Python Dictionary

Dictionary in Python

Python dictionaries are a vital data structure in the Python programming language, allowing efficient storage and retrieval of key-value pairs. With their intuitive syntax and extensive functionality, Python dictionaries play a crucial role in various applications. This comprehensive guide dives deep into Python dictionaries, covering essential aspects like dictionary creation, accessing and modifying elements, looping through keys and values, and advanced techniques such as dictionary comprehension.

Python dictionaries are key-value pairs used to store data, much like a map. Unlike other data types that hold only a single value as an element, dictionaries allow you to store multiple values in one element. A dictionary in Python is created by enclosing a sequence of elements in curly braces, separated by commas. Each key-value pair is separated by a colon. Keys in a dictionary must be unique and immutable, while values can be of any data type and can be duplicated. Dictionaries can also be created using the built-in function dict(), with an empty dictionary created using empty curly braces {}.

For example:


# 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)

# 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)

Adding elements to a dictionary can be done in multiple ways. You can add one value at a time by defining a value along with the key, or you can update an existing value using the built-in update() method. Nested key-values can also be added to an existing dictionary. If the key-value already exists, the value gets updated; otherwise, a new key with the value is added to the dictionary.

For example:

# 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 a set of values to a single key
Dict[‘Value_set’] = 2, 3, 4
print(“\nDictionary after adding 3 elements: “)
print(Dict)# Updating an existing key’s value
Dict[2] = ‘Welcome’
print(“\nUpdated key value: “)
print(Dict)# Adding a nested key-value to a dictionary
Dict[5] = {‘Nested’: {‘1’: ‘Life’, ‘2’: ‘Geeks’}}
print(“\nAdding a nested key: “)
print(Dict)

To access items in a dictionary, refer to its key name.

Overall, the time complexity for creating a dictionary is O(len(dict)), and the space complexity is O(n). Adding elements to a dictionary has a time complexity of O(1) or O(n) and a space complexity of O(1).

Creating a Dictionary

In Python, a Dictionary can be created by placing a sequence of elements within curly {} braces, separated by ‘comma’. Dictionary holds a pair of values, one being the Key and the other corresponding pair element being its Key:value. Values in a dictionary can be of any datatype and can be duplicated, whereas keys can’t be repeated and must be immutable.
Note – Dictionary keys are case sensitive, same name but different cases of Key will be treated distinctly.

# 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)

# 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:

 

Dictionary with the use of Integer Keys: 
{1: 'Geeks', 2: 'For', 3: 'Geeks'}

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

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

# 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'}

 

Nested Dictionary:

 

 

# 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

In Python 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.

# 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, 5: {'Nested': {'1': 'Life', '2': 'Geeks'}}, 'Value_set': (2, 3, 4)}

 

Accessing elements from a Dictionary

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

# 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 acessing the element from a dictionary.

# 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 element of a nested dictionary

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

# 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

 

Removing Elements from Dictionary

 

Using del keyword

In Python Dictionary, deletion of keys can be done by using the del keyword. Using del keyword, specific values from a dictionary as well as whole dictionary can be deleted. Items in a Nested dictionary can also be deleted by using del keyword and providing specific nested key and particular key to be deleted from that nested Dictionary.
Note- del Dict will delete the entire dictionary and hence printing it after deletion will raise an Error.

# Initial Dictionary
Dict = { 5 : 'Welcome', 6 : 'To', 7 : 'Geeks',
        'A' : {1 : 'Geeks', 2 : 'For', 3 : 'Geeks'},
        'B' : {1 : 'Geeks', 2 : 'Life'}}
print("Initial Dictionary: ")
print(Dict)
# Deleting a Key value
del Dict[6]
print("\nDeleting a specific key: ")
print(Dict)
# Deleting a Key from
# Nested Dictionary
del Dict['A'][2]
print("\nDeleting a key from Nested Dictionary: ")
print(Dict)



Output:

Initial Dictionary: 
{'A': {1: 'Geeks', 2: 'For', 3: 'Geeks'}, 'B': {1: 'Geeks', 2: 'Life'}, 5: 'Welcome', 6: 'To', 7: 'Geeks'}

Deleting a specific key: 
{'A': {1: 'Geeks', 2: 'For', 3: 'Geeks'}, 'B': {1: 'Geeks', 2: 'Life'}, 5: 'Welcome', 7: 'Geeks'}

Deleting a key from Nested Dictionary: 
{'A': {1: 'Geeks', 3: 'Geeks'}, 'B': {1: 'Geeks', 2: 'Life'}, 5: 'Welcome', 7: 'Geeks'}

 

Using pop() method

Pop() method is used to return and delete the value of the key specified.

# Creating a Dictionary
Dict = {1: 'Geeks', 'name': 'For', 3: 'Geeks'}
# Deleting a key
# using pop() method
pop_ele = Dict.pop(1)
print('\nDictionary after deletion: ' + str(Dict))
print('Value associated to poped key is: ' + str(pop_ele))

Output:

Dictionary after deletion: {3: 'Geeks', 'name': 'For'}
Value associated to poped key is: Geeks

 

Using popitem() method

The popitem() returns and removes an arbitrary element (key, value) pair from the dictionary.

# Creating Dictionary
Dict = {1: 'Geeks', 'name': 'For', 3: 'Geeks'}
# Deleting an arbitrary key
# using popitem() function
pop_ele = Dict.popitem()
print("\nDictionary after deletion: " + str(Dict))
print("The arbitrary pair returned is: " + str(pop_ele))

Output:

 

Dictionary after deletion: {3: 'Geeks', 'name': 'For'}
The arbitrary pair returned is: (1, 'Geeks')

 

Using clear() method

All the items from a dictionary can be deleted at once by using clear() method.



# Creating a Dictionary
Dict = {1: 'Geeks', 'name': 'For', 3: 'Geeks'}
# Deleting entire Dictionary
Dict.clear()
print("\nDeleting Entire Dictionary: ")
print(Dict)

Output:

Deleting Entire Dictionary: 
{}