Merge Two Dictionaries in Python

Often we come across scenarios where we need to merge two dictionaries in Python to create a bigger dictionary to perform operations based on our requirements. Various methods can be implemented, but a few of the efficient ones are discussed below in the article with an elaborative explanation of the detailed code example to get a better glance at the concept around merging two dictionaries in Python.

How to Merge Two Dictionaries in Python?

Dictionaries in Python play a key part when working with data, such as data loaded from the internet. Sometimes there might arise scenarios where we have to merge the two dictionaries as one and use this bigger dictionary to perform other operations. To start creating this bigger dictionary we shall be deep diving into methods listed below to understand how we can efficiently merge two dictionaries in Python.

• Using the dict.update() method
• Using ** operator in Python
• Using the | (Union) operator
• Using loops and keys() method
• Using dict constructor
• Using the dict() constructor with the union operator (|)
• Using reduce()

Let us start to discuss each of them with code examples and elaborative explanations as we move ahead in the article.

Below are two sample dictionaries that we shall be used to merge two dictionaries with the various methods listed above.

How we initialliy have the two dictionaries is show below:

With various methods, we shall discuss below, we can obatin the following output of merge two dictionaries in Python:

Output:

Merge Two Dictionaries in Python: Using the dict.update()

With the dict.update() method, we can merge two dictionaries in Python as in this method the elements of one dictionary are merged with the elements from the other dictionary in place and overwrites the existing keys. Here the current dictionary also gets modified, hence it is advised to create a copy of this dictionary before actually operating on the dictionary.

Output:

• Time Complexity: O(n), n is the number of keys in the second dictionary.
• Space Complexity: O(1), as we are updating the first dictionary in place.

Merge Two Dictionaries in Python: Using the ** Operator

Using the ** operator is a concise and intuitive way to merge two or more dictionaries in Python. This method unpacks the key-value pairs of each dictionary into a new dictionary, effectively combining them. It's particularly useful when you need to quickly integrate the contents of several dictionaries, with the rightmost dictionary's values taking precedence in case of key conflicts.

Output:

• Time Complexity: O(n + m), n and m are the sizes of the two dictionaries being merged. This is because it creates a new dictionary containing all items from both dictionaries.
• Space Complexity: O(n + m), as it creates a new dictionary containing all the items from the two original dictionaries.

Merge Two Dictionaries in Python: Using the | (Union) Operator

In Python, the | (Union) operator is a convenient way to merge two dictionaries. It combines the keys and values from both dictionaries into a new dictionary, where values from the second dictionary overwrite those from the first if there are duplicate keys. This method provides a readable way to join dictionaries without the need for loops or the update() method.

Output:

• Time Complexity: O(n + m), similar to the unpacking operator, as it creates a new dictionary with all items from the first and second dictionaries.
• Space Complexity: O(n + m), because it results in a new dictionary with combined items from both dictionaries.

Merge Two Dictionaries in Python: Using loops and keys()

This method involves iterating over the keys of the second dictionary and adding each key-value pair to the first dictionary. This method is straightforward and gives you control over how you want to handle conflicts between dictionaries.

Example:

Output:

• Time Complexity: O(n), n is the number of keys in the second dictionary.
• Space Complexity: O(1), as we are updating the first dictionary in place.

Merge Two Dictionaries in Python: Using dict Constructor

This method generates a new dictionary by combining the key-value pairs of both dictionaries using the dict() constructor and the ** operator, which unpacks the key-value pairs of each dictionary.

Example:

Output:

• Time Complexity: O(n + m), n and m are the number of keys in the first and second dictionaries, respectively.
• Space Complexity: O(n + m), as a new dictionary is created.

Merge Two Dictionaries in Python: Using the dict() Constructor with the Union Operator (|)

This method uses the union operator (|) to merge two dictionaries into a new one. It is the most concise method and was introduced in Python 3.9.

Example:

Output:

• Time Complexity: O(n + m), n and m are the number of keys in the first and second dictionaries, respectively.
• Space Complexity: O(n + m), as a new dictionary is created.

Merge Two Dictionaries in Python: Using reduce()

This method uses the reduce() function from the functools module to apply a function of two arguments cumulatively to the items of an iterable, from left to right, so as to reduce the iterable to a single value. In this case, the function merges dictionaries.

Example:

Output:

• Time Complexity: O(n + m), where n and m are the number of keys in the first and second dictionaries, respectively. This is because each merge operation with the ** operator is O(n + m).
• Space Complexity: O(n + m) for the final merged dictionary. If there are more dictionaries in the iterable, temporary dictionaries will be created during the reduction process, which may increase the space complexity.