Python Dictionary - GeeksforGeeks

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("\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 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) 
  
# Creating a Nested Dictionary 
Dict = {1: {'A' : 'Geeks', 'B' : 'For', 'C' : 'Geeks'}, 
        2: {'D' : 'Welcome', 'E' : 'To', 'F' : 'Geeks'}} 
print("\nNested Dictionary: ") 
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: 
{1: {'B': 'For', 'C': 'Geeks', 'A': 'Geeks'}, 2: {'F': 'Geeks', 'E': 'To', 'D': 'Welcome'}}

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)}

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("\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) 
  
# Deleting a Key  
# using pop() 
Dict.pop(5) 
print("\nPopping specific element: ") 
print(Dict) 
  
# Deleting a Key 
# using popitem() 
Dict.popitem() 
print("\nPops first element: ") 
print(Dict) 
  
# Deleting entire Dictionary 
Dict.clear() 
print("\nDeleting 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.
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.