Python

We can use List, Tuple and Set to represent a group of individual objects as a single entity.

If we want to represent a group of objects as key-value pairs, then we should use a Dictionary.

A Dictionary is one of Python's built-in collection data types.

Some common examples of Dictionary are:

Roll Number → Student Name
Phone Number → Address
IP Address → Domain Name

A Dictionary has the following characteristics:

Data is stored as key-value pairs.
Duplicate keys are not allowed.
Duplicate values are allowed.
Heterogeneous objects are allowed for both keys and values.
Insertion order is preserved (Python 3.7+).
Dictionary objects are mutable.
Dictionary objects are dynamic.
Indexing and slicing are not applicable.

Note: In C++ and Java, a Dictionary is known as a Map, whereas in Perl and Ruby, it is known as a Hash.

Every element in a Dictionary consists of two parts:

A Key
A Value

🐍Code Cell

1	dictionary = {
2	key1: value1,
3	key2: value2
4	}

Output

No output captured.

A Dictionary cannot contain duplicate keys.

If we insert an entry with an existing key, the old value is replaced by the new value.

🐍Code Cell

1	d = {
2	101: "durga",
3	102: "ravi",
4	103: "shiva"
5	}
6
7	d[101] = "sunny"
8
9	print(d)

Output

{101: 'sunny', 102: 'ravi', 103: 'shiva'}

The key 101 already exists in the Dictionary.

Therefore, its old value "durga" is replaced with "sunny".

Unlike keys, duplicate values are allowed in a Dictionary.

🐍Code Cell

1	d = {
2	101: "durga",
3	102: "durga",
4	103: "shiva"
5	}
6
7	print(d)

Output

{101: 'durga', 102: 'durga', 103: 'shiva'}

A Dictionary can store different types of keys and values.

🐍Code Cell

1	d = {
2	100: "Durga",
3	"A": 10,
4	10.5: "Python",
5	True: "Yes"
6	}
7
8	print(d)

Output

{100: 'Durga', 'A': 10, 10.5: 'Python', True: 'Yes'}

Since Python 3.7, a Dictionary preserves the insertion order.

When we iterate a Dictionary or print it, the key-value pairs appear in the same order in which they were inserted.

The keys() method also returns the keys in insertion order.

🐍Code Cell

1	d = {
2	100: "durga",
3	200: "ravi",
4	300: "shiva"
5	}
6
7	print(d)
8
9	print(d.keys())

Output

{100: 'durga', 200: 'ravi', 300: 'shiva'}
dict_keys([100, 200, 300])

Dictionary objects are mutable.

After creating a Dictionary, we can:

Add new key-value pairs.
Modify existing values.
Remove key-value pairs.

The size of a Dictionary can be increased or decreased whenever required.

Dictionary elements are accessed using keys, not indexes.

Therefore:

Indexing is not supported.
Slicing is not supported.

There are different ways to create Dictionary objects in Python.

We can create an empty Dictionary in two ways.

🐍Code Cell

1	d = {}
2
3	# or
4
5	d = dict()

Output

No output captured.

🐍Code Cell

1	d = {}
2
3	print(d)
4
5	print(type(d))

Output

{}

After creating an empty Dictionary, we can add key-value pairs one by one.

🐍Code Cell

1	d = {}
2
3	d[100] = "durga"
4	d[200] = "ravi"
5	d[300] = "shiva"
6
7	print(d)

Output

{100: 'durga', 200: 'ravi', 300: 'shiva'}

If the data is already known, we can directly create a Dictionary.

🐍Code Cell

1	d = {
2	key1: value1,
3	key2: value2
4	}

Output

No output captured.

🐍Code Cell

1	d = {
2	100: "durga",
3	200: "ravi",
4	300: "shiva"
5	}
6
7	print(d)

Output

{100: 'durga', 200: 'ravi', 300: 'shiva'}

Dictionary elements are accessed by using keys.

Unlike Lists and Tuples, Dictionary objects do not support indexing and slicing.

