Python Dictionary
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
# Creating a Dictionary # with Integer Keys Dict = { 1 : 'Geeks' , 2 : 'For' , 3 : 'Geeks' } print ( "\nDictionary with the use of Integer Keys: " ) print ( Dict ) # Creating a Dictionary # with Mixed keys 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
# Creating an empty Dictionary Dict = {} print ( "Empty Dictionary: " ) print ( Dict ) # Creating a Dictionary # with dict() method Dict = dict ({ 1 : 'Geeks' , 2 : 'For' , 3 : 'Geeks' }) print ( "\nDictionary with the use of dict(): " ) print ( Dict ) # Creating a Dictionary # with each item as a Pair 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
# Creating a Nested Dictionary # as shown in the below image 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
# Creating an empty Dictionary Dict = {} print ( "Empty Dictionary: " ) print ( Dict ) # Adding elements one at a time Dict [ 0 ] = 'Geeks' Dict [ 2 ] = 'For' Dict [ 3 ] = 1 print ( "\nDictionary after adding 3 elements: " ) print ( Dict ) # Adding set of values # to a single Key Dict [ 'Value_set' ] = 2 , 3 , 4 print ( "\nDictionary after adding 3 elements: " ) print ( Dict ) # Updating existing Key's Value Dict [ 2 ] = 'Welcome' print ( "\nUpdated key value: " ) print ( Dict ) # Adding Nested Key value to Dictionary 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
# Python program to demonstrate # accessing a element from a Dictionary # Creating a Dictionary Dict = { 1 : 'Geeks' , 'name' : 'For' , 3 : 'Geeks' } # accessing a element using key print ( "Accessing a element using key:" ) print ( Dict [ 'name' ]) # accessing a element using key 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
# Creating a Dictionary Dict = { 1 : 'Geeks' , 'name' : 'For' , 3 : 'Geeks' } # accessing a element using get() # method 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
# Creating a Dictionary Dict = { 'Dict1' : { 1 : 'Geeks' }, 'Dict2' : { 'Name' : 'For' }} # Accessing element using key print ( Dict [ 'Dict1' ]) print ( Dict [ 'Dict1' ][ 1 ]) print ( Dict [ 'Dict2' ][ 'Name' ]) |
Output:
{1: 'Geeks'} Geeks For
Deleting Elements using del Keyword
The items of the dictionary can be deleted by using the del keyword as given below.
Python3
# Python program to demonstrate # Deleting Elements using del Keyword # Creating a Dictionary Dict = { 1 : 'Geeks' , 'name' : 'For' , 3 : 'Geeks' } print ( "Dictionary =" ) print ( Dict ) #Deleting some of the Dictionar data del ( Dict [ 1 ]) print ( "Data after deletion Dictionary=" ) print ( Dict ) |
Output
Dictionary ={1: 'Geeks', 'name': 'For', 3: 'Geeks'} Data after deletion Dictionary={'name': 'For', 3: 'Geeks'}
Dictionary methods
Method | Description |
dic.clear() | Remove all the elements from the dictionary |
dict.copy() | Returns a copy of the dictionary |
dict.get(key, default = “None”) | Returns the value of specified key |
dict.items() | Returns a list containing a tuple for each key value pair |
dict.keys() | Returns a list containing dictionary’s keys |
dict.update(dict2) | Updates dictionary with specified key-value pairs |
dict.values() | Returns a list of all the values of dictionary |
pop() | Remove the element with specified key |
popItem() | Removes the last inserted key-value pair |
dict.setdefault(key,default= “None”) | set the key to the default value if the key is not specified in the dictionary |
dict.has_key(key) | returns true if the dictionary contains the specified key. |
dict.get(key, default = “None”) | used to get the value specified for the passed key. |
Python3
# demo for all dictionary methods dict1 = { 1 : "Python" , 2 : "Java" , 3 : "Ruby" , 4 : "Scala" } # copy() method dict2 = dict1.copy() print (dict2) # clear() method dict1.clear() print (dict1) # get() method print (dict2.get( 1 )) # items() method print (dict2.items()) # keys() method print (dict2.keys()) # pop() method dict2.pop( 4 ) print (dict2) # popitem() method dict2.popitem() print (dict2) # update() method dict2.update({ 3 : "Scala" }) print (dict2) # values() method 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'])
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