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Python | Add new keys to a dictionary
  • Difficulty Level : Easy
  • Last Updated : 21 Nov, 2018

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’. 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.

Let’s see all different ways of adding new keys to a dictionary.

Create a dictionary first.




# Let's create a dictionary, the functional way
  
# Create your dictionary class
class my_dictionary(dict):
  
    # __init__ function
    def __init__(self):
        self = dict()
          
    # Function to add key:value
    def add(self, key, value):
        self[key] = value
  
# Main Function
dict_obj = my_dictionary()
  
dict_obj.add(1, 'Geeks')
dict_obj.add(2, 'forGeeks')
  
print(dict_obj)

Output:



{1: 'Geeks', 2: 'forGeeks'}

Method #1: Using Subscript notation

This method will create a new key\value pair on a dictionary by assigning a value to that key. If the key doesn’t exist, it will be added and will point to that value. If the key exists, the current value it points to will be overwritten.




dict = {'key1':'geeks', 'key2':'fill_me'}
print("Current Dict is: ", dict)
  
# using the subscript notation
# Dictionary_Name[New_Key_Name] = New_Key_Value
dict['key2'] = 'for'
dict['key3'] = 'geeks'
print("Updated Dict is: ", dict)

Output:

Current Dict is:  {'key1': 'geeks', 'key2': 'fill_me'}
Updated Dict is:  {'key3': 'geeks', 'key1': 'geeks', 'key2': 'for'}

 
Method #2: Using update() method

When we have to update/add a lots of key/value to dictionary, update() method is suitable.




dict = {'key1':'geeks', 'key2':'for'}
print("Current Dict is: ", dict)
  
# adding key3
dict.update({'key3':'geeks'})
print("Updated Dict is: ", dict)
  
# adding dict1 (key4 and key5) to dict
dict1 = {'key4':'is', 'key5':'fabulous'}
dict.update(dict1)
print(dict)
  
# by assigning 
dict.update(newkey1 ='portal')
print(dict)

Output:

Current Dict is:  {'key2': 'for', 'key1': 'geeks'}
Updated Dict is:  {'key2': 'for', 'key3': 'geeks', 'key1': 'geeks'}

{'key4': 'is', 'key2': 'for', 'key5': 'fabulous', 'key3': 'geeks', 'key1': 'geeks'}

{'key3': 'geeks', 'newkey1': 'portal', 'key1': 'geeks',
        'key4': 'is', 'key2': 'for', 'key5': 'fabulous'}

 
Method #3: __setitem__ method to add a key-value pair to a dict

Using __setitem__ method should be avoided because of its poor performance(computationally inefficient).




dict = {'key1':'geeks', 'key2':'for'}
  
# using __setitem__ method
dict.__setitem__('newkey2', 'GEEK')
print(dict)

Output:

{'key2': 'for', 'newkey2': 'GEEK', 'key1': 'geeks'}

 
Method #4: Using * operator

Using this method we can merge old dictionary and new key/value pair in another dictionary.




dict = {'a': 1, 'b': 2}
  
# will create a new dictionary
new_dict = {**dict, **{'c': 3}}
  
print(dict)
print(new_dict)

Output:

{'b': 2, 'a': 1}
{'b': 2, 'c': 3, 'a': 1}

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