A dictionary in Python is an unordered, changeable collection of data values that holds key-value pairs, with keys being unique and immutable. As of Python 3.7, dictionaries maintain the order of items, and various methods are available for accessing, modifying, and iterating over them. The document also compares dictionaries with lists, highlighting key differences in syntax, ordering, and methods.

1. Dictionary in Python

2. What is a Dictionary in Python?
• A Dictionary in Python is the unordered, and changeable collection of data values that holds
key-value pairs.
• Each key-value pair in the dictionary maps the key to its associated value.
• Python Dictionary is classified into two elements: Keys and Values.
• Keys will be a single element.
• Values can be a list or list within a list, numbers, etc.

3. Cont..
• Dictionary is ordered or unordered?
• As of Python version 3.7, dictionaries are ordered. In Python 3.6 and earlier, dictionaries
are unordered.
• When we say that dictionaries are ordered, it means that the items have a defined order,
and that order will not change.
• Unordered means that the items does not have a defined order, you cannot refer to
an item by using an index.
• Dictionary is Changeable:
• Dictionaries are changeable, meaning that we can change, add or remove items
after the dictionary has been created.
• Duplicates Not Allowed in dictionary:
• Dictionaries cannot have two items with the same key

4. How to Create a Dictionary in Python?
• Dictionary is created with curly brackets, inside these curly brackets, keys and values are declared. Each key is
separated from its value by a colon (:), while commas separate each element.
• A Dictionary in python is declared by enclosing a comma-separated list of key-value pairs using curly braces({}).
• Dictionaries are used to store data values in key:value pairs.
#Creating an empty dictionary
d = {}
#Creating an empty dictionary with the dict function
d = dict()

5. Cont…
# Initializing a dictionary with values
a = {"a":1 ,"b":2}
b = {'fruit': 'apple', ‘color': ‘red’}
c = {'name': ‘Suchith', 'age’: 20}
d = {'name': ‘Ishanth’, ‘sem’: [1,2,3,7]}
e = {‘branch’:‘CSE’, ‘course’:[‘c’,’java’,’python’]}
f = {'a':1, 'b':2, 'c':3, 'd’:2}
# For Sequence of Key-Value Pairs
g = dict([(1, ‘Apple'), (2, 'Orange'), (3, 'Kiwi')])

6. Properties of Dictionary Keys
• Keys must be unique: More than one entry per key is not allowed ( no
duplicate key is allowed)
• The values in the dictionary can be of any type, while the keys must be
immutable like numbers, tuples, or strings.
• Dictionary keys are case sensitive- Same key name but with the different
cases are treated as different keys in Python dictionaries.

7. How to Access Python Dictionary items?
• Since the dictionary is an unordered collection, we can access the values using their
keys.
• It can be done by placing the key in the square brackets or using the get function.
• Ex: Dname[key] or Dname.get(key)
• If we access a non-existing one,
• The get function returns none.
• The key inside a square brackets method throw an error.

8. Cont..
# Accessing dictionary items
>>> a = {'name': 'Kevin', 'age': 25, 'job': 'Developer’}
# Print
>>> print(a)
{'name': 'Kevin', 'age': 25, 'job': 'Developer’}
>>> print("nName :",a['name’])
Name : Kevin
>>> print("Age : ", a['age’])
Age : 25
>>> print("Job : ", a['job’])
Job : Developer

9. Cont..
# Accessing dictionary items
>>> a = {'name': 'Kevin', 'age': 25, 'job': 'Developer’}
>>> len(a) #length of dictionary
3
# using get()
>>> print("Name : ", a.get('name’))
Name : Kevin
>>> print("Age : ", a.get('age’))
Age : 25
>>> print("Job : ", a.get('job’))
Job : Developer

