Python Dictionary: Definition, Keys, Values, and Examples

✍️Introduction

A dictionary in Python is used to store data in key–value pairs.

It is very useful when you want to store related information like a person’s details, product info, or student records.

Dictionaries are written using curly braces {}.
Each key in a dictionary must be unique.
Dictionaries are mutable, which means we can change their values.

They allow fast data access using keys instead of indexes.

What is a Dictionary in Python?

A dictionary is a data structure that stores values in key–value pairs, where each key is used to access its value.

A dictionary is:

Dictionaries are ordered (from Python 3.7+).
They are changeable (mutable), so values can be updated.

Keys must be unique, but values can be duplicated.

Example:

student = { # Create a dictionary

"name": "Rahul", # Key: name, Value: Rahul

"age": 20, # Key: age, Value: 20

"course": "Python" # Key: course, Value: Python

}

print(student) # Print the dictionary

Output:

{'name': 'Rahul', 'age': 20, 'course': 'Python'}

"Code executed successfully"  

Note:Dictionaries are commonly used to store structured data like student records or user details.

Dictionary Syntax :

Dictionary syntax defines how to create a dictionary using key–value pairs inside curly brackets {}.

A dictionary is created using { }.
Each element consists of a key and a value separated by a colon :.
Multiple key–value pairs are separated by commas ,.

Keys must be unique and usually written as strings or numbers.

Syntax :


dictionary_name = {
    key1: value1,
    key2: value2
}

Example:

student = {                        # Dictionary name
    "name": "Rutuja",        # Key: name, Value: Rahul
    "age": 20                     # Key: age, Value: 20
}

print(student)                  # Print dictionary

Output:
{'name': 'Rutuja', 'age': 20}

"Code executed successfully"  

Note:Keys are used to access values quickly in a dictionary.

Access Dictionary Values :

Dictionary values can be accessed using their keys.

We use the key inside square brackets [ ] to access a value.
The get() method is another way to access values safely.
If the key does not exist, get() returns None instead of an error.

Using [] gives an error if the key is missing.

Example:


student = {                              # Create dictionary
    "name": "Rutuja",
    "age": 20,
    "course": "Python"
}

print(student["name"])          # Access value using key "name"
print(student.get("age"))       # Access value using get() method


Output:
Rutuja
20

"Code executed successfully"  

Note:get() is safer than [] because it does not crash the program if the key is missing.
Change Dictionary Values :
Dictionary values can be changed by using the key and assigning a new value.
Dictionaries are mutable, so their values can be updated.
We use the key inside square brackets [ ] to modify the value.
If the key exists, the value is updated.
If the key does not exist, a new key–value pair is created.
Example:

student = {                 # Create dictionary
    "name": "Rutuja",
    "age": 20,
    "course": "Python"
}

student["age"] = 21         # Change value of key "age"

print(student)              # Print updated dictionary
Output:
{'name': 'Rutuja', 'age': 21, 'course': 'Python'}

"Code executed successfully"  

Note:Dictionaries allow easy updating of values using keys.
Add New Items:
New items can be added to a dictionary by assigning a value to a new key.
Dictionaries are dynamic, so new key–value pairs can be added anytime.
If the key does not exist, it will be created automatically.
If the key already exists, its value will be updated.
Example:
student = {                 # Create dictionary
    "name": "Rutuja",
    "age": 20
}

student["city"] = "Pune"    # Add new key "city" with value "Pune"

print(student)              # Print updated dictionary

Output:
{'name': 'Rutuja', 'age': 20, 'city': 'Pune'}

"Code executed successfully"  

Note:Adding new keys makes dictionaries flexible for storing extra data.

Remove Items from Dictionary :

Items can be removed from a dictionary using different methods like pop(), popitem(), del, and clear().

Dictionaries allow deleting specific or all items.
pop() removes an item using its key.
popitem() removes the last inserted item.
del removes a specific key–value pair.
clear() removes all items from the dictionary.

Example:


student = {                         # Create dictionary
    "name": "Rahul",
    "age": 20,
    "course": "Python",
    "city": "Pune"
}

student.pop("age")              # Removes key "age"
print(student)                      # Print dictionary

student.popitem()                # Removes last inserted item ("city")
print(student)                       # Print dictionary

del student["course"]           # Deletes key "course"
print(student)                       # Print dictionary      
 
student.clear()                      # Removes all items
print(student)                       # Print dictionary

Output:
{'name': 'Rahul', 'course': 'Python', 'city': 'Pune'}
{'name': 'Rahul', 'course': 'Python'}
{'name': 'Rahul'}
{}

"Code executed successfully"  
Note:
pop() needs a key.
del gives an error if the key does not exist.
clear() makes the dictionary empty.

Dictionary Length :

The len() function is used to count the number of key–value pairs in a dictionary.

It returns the total number of items in the dictionary.
Each key–value pair is counted as one item.

Useful for checking how much data is stored.