If the specified key is available, Python returns the corresponding value.

If the key is not available, Python raises a KeyError.

🐍Code Cell

1	dictionary_name[key]

Output

No output captured.

🐍Code Cell

1	d = {
2	100: "Durga",
3	200: "Ravi",
4	300: "Shiva"
5	}
6
7	print(d[100])
8
9	print(d[300])

Output

Durga
Shiva

The Dictionary searches for the specified key.

If the key exists, the corresponding value is returned.

🐍Code Cell

1	d = {
2	100: "Durga",
3	200: "Ravi"
4	}
5
6	print(d[300])

Output

KeyError: 300

If we try to access a key that is not present in the Dictionary, Python raises a KeyError.

This error indicates that the specified key does not exist.

Before accessing a value, we can check whether the key is available in the Dictionary.

In Python 3, the recommended approach is to use the in operator.

🐍Code Cell

1	d = {
2	100: "Durga",
3	200: "Ravi",
4	300: "Shiva"
5	}
6
7	if 200 in d:
8	print(d[200])
9
10	if 500 in d:
11	print(d[500])
12	else:
13	print("Key Not Found")

Output

Ravi
Key Not Found

In Python 2, the has_key() method was used to check whether a key exists.

This method has been removed from Python 3.

In Python 3, always use the in operator.

We can add a new key-value pair simply by assigning a value to a new key.

If the key does not exist, a new entry is created.

🐍Code Cell

1	dictionary_name[new_key] = value

Output

No output captured.

🐍Code Cell

1	d = {}
2
3	d[100] = "Durga"
4	d[200] = "Ravi"
5	d[300] = "Shiva"
6
7	print(d)

Output

{100: 'Durga', 200: 'Ravi', 300: 'Shiva'}

Initially the Dictionary is empty.

Each assignment creates a new key-value pair.

If the specified key already exists, assigning a new value updates the existing value.

No duplicate key is created.

🐍Code Cell

1	dictionary_name[existing_key] = new_value

Output

No output captured.

🐍Code Cell

1	d = {
2	100: "Durga",
3	200: "Ravi",
4	300: "Shiva"
5	}
6
7	d[200] = "Sunny"
8
9	print(d)

Output

{100: 'Durga', 200: 'Sunny', 300: 'Shiva'}

The key 200 already exists.

Therefore, only its value is updated.

🐍Code Cell

1	d = {}
2
3	d[101] = "Amit"
4	d[102] = "Rahul"
5
6	d[102] = "Karan"
7
8	d[103] = "Vijay"
9
10	print(d)

Output

{101: 'Amit', 102: 'Karan', 103: 'Vijay'}

The following program stores student details in a Dictionary.

The Roll Number is used as the key and the Student Name is used as the value.

🐍Code Cell

1	students = {}
2
3	students[101] = "Rahul"
4	students[102] = "Amit"
5	students[103] = "Neha"
6
7	print(students)
8
9	print("Student with Roll No 102 =", students[102])
10
11	students[102] = "Priya"
12
13	print("Updated Dictionary:")
14
15	print(students)

Output

{101: 'Rahul', 102: 'Amit', 103: 'Neha'}
Student with Roll No 102 = Amit
Updated Dictionary:
{101: 'Rahul', 102: 'Priya', 103: 'Neha'}

Python provides several built-in functions and methods to work with Dictionary objects.

In this part, we will learn the following:

dict()
len()
get()
pop()
popitem()

The dict() function is used to create a Dictionary object.

It creates an empty Dictionary if no argument is provided.

It can also convert suitable objects into a Dictionary.

🐍Code Cell

1	d = dict()
2
3	# or
4
5	d = dict(iterable)

Output

No output captured.

🐍Code Cell

1	d = dict()
2
3	print(d)
4
5	print(type(d))

Output

{}

🐍Code Cell

1	d = dict([
2	(100, "Durga"),
3	(200, "Ravi"),
4	(300, "Shiva")
5	])
6
7	print(d)

Output

{100: 'Durga', 200: 'Ravi', 300: 'Shiva'}

The len() function returns the total number of key-value pairs available in a Dictionary.

