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Python Dictionary

Dictionary in Python is an unordered collection of data values, used to store data values like a map, which unlike other Data Types that hold only single value as an element, Dictionary holds key:value pair. Key value is provided in the dictionary to make it more optimized. Each key-value pair in a Dictionary is separated by a colon :, whereas each key is separated by a ‘comma’.

A Dictionary in Python works similar to the Dictionary in a real world. Keys of a Dictionary must be unique and of immutable data type such as Strings, Integers and tuples, but the key-values can be repeated and be of any type.

Note – Keys in a dictionary doesn’t allows Polymorphism.

Creating a Dictionary

In Python, a Dictionary can be created by placing 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.

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

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

# Creating an empty Dictionary
Dict = {}
print("Empty Dictionary: ")
print(Dict)
  
# Creating a Dictionary 
# with Integer Keys
Dict = {1: 'Geeks', 2: 'For', 3: 'Geeks'}
print(" Dictionary with the use of Integer Keys: ")
print(Dict)
  
# Creating a Dictionary 
# with Mixed keys
Dict = {'Name': 'Geeks', 1: [1, 2, 3, 4]}
print(" Dictionary with the use of Mixed Keys: ")
print(Dict)
  
# Creating a Dictionary
# with dict() method
Dict = dict({1: 'Geeks', 2: 'For', 3:'Geeks'})
print(" Dictionary with the use of dict(): ")
print(Dict)
  
# Creating a Dictionary
# with each item as a Pair
Dict = dict([(1, 'Geeks'), (2, 'For')])
print(" Dictionary with each item as a pair: ")
print(Dict)

Output:

Empty Dictionary: 
{}

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 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(" Dictionary after adding 3 elements: ")
print(Dict)
  
# Adding set of values 
# to a single Key
Dict['Value_set'] = 2, 3, 4
print(" Dictionary after adding 3 elements: ")
print(Dict)
  
# Updating existing Key's Value
Dict[2] = 'Welcome'
print(" Updated key value: ")
print(Dict)
  
# Adding Nested Key value to Dictionary
Dict[5] = {'Nested' :{'1' : 'Life', '2' : 'Geeks'}}
print(" Adding 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.There is also a method called get() that will also help in acessing the element from a dictionary.

# Python program to demonstrate  
# accesing a element from a Dictionary 
  
# Creating a Dictionary 
Dict = {1: 'Geeks', 'name': 'For', 3: 'Geeks'}
  
# accessing a element using key
print("Acessing a element using key:")
print(Dict['name'])
  
# accessing a element using key
print("Acessing a element using key:")
print(Dict[1])
  
# accessing a element using get()
# method
print("Acessing a element using get:")
print(Dict.get(3))

Output:

Acessing a element using key:
For

Acessing a element using key:
Geeks

Acessing a element using get:
Geeks

Removing Elements from Dictionary

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. Other functions like pop() and popitem() can also be used for deleting specific values and arbitrary values from a Dictionary. All the items from a dictionary can be deleted at once by using clear() method. 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(" Deleting a specific key: ")
print(Dict)
  
# Deleting a Key from
# Nested Dictionary
del Dict['A'][2]
print(" Deleting a key from Nested Dictionary: ")
print(Dict)
  
# Deleting a Key 
# using pop()
Dict.pop(5)
print(" Popping specific element: ")
print(Dict)
  
# Deleting a Key
# using popitem()
Dict.popitem()
print(" Pops first element: ")
print(Dict)
  
# Deleting entire Dictionary
Dict.clear()
print(" Deleting Entire 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'}

Popping specific element: 
{'A': {1: 'Geeks', 3: 'Geeks'}, 'B': {1: 'Geeks', 2: 'Life'}, 7: 'Geeks'}

Pops first element: 
{'B': {1: 'Geeks', 2: 'Life'}, 7: 'Geeks'}

Deleting Entire Dictionary: 
{}

Dictionary Methods

Methods Description
copy() They copy() method returns a shallow copy of the dictionary.
clear() The clear() method removes all items from the dictionary.
pop() Removes and returns an element from a dictionary having the given key.
popitem() Removes the arbitrary key-value pair from the dictionary and returns it as tuple.
get() It is a conventional method to access a value for a key.
dictionary_name.values() returns a list of all the values available in a given dictionary.
str() Produces a printable string representation of a dictionary.
update() Adds dictionary dict2’s key-values pairs to dict
setdefault() Set dict[key]=default if key is not already in dict
keys() Returns list of dictionary dict’s keys
items() Returns a list of dict’s (key, value) tuple pairs
has_key() Returns true if key in dictionary dict, false otherwise
fromkeys() Create a new dictionary with keys from seq and values set to value.
type() Returns the type of the passed variable.
cmp() Compares elements of both dict.


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This article is attributed to GeeksforGeeks.org

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