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How to create a Dictionary in Python

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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.

Create a Dictionary in Python

In Python, a dictionary can be created by placing a sequence of elements within curly {} braces, separated by a ‘comma’. Let us see a few examples to see how we can create a dictionary in Python.

Define a Dictionary with Items

In this example, we first declared an empty dictionary D, then added the elements from the Python list L into the dictionary. The first element of each of the sublists is the key and the second element is the value. We will store the key-value pair dynamically.

Python3




# 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:
        D[L[i][0]].append(L[i][1])
     
    # If the key is not present
    # in the dictionary then add
    # the key-value pair
    else:
        D[L[i][0]]= []
        D[L[i][0]].append(L[i][1])
         
print(D)


Output:

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

An Overview of Keys and Values

In this example, we will add another element to the existing dictionary in Python. We are provided with the key and value separately and will add this pair to the dictionary my_dict.

Python3




# 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
try:
    my_dict[key_ref].append(my_list_of_things)
     
except KeyError:
    my_dict = {**my_dict, **{key_ref: my_list_of_things}}
     
print(my_dict)


Output:

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

Built-in Dictionary Functions Methods in Python

A dictionary in Python can also be created by the built-in function dict(). In this example, we first created an empty dictionary using curly braces {}. Then we used the dict() method and passed a list to it.

Python3




# Creating an empty Dictionary
Dict = {}
my_list = [(1, 'Geeks'), (2, 'For')]
print(my_list)
 
# Creating a Dictionary
# with each item as a Pair
print("\nDictionary with the use of dict(): ")
Dict = dict(my_list)
print(Dict)


Output:

[(1, 'Geeks'), (2, 'For')]

Dictionary with the use of dict(): 
{1: 'Geeks', 2: 'For'}


Last Updated : 04 Jul, 2023
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