🐍Code Cell

1	len(dictionary)

Output

No output captured.

🐍Code Cell

1	d = {
2	100: "Durga",
3	200: "Ravi",
4	300: "Shiva"
5	}
6
7	print(len(d))

Output

🐍Code Cell

1	d = {}
2
3	print(len(d))

Output

The get() method returns the value associated with the specified key.

If the key is not available, it returns None instead of raising a KeyError.

🐍Code Cell

1	dictionary.get(key)
2
3	dictionary.get(key, default_value)

Output

No output captured.

🐍Code Cell

1	d = {
2	100: "Durga",
3	200: "Ravi"
4	}
5
6	print(d.get(100))

Output

Durga

🐍Code Cell

1	d = {
2	100: "Durga",
3	200: "Ravi"
4	}
5
6	print(d.get(300))

Output

None

🐍Code Cell

1	d = {
2	100: "Durga",
3	200: "Ravi"
4	}
5
6	print(d.get(300, "Key Not Found"))

Output

Key Not Found

Prevents KeyError.
Returns None if the key is not available.
A custom default value can also be returned.

The pop() method removes the specified key from the Dictionary.

It also returns the corresponding value.

If the key is not available, Python raises a KeyError.

🐍Code Cell

1	dictionary.pop(key)

Output

No output captured.

🐍Code Cell

1	d = {
2	100: "Durga",
3	200: "Ravi",
4	300: "Shiva"
5	}
6
7	print(d.pop(200))
8
9	print(d)

Output

Ravi
{100: 'Durga', 300: 'Shiva'}

🐍Code Cell

1	d = {
2	100: "Durga"
3	}
4
5	print(d.pop(200))

Output

KeyError: 200

The popitem() method removes and returns one key-value pair from the Dictionary.

The returned value is in the form of a tuple.

🐍Code Cell

1	dictionary.popitem()

Output

No output captured.

🐍Code Cell

1	d = {
2	100: "Durga",
3	200: "Ravi",
4	300: "Shiva"
5	}
6
7	print(d.popitem())
8
9	print(d)

Output

(300, 'Shiva')
{100: 'Durga', 200: 'Ravi'}

The removed item is returned as a tuple containing the key and its corresponding value.

The Dictionary is modified after removing the item.

Function / Method	Purpose
`dict()`	Create a Dictionary.
`len()`	Returns the number of key-value pairs.
`get()`	Returns the value safely.
`pop()`	Removes the specified key and returns its value.
`popitem()`	Removes and returns one key-value pair.

These Dictionary functions are commonly used in:

Student management systems.
Employee databases.
Configuration files.
API response processing.
Inventory management applications.

Python provides several advanced Dictionary methods to access keys, values, items and to modify Dictionary objects.

In this part, we will learn:

keys()
values()
items()
copy()
setdefault()
update()

The keys() method returns all keys present in the Dictionary.

The returned object can be used for iteration.

🐍Code Cell

1	dictionary.keys()

Output

No output captured.

🐍Code Cell

1	d = {
2	100: "Durga",
3	200: "Ravi",
4	300: "Shiva"
5	}
6
7	print(d.keys())

Output

dict_keys([100, 200, 300])

The values() method returns all values present in the Dictionary.

🐍Code Cell

1	dictionary.values()

Output

No output captured.

🐍Code Cell

1	d = {
2	100: "Durga",
3	200: "Ravi",
4	300: "Shiva"
5	}
6
7	print(d.values())

Output

dict_values(['Durga', 'Ravi', 'Shiva'])

The items() method returns all key-value pairs.

Each item is returned as a tuple.

🐍Code Cell

1	dictionary.items()

Output

No output captured.