10. Insert and Update Dictionary items
• Remember, dicts are mutable, so you can insert or update any element at any point in time. Use the below
syntax to insert or update values.
DName[key] = value
• If a key is present, it updates the value. If DName does not have the key, then it inserts the new one with
the given.
>>> da = {'name': 'Kevin', 'age': 25}
# Print Elements
>>> print("Items : ", da)
Items : {'name': 'Kevin', 'age': 25}
# Add an Item
>>> da['job'] = 'Programmer'
>>> print("nItems : ", da)
Items : {'name': 'Kevin', 'age': 25, 'job': 'Programmer’}
# Update an Item
>>> da['name'] = 'Python'
>>> print("nItems : ", da)
Items : {'name': 'Python', 'age': 25, 'job': 'Programmer'}

11. Methods in Python Dictionary
Functions Description
clear() Removes all Items
copy() Returns Shallow Copy of a Dictionary
fromkeys() Creates a dictionary from the given sequence
get() Find the value of a key
items() Returns view of dictionary’s (key, value) pair
keys() Prints the keys
popitem() Remove the last element of the dictionary
setdefault() Inserts Key With a Value if Key is not Present
pop() Removes and returns element having given key
values() Returns view of all values in the dictionary
update() Updates the Dictionary

12. Cont..
keys(): The keys() method returns a list of keys in a Python dictionary.
>>> d ={'name': 'Python', 'age': 25, 'job': 'Programmer’}
>>> d.keys()
dict_keys(['name', 'age', 'job'])
values(): The values() method returns a list of values in the dictionary.
>>> d ={'name': 'Python', 'age': 25, 'job': 'Programmer’}
>>> dict4.values()
dict_values(['Python', 25, 'Programmer'])

13. Cont..
items(): This method returns a list of key-value pairs.
>>> d ={'name': 'Python', 'age': 25, 'job': 'Programmer’}
>>> d.items()
dict_items([('name','Python'),('age',25),('job','Programmer')])
get(): It takes one to two arguments. While the first is the key to search for,
the second is the value to return if the key isn’t found. The default value for
this second argument is None.
>>> d ={'name': 'Python', 'age': 25, 'job': 'Programmer’}
>>> d.get('job')
'Programmer'
>>> d.get("role")
>>> d.get("role","Not Present")
'Not Present'

14. Cont..
copy(): Python dictionary method copy() returns a shallow copy of the dictionary.
>>> d ={'name': 'Python', 'age': 25, 'job': 'Programmer’}
>>> d2=d.copy()
>>> d
{'name': 'Python', 'age': 25, 'job': 'Programmer’}
>>> d2
{'name': 'Python', 'age': 25, 'job': 'Programmer’}
>>> id(d)
3218144153792
>>> id(d2)
3218150637248

15. Cont..
pop(): This method is used to remove and display an item from the dictionary. It takes
one or two arguments. The first is the key to be deleted, while the second is the value
that’s returned if the key isn’t found.
>>> d ={'name': 'Python', 'age': 25, 'job': 'Programmer’}
>>> d.pop("age")
25
>>> d
{'name': 'Python', 'job': 'Programmer'}
>>> d.pop("role")
Traceback (most recent call last):
File "<pyshell#31>", line 1, in <module>
d.pop("role")
KeyError: 'role'
>>> d.pop("role",-1)
-1

16. Cont..
popitem():The popitem() method removes and returns the item that was last inserted into
the dictionary.
>>> d ={'name': 'Python', 'job': 'Programmer’, 'age’: 26}
>>> d.popitem()
('age', 26)
fromkeys(): Python dictionary method fromkeys() creates a new dictionary
with keys from seq and values set to value.
>>> d=fromkeys({1,2,3,4,7},0)
>>> d
{1: 0, 2: 0, 3: 0, 4: 0, 7: 0}
>>> seq = ('name', 'age', ‘job’)
>>> d2 = d2.fromkeys(seq)
>>> d2
{'age': None, 'name': None, ‘job': None}

17. Cont..
update():The update() method takes another dictionary as an argument. Then it updates
the dictionary to hold values from the other dictionary that it doesn’t already.
>>> d1 ={'name': 'Python', 'job': 'Programmer’}
>>> d2 ={'age’: 26}
>>> d1.update(d2)
>>> print(d1)
{'name': 'python', 'job': 'programmer', 'age': 25}
>>> d1 ={'name': 'Python', 'job': 'Programmer’}
>>> d2 ={'age’: 26, 'name’: ‘Java'}
>>> d1.update(d2)
>>> print(d1)
{'name’: ‘Java', 'job': 'programmer', 'age': 25}
clear(): Removes all elements of dictionary.
>>> d ={'name': 'Python', 'job': 'Programmer’, 'age’: 26}
>>> d.clear()
{}