Example:


student = {                 # Create dictionary
    "name": "Rutuja",
    "age": 20,
    "city": "Pune"
}

print(len(student))         # Prints number of items in dictionary

Output:
3

"Code executed successfully"  

Note:len() counts keys, not characters or values.
Loop Through Dictionary:
Looping through a dictionary allows us to access keys, values, or both using loops.
A for loop is used to iterate through dictionary items.
By default, looping returns keys.
.values() returns all values.
.items() returns both key and value pairs.
Useful for printing or processing dictionary data.
Example:
student = {                     # Create dictionary
    "name": "Rutuja",
    "age": 20,
    "city": "Pune"
}

# Loop through keys
for key in student:             # Iterates through keys
    print(key)                  # Prints each key

# Loop through values
for value in student.values():  # Gets all values
    print(value)                # Prints each value

# Loop through keys and values
for key, value in student.items():  # Gets key-value pairs
    print(key, ":", value)          # Prints key and value

Output:
name
age
city
Rutuja
20
Pune
name : Rutuja
age : 20
city : Pune

"Code executed successfully"  

Note:

student → keys
student.values() → values
student.items() → key + value pairs

Dictionary Methods (Common) :

Dictionary methods are built-in functions used to perform operations like accessing, updating, and deleting dictionary data.

Methods make dictionaries easier to use and manage.
They help in retrieving keys, values, and items quickly.
Some methods modify the dictionary, while others only read data.
These methods are commonly used in real-world programs and projects.

Method	Description
keys()	Returns keys
values()	Returns values
items()	Returns key–value pairs
get()	Access value
update()	Update dictionary
pop()	Remove item
clear()	Remove all

Note:Dictionary methods help in fast and easy data manipulation.

Check if Key Exists :

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

The in keyword searches for a key in the dictionary.
It returns True if the key exists, otherwise False.
This helps avoid errors when accessing dictionary values.

It is commonly used before using student["key"].

Example:


student = {                 # Create dictionary
    "name": "Rutuja",
    "age": 20
}

if "name" in student:       # Check if key "name" exists
    print("Key exists")     # Runs if key is found

Output:
Key exists

"Code executed successfully"  

Note:Using in prevents errors like KeyError when accessing dictionary values.

Nested Dictionary :

A nested dictionary is a dictionary inside another dictionary.

It is used to store multiple records in a structured way.
Each key can contain another dictionary as its value.
Useful for storing student data, employee records, or product details.

Data can be accessed using multiple keys.

Example:


students = {
    "s1": {"name": "Aniket", "age": 20},   # Dictionary inside dictionary
    "s2": {"name": "Om", "age": 19}    # Another nested dictionary
}

print(students["s1"]["name"])  # Access name of student s1
print(students["s2"]["age"])   # Access age of student s2

Output:
Aniket
19

"Code executed successfully"  

Note:Nested dictionaries help organize complex data in a structured format.

Dictionary with Different Data Types :

A dictionary can store different types of values such as strings, numbers, and booleans in a single structure.

Keys are usually strings, but values can be of any data type.
Dictionaries can store integers, floats, strings, lists, and booleans together.
This makes dictionaries very useful for storing mixed data like program settings or user information.
Each value is accessed using its key.

Example:

data = {

"name": "Python", # String value

"version": 3.12, # Float value

"is_easy": True # Boolean value

}

print(data["name"]) # Prints string value

print(data["version"]) # Prints float value

print(data["is_easy"]) # Prints boolean value

Output:

Python 3.12 True

"Code executed successfully"  

Note:Dictionaries allow storing multiple data types in one place, unlike arrays.

Convert Dictionary to List :

A dictionary can be converted into a list by using the list() function along with dictionary methods like keys(), values(), or items().

list() converts them into a list format.
This is useful when you want to process dictionary data like a list.
You can also convert values and key–value pairs into lists.

Example:


student = {                                   # Create dictionary
    "name": "Rahul",
    "age": 20,
    "city": "Pune"
}

keys_list = list(student.keys())   # Convert dictionary keys into a list
print(keys_list)                           # Print list

Output:

['name', 'age', 'city']

"Code executed successfully"  

Note:
You can also use:

list(student.values()) → List of values

list(student.items()) → List of key–value pairs

Difference Between List, Tuple, Set, Dictionary :

List, Tuple, Set, and Dictionary are the main data structures in Python used to store collections of data.

Feature	List	Tuple	Set	Dictionary
Ordered	Yes	Yes	No	Yes
Changeable	Yes	No	Yes	Yes
Duplicates	Yes	Yes	No	Keys ❌
Indexing	Yes	Yes	No	Keys

Note:

List → General purpose collection
Tuple → Fixed data
Set → Unique values
Dictionary → Key–value data storage

Real-Life Example :

A dictionary can be used to store real-life data such as product details in key–value pairs.


product = {
    "name": "Laptop",     # Product name
    "price": 50000,       # Product price
    "brand": "HP"         # Product brand
}

print(product["name"])    # Print product name
print(product["price"])   # Print product price
print(product["brand"])   # Print product brand

Output:
Laptop
50000
HP

"Code executed successfully"

❌ Common Mistakes :

Using duplicate keys
Accessing non-existing key without get()
Confusing list indexing with dictionary keys

Conclusion :

Python dictionaries are powerful and widely used in real-world applications.
They make data handling easy, readable, and efficient.

💬 Quick Question

What will be the output?


d = {"a":1, "b":2, "a":3}
print(d)

Comment your answer 👇😊

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