🐍Code Cell

1	d = {
2	100: "Durga",
3	200: "Ravi",
4	300: "Shiva"
5	}
6
7	print(d.items())

Output

dict_items([(100, 'Durga'), (200, 'Ravi'), (300, 'Shiva')])

Method	Returns
`keys()`	All keys.
`values()`	All values.
`items()`	Key-value pairs as tuples.

The copy() method creates a copy of the Dictionary.

The copied Dictionary is a new object.

The original Dictionary remains unchanged.

🐍Code Cell

1	new_dictionary = dictionary.copy()

Output

No output captured.

🐍Code Cell

1	d = {
2	100: "Durga",
3	200: "Ravi"
4	}
5
6	d1 = d.copy()
7
8	print(d1)

Output

{100: 'Durga', 200: 'Ravi'}

The setdefault() method returns the value associated with the specified key.

If the key is not available, it inserts the key with the specified default value and returns that value.

🐍Code Cell

1	dictionary.setdefault(key)
2
3	dictionary.setdefault(key, default_value)

Output

No output captured.

🐍Code Cell

1	d = {
2	100: "Durga",
3	200: "Ravi"
4	}
5
6	print(d.setdefault(100, "Sunny"))
7
8	print(d)

Output

Durga
{100: 'Durga', 200: 'Ravi'}

🐍Code Cell

1	d = {
2	100: "Durga",
3	200: "Ravi"
4	}
5
6	print(d.setdefault(300, "Shiva"))
7
8	print(d)

Output

Shiva
{100: 'Durga', 200: 'Ravi', 300: 'Shiva'}

The update() method adds multiple key-value pairs from another Dictionary.

If a key already exists, its value is updated.

🐍Code Cell

1	dictionary.update(other_dictionary)

Output

No output captured.

🐍Code Cell

1	d1 = {
2	100: "Durga",
3	200: "Ravi"
4	}
5
6	d2 = {
7	200: "Sunny",
8	300: "Shiva",
9	400: "Pavan"
10	}
11
12	d1.update(d2)
13
14	print(d1)

Output

{
    100: 'Durga',
    200: 'Sunny',
    300: 'Shiva',
    400: 'Pavan'
}

The key 200 already exists in the first Dictionary.

Therefore, its value is updated.

The remaining key-value pairs are added as new entries.

Method	Purpose
`keys()`	Returns all keys.
`values()`	Returns all values.
`items()`	Returns key-value pairs.
`copy()`	Creates a copy of the Dictionary.
`setdefault()`	Returns a value or inserts a new key.
`update()`	Adds or updates multiple key-value pairs.

These methods are commonly used in:

Student information systems.
Inventory management.
Employee databases.
API response processing.
Configuration management.

We can traverse a Dictionary by using a for loop.

By default, when we iterate over a Dictionary, only the keys are returned.

If we want the corresponding values, we can access them by using the key.

🐍Code Cell

1	d = {
2	100: "durga",
3	200: "ravi",
4	300: "shiva"
5	}
6
7	for k in d:
8	print(k)

Output

100
200
300

The for loop traverses the Dictionary.

By default, each iteration returns only the key.

The values are not displayed unless they are accessed explicitly.

We can use the keys() method to traverse all keys present in the Dictionary.

The keys() method returns a view object containing all keys.

🐍Code Cell

1	dictionary.keys()

Output

No output captured.

🐍Code Cell

1	d = {
2	100: "durga",
3	200: "ravi",
4	300: "shiva"
5	}
6
7	print(d.keys())
8
9	for k in d.keys():
10	print(k)

Output

dict_keys([100, 200, 300])
100
200
300

The keys() method returns all keys in the Dictionary.

The for loop prints each key one by one.

We can use the values() method to traverse all values present in the Dictionary.

The values() method returns a view object containing all values.

🐍Code Cell

1	dictionary.values()

Output

No output captured.

🐍Code Cell

1	d = {
2	100: "durga",
3	200: "ravi",
4	300: "shiva"
5	}
6
7	print(d.values())
8
9	for v in d.values():
10	print(v)

Output

dict_values(['durga', 'ravi', 'shiva'])
durga
ravi
shiva

The values() method returns all values stored in the Dictionary.

