)

print(students["student1"]["name"])

Output:
Punitha

Dictionary with Different Data Types

• Dictionary values can have different data types.

student = {
    "name": "Punitha",
    "age": 22,
    "marks": 85.5,
    "passed": True
}
print(student)

Dictionary Comprehension

• Dictionary comprehension provides a short way to create a dictionary.

numbers = [1, 2, 3, 4, 5]
squares = {num: num ** 2 for num in numbers}
print(squares)

Output:
{1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

Converting List to Dictionary

• We can use dict() with pairs.

data = [
    ("name", "Punitha"),
    ("age", 22),
    ("city", "Chennai")
]
student = dict(data)
print(student)

Output:
{'name': 'Punitha', 'age': 22, 'city': 'Chennai'}

Dictionary vs List

List                        Dictionary

Stores values               Stores key-value pairs
Uses []                     Uses {}
Access using index      Access using key
Example students[0]     Example student["name"]
Ordered                     Ordered by insertion in modern Python
Duplicates allowed      Keys must be unique

Dictionary vs Set

Dictionary                Set

Stores key-value pairs        Stores values
{key: value}                  {value}
Access using key          No indexing
Keys must be unique       Values must be unique
Used for structured data      Used for unique data