Dictionary Merge & Update Operators in Python

Dictionaries

Dictionaries (dict) are used to store the data based on a key value. Unlike Lists and Tuples, data in the dictionaries isn't stored based on the index. And, data in the dictionaries are specified using curly brackets with the key and it's corresponding value ({key1 : value1, key2 : value2,...}. Click Here to see more about dictionaries. 

In this post, we will see more about the dictionary merge and update operators introduced in Python 3.9 along with the equivalent methods to do the same before 3.9.
 

Merge (|) Operator

Merge operator would merge the elements from both the dictionaries into a new dictionary. This wouldn't update either of these dictionaries. 

If there are any common keys between both the dictionaries only one would be present in the resulting dictionary as no duplicate keys are allowed in dictionaries.

Dictionary Merge Operator in Python

In the above example, 
  • Line - 5: merge (|) operator merges the elements from both the dictionaries (first_dictionary and second_dictionary) and adds the data to the result dictionary (new_dictionary).
      new_dictionary = first_dictionary | second_dictionary   

If there are duplicate keys in both the dictionaries, data from the second dictionary would be updated in the result dictionary.

Key value '3' has same value 'THREE', so we wouldn't know data from which dictionary the resulting data is from. Below example shows how the data is populated in the result dictionary.

Dictionary merge operator in Python

This operator is same as using {**d1, **d2} for merging dictionaries. 

Let's have a look at how merging works for merging the keys alone and both keys and values. 

Merge two dictionaries in Python

In the above example, 
  • Line - 5: Using single asterisk before the dictionaries inside curly brackets would merge the keys from both the dictionaries and returns a set. 
  • Line - 6: Using two asterisks before the dictionaries inside curly brackets would merge both the key and value and returns a dictionary. 

Update (|=) Operator

Update operator would append the elements from the dictionary on the right side to the dictionary on the left side.

By just using the merge operator, data in the dictionary can be updated as below. 

Update data in a dictionary in Python

In the above example, 
  • Line - 5: We are using merge operator to merge the data from the two dictionaries and assign the data to first_dictionary. 
Instead of using the merge operator, we can use update (|=) operator to do the same. 

Update data in a dictionary in Python

In the above example, 
  • Line - 5: We are using update (|=) operator to update the dictionary on the right side to the dictionary on the left side. And, data in the right side dictionary won't be updated. 
This is same as using dictionary update() method. 

Dictionary update method in Python

These operators make the dictionary merge and update operations easier.


Hope the above info was useful in understanding Dictionary Merge and Update operators. 


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