Python | Merging two Dictionaries

In Python, a dictionary is a data structure that contains the element in the key-value pair in which keys are used to access the values in the dictionary. Python has some inbuilt dictionaries like defaultdict. In this article, we will see various ways to merge two dictionaries.

Example

Input: dict1 = {'a': 10, 'b': 8}
           dict2 = {'d': 6, 'c': 4}
Output: {'a': 10, 'b': 8, 'd': 6, 'c': 4}

Merging Two Dictionaries in Python

There are various ways in which Dictionaries can be merged by using various functions and constructors in Python. Below are some following ways:

• Using update()
• Using unpacking operator
• Using merge | operator
• Using loops and keys() method
• Using dict constructor
• Using the dict() constructor with the union operator (|)
• Using reduce()

Python update()

By using the method update() in Python, one list can be merged into another. But in this, the second list is merged into the first list and no new list is created. It returns None. In this example, we are using the update function to merge two dictionaries.

Python3

# Python code to merge dict using update() method
def Merge(dict1, dict2):
    return(dict2.update(dict1))


# Driver code
dict1 = {'a': 10, 'b': 8}
dict2 = {'d': 6, 'c': 4}

# This returns None
print(Merge(dict1, dict2))

# changes made in dict2
print(dict2)

Output:

None
{'c': 4, 'a': 10, 'b': 8, 'd': 6}

Time complexity: O(1) 
Auxiliary space: O(1)

Python unpacking operator

Using ** [double star] is a shortcut that allows you to pass multiple arguments to a function directly using a dictionary. For more information refer **kwargs in Python. Using this we first pass all the elements of the first dictionary into the third one and then pass the second dictionary into the third. This will replace the duplicate keys of the first dictionary.

Python3

# Python code to merge dict using a single
# expression
def Merge(dict1, dict2):
    res = {**dict1, **dict2}
    return res

# Driver code
dict1 = {'a': 10, 'b': 8}
dict2 = {'d': 6, 'c': 4}
dict3 = Merge(dict1, dict2)
print(dict3)

Output

{'a': 10, 'b': 8, 'd': 6, 'c': 4}

Time complexity: O(1)
Auxiliary complexity: O(n)

Python Merge Dictionaries Using | in Python 3.9

In the latest update of python now we can use “|” operator to merge two dictionaries. It is a very convenient method to merge dictionaries. In this example, we are using | operator to merge two dictionaries.

Python3

# code
# Python code to merge dict using a single
# expression
def Merge(dict1, dict2):
    res = dict1 | dict2
    return res

# Driver code
dict1 = {'x': 10, 'y': 8}
dict2 = {'a': 6, 'b': 4}
dict3 = Merge(dict1, dict2)
print(dict3)

# This code is contributed by virentanti16

Output:

{'x': 10, 'a': 6,  'b': 4, 'y': 8}

Time complexity: O(1)
Auxiliary space: O(N)

Using for loop and keys() method

In this example, we are using loop and key() method to merge two dictionaries.

Python3

# code
# Python code to merge dictionary
def Merge(dict1, dict2):
    for i in dict2.keys():
        dict1[i]=dict2[i]
    return dict1

# Driver code
dict1 = {'x': 10, 'y': 8}
dict2 = {'a': 6, 'b': 4}
dict3 = Merge(dict1, dict2)
print(dict3)

# This code is contributed by Bhavya Koganti

Output

{'x': 10, 'y': 8, 'a': 6, 'b': 4}

Python Merge Dictionaries Using ChainMap Class

In this example, we are merging dictionaries in Python by using the built-in ChainMap class from the collections module. This class allows you to create a single view of multiple dictionaries, and any updates or changes made to the ChainMap will be reflected in the underlying dictionaries.

Python3

from collections import ChainMap

# create the dictionaries to be merged
dict1 = {'a': 1, 'b': 2}
dict2 = {'c': 3, 'd': 4}

# create a ChainMap with the dictionaries as elements
merged_dict = ChainMap(dict1, dict2)

# access and modify elements in the merged dictionary
print(merged_dict['a'])  # prints 1
print(merged_dict['c'])  # prints 3
merged_dict['c'] = 5  # updates value in dict2
print(merged_dict['c'])  # prints 5

# add a new key-value pair to the merged dictionary
merged_dict['e'] = 6  # updates dict1
print(merged_dict['e'])  # prints 6

Output

1
3
5
6

Merge Two Dictionaries in Python Using dict constructor:

In this example, we are using dict constructor to merge two dictionaries.

Python3

def merge_dictionaries(dict1, dict2):
    merged_dict = dict1.copy()
    merged_dict.update(dict2)
    return merged_dict

# Driver code
dict1 = {'x': 10, 'y': 8}
dict2 = {'a': 6, 'b': 4}

print(merge_dictionaries(dict1, dict2))

Output

{'x': 10, 'y': 8, 'a': 6, 'b': 4}

Time Complexity: O(N)
Auxiliary Space: O(N)

Python Merge Dictionaries Using dict() constructor and union operator (|)

This method uses the dict() constructor with the union operator (|) to merge two dictionaries. The union operator combines the keys and values of the two dictionaries, and any common keys in the two dictionaries take the value from the second dictionary.

Python3

# method to merge two dictionaries using the dict() constructor with the union operator (|)
def Merge(dict1, dict2):
    # create a new dictionary by merging the items of the two dictionaries using the union operator (|)
    merged_dict = dict(dict1.items() | dict2.items())
    # return the merged dictionary
    return merged_dict

# Driver code
dict1 = {'a': 10, 'b': 8}
dict2 = {'d': 6, 'c': 4}

# merge the two dictionaries using the Merge() function
merged_dict = Merge(dict1, dict2)

# print the merged dictionary
print(merged_dict)

Output

{'d': 6, 'b': 8, 'c': 4, 'a': 10}

Time complexity: O(n), where n is the total number of key-value pairs in both dictionaries.
Auxiliary Space: O(n), where n is the total number of key-value pairs in both dictionaries

Python Merge Two Dictionaries Using reduce():

In this example, we are merging two dictionaries using reduce() function. In this method, we define a merge function then takes two dictionaries as arguments and returns their merge.

Python3

from functools import reduce

def merge_dictionaries(dict1, dict2):
    merged_dict = dict1.copy()
    merged_dict.update(dict2)
    return merged_dict


dict1 = {'a': 10, 'b': 8}
dict2 = {'d': 6, 'c': 4}

dict_list = [dict1, dict2]  # Put the dictionaries into a list

result_dict = reduce(merge_dictionaries, dict_list)

print(result_dict)
#This code is contributed by Rayudu.

Output

{'a': 10, 'b': 8, 'd': 6, 'c': 4}

Time complexity :O(n), where n is the number of dictionaries in the dict_list list.
Auxiliary complexity : O(m), where m is the total number of key-value pairs in all the dictionaries.