The for loop prints each value one by one.

We can use the items() method to traverse both keys and values together.

The items() method returns key-value pairs as tuples.

🐍Code Cell

1	dictionary.items()

Output

No output captured.

🐍Code Cell

1	d = {
2	100: "durga",
3	200: "ravi",
4	300: "shiva"
5	}
6
7	for k, v in d.items():
8	print(k, "--", v)

Output

100 -- durga
200 -- ravi
300 -- shiva

The items() method returns each Dictionary item as a tuple.

The first variable stores the key and the second variable stores the corresponding value.

This is the most commonly used method for traversing a Dictionary when both keys and values are required.

Method	Returns	Best Use
`for k in d`	Keys	Traverse only keys.
`d.keys()`	All keys	Display or process only keys.
`d.values()`	All values	Display or process only values.
`d.items()`	Key-value tuples	Access both keys and values together.

Dictionary traversal is commonly used in:

Displaying student records.
Processing employee information.
Reading configuration settings.
Generating reports.
Processing API response data.

In many real-world applications, we need to calculate the total of all numeric values stored in a Dictionary.

Python provides the built-in sum() function, which makes this task very simple.

In this program, the Dictionary is entered from the keyboard, and the sum of all its values is displayed.

Write a Python program to take a Dictionary from the keyboard and print the sum of all its values.

🐍Code Cell

1	d = eval(input("Enter dictionary: "))
2
3	s = sum(d.values())
4
5	print("Sum =", s)

Output

No output captured.

Let us understand the program step by step.

input() reads the Dictionary as a string.
eval() converts the entered string into an actual Dictionary object.
The Dictionary is stored in the variable d.

🐍Code Cell

1	d = eval(input("Enter dictionary: "))

Output

No output captured.

🐍Code Cell

1	{'A':100,'B':200,'C':300}

Output

No output captured.

🐍Code Cell

1	d = {
2	'A': 100,
3	'B': 200,
4	'C': 300
5	}

Output

No output captured.

The values() method returns all values present in the Dictionary.

🐍Code Cell

1	d.values()

Output

No output captured.

🐍Code Cell

1	d = {
2	'A': 100,
3	'B': 200,
4	'C': 300
5	}
6
7	print(d.values())

Output

dict_values([100, 200, 300])

The sum() function adds all numeric values returned by d.values().

The result is stored in the variable s.

🐍Code Cell

1	s = sum(d.values())

Output

No output captured.

🐍Code Cell

1	100 + 200 + 300 = 600

Output

No output captured.

🐍Code Cell

1	d = {
2	'A': 100,
3	'B': 200,
4	'C': 300
5	}
6
7	s = sum(d.values())
8
9	print("Sum =", s)

Output

Sum = 600

Step	Operation
1	Read the Dictionary from the keyboard.
2	Retrieve all values using `values()`.
3	Calculate the total using `sum()`.
4	Display the final sum.

🐍Code Cell

1	d = {
2	"Math": 80,
3	"Science": 90,
4	"English": 70
5	}
6
7	print(sum(d.values()))

Output

This program is useful in many real-world situations.

Calculating total student marks.
Finding the total salary of employees.
Calculating total sales.
Summing product quantities.
Preparing reports.

One of the most common applications of a Dictionary is counting the frequency of characters in a string.

In this program, each character is stored as a key and its number of occurrences is stored as the corresponding value.

If the character already exists in the Dictionary, its count is increased by 1. Otherwise, a new key is created with the value 1.

Write a Python program to count the occurrence of each character present in a given string by using a Dictionary.

🐍Code Cell

1	word = input("Enter Any String : ")
2
3	d = {}
4
5	for ch in word:
6	d[ch] = d.get(ch, 0) + 1
7
8	for k, v in d.items():
9	print(k, "occurred", v, "times")

Output

No output captured.

Let us understand the program step by step.

