How do you create a dictionary in python?

Dictionary in Python is a collection of keys values, used to store data values like a map, which, unlike other data types which hold only a single value as an element.

Example of Dictionary in Python

Dictionary holds key:value pair. Key-Value is provided in the dictionary to make it more optimized.

Python3

Dict = {1: 'Geeks', 2: 'For', 3: 'Geeks'}

print[Dict]

Output:

{1: 'Geeks', 2: 'For', 3: 'Geeks'}

Creating a Dictionary

In Python, a dictionary can be created by placing a sequence of elements within curly {} braces, separated by ‘comma’. Dictionary holds pairs of values, one being the Key and the other corresponding pair element being its Key:value. Values in a dictionary can be of any data type and can be duplicated, whereas keys can’t be repeated and must be immutable.

Note – Dictionary keys are case sensitive, the same name but different cases of Key will be treated distinctly.

Python3

Dict = {1: 'Geeks', 2: 'For', 3: 'Geeks'}

print["\nDictionary with the use of Integer Keys: "]

print[Dict]

Dict = {'Name': 'Geeks', 1: [1, 2, 3, 4]}

print["\nDictionary with the use of Mixed Keys: "]

print[Dict]

Output:

Dictionary with the use of Integer Keys: 
{1: 'Geeks', 2: 'For', 3: 'Geeks'}
Dictionary with the use of Mixed Keys: 
{'Name': 'Geeks', 1: [1, 2, 3, 4]}

Dictionary can also be created by the built-in function dict[]. An empty dictionary can be created by just placing to curly braces{}.

Python3

Dict = {}

print["Empty Dictionary: "]

print[Dict]

Dict = dict[{1: 'Geeks', 2: 'For', 3: 'Geeks'}]

print["\nDictionary with the use of dict[]: "]

print[Dict]

Dict = dict[[[1, 'Geeks'], [2, 'For']]]

print["\nDictionary with each item as a pair: "]

print[Dict]

Output:

Empty Dictionary: 
{}
Dictionary with the use of dict[]: 
{1: 'Geeks', 2: 'For', 3: 'Geeks'}
Dictionary with each item as a pair: 
{1: 'Geeks', 2: 'For'}

Complexities for Creating a Dictionary:

Time complexity: O[len[dict]]
Space complexity: O[n]

Nested Dictionary

Python3

Dict = {1: 'Geeks', 2: 'For',

3: {'A': 'Welcome', 'B': 'To', 'C': 'Geeks'}}

print[Dict]

Output:

{1: 'Geeks', 2: 'For', 3: {'A': 'Welcome', 'B': 'To', 'C': 'Geeks'}}

Adding elements to a Dictionary

Addition of elements can be done in multiple ways. One value at a time can be added to a Dictionary by defining value along with the key e.g. Dict[Key] = ‘Value’. Updating an existing value in a Dictionary can be done by using the built-in update[] method. Nested key values can also be added to an existing Dictionary.

Note- While adding a value, if the key-value already exists, the value gets updated otherwise a new Key with the value is added to the Dictionary.

Python3

Dict = {}

print["Empty Dictionary: "]

print[Dict]

Dict[0] = 'Geeks'

Dict[2] = 'For'

Dict[3] = 1

print["\nDictionary after adding 3 elements: "]

print[Dict]

Dict['Value_set'] = 2, 3, 4

print["\nDictionary after adding 3 elements: "]

print[Dict]

Dict[2] = 'Welcome'

print["\nUpdated key value: "]

print[Dict]

Dict[5] = {'Nested': {'1': 'Life', '2': 'Geeks'}}

print["\nAdding a Nested Key: "]

print[Dict]

Output:

Empty Dictionary: 
{}
Dictionary after adding 3 elements: 
{0: 'Geeks', 2: 'For', 3: 1}
Dictionary after adding 3 elements: 
{0: 'Geeks', 2: 'For', 3: 1, 'Value_set': [2, 3, 4]}
Updated key value: 
{0: 'Geeks', 2: 'Welcome', 3: 1, 'Value_set': [2, 3, 4]}
Adding a Nested Key: 
{0: 'Geeks', 2: 'Welcome', 3: 1, 'Value_set': [2, 3, 4], 5: 
{'Nested': {'1': 'Life', '2': 'Geeks'}}}

Complexities for Adding elements in a Dictionary:

Time complexity: O[1]/O[n]

Space complexity: O[1]

Accessing elements of a Dictionary

In order to access the items of a dictionary refer to its key name. Key can be used inside square brackets.

Python3

Dict = {1: 'Geeks', 'name': 'For', 3: 'Geeks'}

print["Accessing a element using key:"]

print[Dict['name']]

print["Accessing a element using key:"]

print[Dict[1]]

Output:

Accessing a element using key:
For
Accessing a element using key:
Geeks

There is also a method called get[] that will also help in accessing the element from a dictionary.This method accepts key as argument and returns the value.

Complexities for Accessing elements in a Dictionary:

Time complexity: O[1]

Space complexity: O[1]

Python3

Dict = {1: 'Geeks', 'name': 'For', 3: 'Geeks'}

print["Accessing a element using get:"]

print[Dict.get[3]]

Output:

Accessing a element using get:
Geeks

Accessing an element of a nested dictionary

In order to access the value of any key in the nested dictionary, use indexing [] syntax.

Python3

Dict = {'Dict1': {1: 'Geeks'},

'Dict2': {'Name': 'For'}}

print[Dict['Dict1']]

print[Dict['Dict1'][1]]

print[Dict['Dict2']['Name']]

Output:

{1: 'Geeks'}
Geeks
For

Dictionary methods

clear[] – Remove all the elements from the dictionary
copy[] – Returns a copy of the dictionary
get[] – Returns the value of specified key
items[] – Returns a list containing a tuple for each key value pair
keys[] – Returns a list containing dictionary’s keys
pop[] – Remove the element with specified key
popitem[] – Removes the last inserted key-value pair
update[] – Updates dictionary with specified key-value pairs
values[] – Returns a list of all the values of dictionary

Python3

dict1 = {1: "Python", 2: "Java", 3: "Ruby", 4: "Scala"}

dict2 = dict1.copy[]

print[dict2]

dict1.clear[]

print[dict1]

print[dict2.get[1]]

print[dict2.items[]]

print[dict2.keys[]]

dict2.pop[4]

print[dict2]

dict2.popitem[]

print[dict2]

dict2.update[{3: "Scala"}]

print[dict2]

print[dict2.values[]]

Output:

{1: 'Python', 2: 'Java', 3: 'Ruby', 4: 'Scala'}
{}
Python
dict_items[[[1, 'Python'], [2, 'Java'], [3, 'Ruby'], [4, 'Scala']]]
dict_keys[[1, 2, 3, 4]]
{1: 'Python', 2: 'Java', 3: 'Ruby'}
{1: 'Python', 2: 'Java'}
{1: 'Python', 2: 'Java', 3: 'Scala'}
dict_values[['Python', 'Java', 'Scala']]