18. Cont..
setdefault():
Returns the value of the specified key in the dictionary. If the key not found, then it adds
the key with the specified defaultvalue. If the defaultvalue is not specified then it set None value.
Syntax:
dict.setdefault(key, defaultValue)
Ex:
romanNums = {'I':1, 'II':2, 'III':3}
romanNums.setdefault('IV')
print(romanNums)
{'I': 1, 'II': 2, 'III': 3, 'IV': None}
romanNums.setdefault('VI', 4)
print(romanNums)
{'I': 1, 'II': 2, 'III': 3, 'IV': 4}

19. Iterating Dictionary
• A dictionary can be iterated using for loop as given below.
# for loop to print all the keys of a dictionary
Employee = {"Name": “Kiran",
"Age": 29,
"salary":25000,
"Company":"GOOGLE"}
for i in Employee:
print(i)
Output:
Name
Age
salary
Company

20. Iterating Dictionary
# for loop to print all the keys of a dictionary
Employee = {"Name": “Kiran", "Age": 29, "salary":25000,
"Company":"GOOGLE"}
Output:
Kiran
29
25000
GOOGLE
for i in Employee.values():
print(i)
for i in Employee:
print(Employee[i])
Output:
Kiran
29
25000
GOOGLE

21. Iterating Dictionary
• A dictionary can be iterated using for loop as given below.
# for loop to print all the keys of a dictionary
Employee = {"Name": “Kiran",
"Age": 29,
"salary":25000,
"Company":"GOOGLE"}
for i in Employee.items():
print(i)
Output:
('Name', 'Kiran')
('Age', 29)
('salary', 25000)
('Company',
'GOOGLE')

22. Difference between List and Dictionary in Python
Comparison
Parameter
List Dictionary
Definition Collection of various elements just like an array.
Collection of elements in the hashed structure as
key-value pairs
Syntax
Placing all the elements inside square brackets [],
separated by commas(,)
Placing all key-value pairs inside curly brackets({}),
separated by a comma. Also, each key and pair is
separated by a semi-colon (:)
Index type Indices are integer values starts from value 0 The keys in the dictionary are of any given data type
Mode of Access We can access the elements using the index value We can access the elements using the keys
Order of
Elements
The default order of elements is always
maintained
No guarantee of maintaining the order
Mutability Lists are mutable in nature
Dictionaries are mutable, but keys do not allow
duplicates
Creation List object is created using list() function Dictionary object is created using dict() function
Sort()
Sort() method sorts the elements in ascending or
descending order
Sort() method sorts the keys in the dictionary by
default
Count()
Count() methods returns the number of elements
appeared in list
Count() method does not exists in dictionation
Reverse() Reverse() method reverse the list elements
Dictionary items cannot be reversed as they are the
key-value pairs

23. Write a Python program to check whether a given key already exists in a
dictionary.
d = {1: 10, 2: 20, 3: 30, 4: 40, 5: 50, 6: 60}
key=int(input(“enter key element to be searched”))
if n in d:
print("key present")
else:
print("Key not present")

24. Write a program to count the number of occurrences of character in a
string.
x = input("Enter a String: ")
count = {}
for ch in x:
count.setdefault(ch, 0)
count[ch] = count[ch] + 1
print(count)
Output:
Enter a String: Kannada
{'K': 1, 'a': 3, 'n': 2, 'd': 1}

25. Write a Python script to print a dictionary where the keys are numbers
between 1 and 15 (both included) and the values are square of keys.
d=dict()
for x in range(1,16):
d[x]=x**2
print(d)
Output:
{1: 1, 2: 4, 3: 9, 4: 16, 5: 25, 6: 36, 7: 49,
8: 64, 9: 81, 10: 100, 11: 121, 12: 144, 13: 169,
14: 196, 15: 225}