The input() function reads a string from the keyboard and stores it in the variable word.

🐍Code Cell

1	word = input("Enter Any String : ")

Output

No output captured.

🐍Code Cell

1	MISSISSIPPI

Output

No output captured.

🐍Code Cell

1

d = {}

Output

No output captured.

The Dictionary d is initially empty.

It will store:

Character → Key
Frequency → Value

🐍Code Cell

1	for ch in word:

Output

No output captured.

The for loop reads one character at a time from the string.

Each character is processed individually.

🐍Code Cell

1	d[ch] = d.get(ch, 0) + 1

Output

No output captured.

The get() method checks whether the character already exists in the Dictionary.

If the character exists, its current count is returned.
If the character does not exist, 0 is returned.
Then 1 is added to the count.

This statement creates a new entry for a new character and updates the count for an existing character.

🐍Code Cell

1	Input : MISS
2
3	After M
4	{'M': 1}
5
6	After I
7	{'M': 1, 'I': 1}
8
9	After S
10	{'M': 1, 'I': 1, 'S': 1}
11
12	After S
13	{'M': 1, 'I': 1, 'S': 2}

Output

No output captured.

🐍Code Cell

1	for k, v in d.items():
2	print(k, "occurred", v, "times")

Output

No output captured.

The items() method returns all key-value pairs.

The first variable stores the character and the second variable stores its frequency.

The program prints the frequency of every character.

🐍Code Cell

1	word = "APPLE"
2
3	d = {}
4
5	for ch in word:
6	d[ch] = d.get(ch, 0) + 1
7
8	for k, v in d.items():
9	print(k, "occurred", v, "times")

Output

A occurred 1 times
P occurred 2 times
L occurred 1 times
E occurred 1 times

Step	Operation
1	Read the input string.
2	Create an empty Dictionary.
3	Traverse every character.
4	Increase the character count using `get()`.
5	Display all characters and their frequencies.

This program is useful in many applications such as:

Word frequency analysis.
Text processing.
Search engines.
Natural Language Processing (NLP).
Data analysis.
Password strength checking.

Dictionary is one of the best data structures for storing data as key-value pairs.

In this program, the student name is stored as the key and the student marks are stored as the corresponding value.

The program allows the user to search for the marks of any student by entering the student's name.

Write a Python program to store student names and marks in a Dictionary and display the marks based on the student name entered by the user.

🐍Code Cell

1	rec = {}
2
3	n = int(input("Enter number of students: "))
4
5	i = 1
6
7	while i <= n:
8	name = input("Enter Student Name: ")
9	marks = input("Enter Student Marks: ")
10
11	rec[name] = marks
12
13	i = i + 1
14
15	while True:
16
17	name = input("Enter Student Name to get Marks: ")
18
19	marks = rec.get(name, "Student Not Found")
20
21	print("The Marks of", name, "are", marks)
22
23	option = input("Do you want to find another student marks[Yes\|No]")
24
25	if option == "No":
26	break
27
28	print("Thanks for using our application")

Output

No output captured.

An empty Dictionary is created.
Student names are stored as keys.
Student marks are stored as values.
The get() method searches for the student's marks.
If the student name is not found, "Student Not Found" is displayed.
The user can search repeatedly until No is entered.

Comprehension concept is applicable for Dictionaries also.

Dictionary Comprehension provides a simple and compact way to create Dictionary objects.

🐍Code Cell

1	dictionary = {
2	key_expression : value_expression
3	for item in iterable
4	}

Output

No output captured.

🐍Code Cell

1	squares = {x: x * x for x in range(1, 6)}
2
3	print(squares)

Output

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

The Dictionary stores:

Key → Number
Value → Square of that number

🐍Code Cell

1	doubles = {x: 2 * x for x in range(1, 6)}
2
3	print(doubles)

Output

{1: 2, 2: 4, 3: 6, 4: 8, 5: 10}

The Dictionary stores:

Key → Number
Value → Double of that number

Nearby lessons

Python - Dictionaries