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How to create a Dictionary in Python

  • Last Updated : 31 Dec, 2019

Dictionaries are the fundamental data structure in Python and are very important for Python programmers. They are an unordered collection of data values, used to store data values like a map. Dictionaries are mutable, which means they can be changed. They offer a time complexity of O(1) and have been heavily optimized for memory overhead and lookup speed efficiency.

Example 1: The first element of each of the sublists is the key and the second element is the value. We want to store the key-value pair dynamically.

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# Python program to demonstrate
# dynamic dictionary creation
# Initialize an empty dictionary
D = {} 
L = [['a', 1], ['b', 2], ['a', 3], ['c', 4]]
# Loop to add key-value pair
# to dictionary
for i in range(len(L)):
    # If the key is already 
    # present in dictionary
    # then append the value 
    # to the list of values
    if L[i][0] in D:
    # If the key is not present
    # in the dictionary then add
    # the key-value pair
        D[L[i][0]]= []


{'a': [1, 3], 'b': [2], 'c': [4]}

Example 2:

# Python program to demonstrate
# dynamic dictionary creation
# Key to be added
key_ref = 'More Nested Things'
my_dict = {
    'Nested Things': [{'name', 'thing one'}, {'name', 'thing two'}]
# Value to be added
my_list_of_things = [{'name', 'thing three'}, {'name', 'thing four'}]
# try-except to take care of errors
# while adding key-value pair
except KeyError:
    my_dict = {**my_dict, **{key_ref: my_list_of_things}}


 'Nested Things': [{'name', 'thing one'}, {'thing two', 'name'}], 
 'More Nested Things': [{'name', 'thing three'}, {'thing four', 'name'}]

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