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] = 1print("\nDictionary after adding 3 elements: ") print(Dict) # Adding set of values # to a single Key Dict['Value_set'] = 2, 3, 4print("\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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