Python union two dictionaries

Python: Dictionary Union Operators Explained

We all have those moments when we want to merge two Python dictionaries together based on their keys. The traditional way to do this was to use the dict.update() method. However starting Python 3.9, the much-awaited dict Union operator (| and |=) was introduced.

So what are these new Union operators and how to use them?

This is what I will discuss in this article.

Merging two Dictionaries using dict.update [Old Way]

Say you want to merge two dictionaries d1 and d2 . Here is how you would do it using the dict.update method

>>> d1 = >>> >>> d1 = >>> d2 = >>> d1.update(d2) >>> d1

A couple of things to notice:

• the update method didn’t create a new dictionary object, it just updated d1 in place
• the keys of the updated d1 is the union of the keys in d1 and d2
• for keys that exist in both d1 and d2 , the values of those keys will be the ones in d2 . In other words, d1 is actually “updated” with d2

Now let’s see how we can do the same thing with the Union operator (|)

Merging two Dictionaries using the Union Operator [Python 3.9+]

Easy and simple, this is how you do it.

As you can see, we get the same result as before. d1 is merged with d2 and keys that exist in both dictionaries are updated with the values of the operand to the right of the union operator.

There is one difference between using the union operator and the update method though.

As you can see from the code above, the union operator actually returns a new dictionary. It doesn’t modify d1 or d2 in place.

So what if you want to update d1 in place instead of creating a new dictionary?

In this case, you can still use the dict.update method, or even better, the augmented assignment union operator (|=)

Merging two Dictionaries using the Augmented Assignment Union Operator [Python 3.9+]

If you want to modify the dictionary d1 in place, here is how to do it using the union operator.

Conclusion

Starting Python 3.9+, you can now use the Union operator to merge two dictionaries together.

To merge two dictionaries in place:

To merge two dictionaries and return the result in a new dictionary object:

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How to Merge Dictionaries in Python

Ashutosh Krishna

In Python, a dictionary is a collection you use to store data in pairs. It is ordered and mutable, and it cannot store duplicate data.

We write a dictionary using curly brackets like this:

Sometimes, we need to merge two or more dictionaries to create a bigger dictionary. For example:

dict_one = < "id": 1, "name": "Ashutosh", "books": ["Python", "DSA"] >dict_two = < "college": "NSEC", "city": "Kolkata", "country": "India" >merged_dict =

In the merged_dict we have the key-value pairs of both dict_one and dict_two . This is what we wish to achieve programmatically.

There are various ways we can do this in Python:

1. Using a for loop
2. Using the dict.update() method
3. Using the ** operator
4. Using the | (Union) operator (for Python 3.9 and above)

Let’s explore each way one after another.

How to Merge Dictionaries in Python Using a For Loop

We can merge two or more dictionaries using for loop like this:

>>> dict_one = < . "id": 1, . "name": "Ashutosh", . >>>> dict_two = < . "books": ["Python", "DSA"], . "college": "NSEC", . >>>> dict_three = < . "city": "Kolkata", . "country": "India" . >>>> for key,value in dict_two.items(): . merged_dictPython union two dictionaries = value . >>> merged_dict >>> for key,value in dict_three.items(): . merged_dictPython union two dictionaries = value . >>> merged_dict

But the problem with this method is that we need to run so many loops to merge the dictionaries.

How to Merge Dictionaries in Python Using the dict.update() Method

If you explore the dict class, there are various methods inside it. One such method is the update() method which you can use to merge one dictionary into another.

>>> dict_one = < . "id": 1, . "name": "Ashutosh", . "books": ["Python", "DSA"] . >>>> dict_two = < . "college": "NSEC", . "city": "Kolkata", . "country": "India" . >>>> dict_one.update(dict_two) >>> dict_one

But the problem when we use the update() method is that it modifies one of the dictionaries. If we wish to create a third dictionary without modifying any of the other dictionaries, we cannot use this method.

Also, you can only use this method to merge two dictionaries at a time. If you wish to merge three dictionaries, you first need to merge the first two, and then merge the third one with the modified dictionary.

>>> dict_one = < . "id": 1, . "name": "Ashutosh", . >>>> dict_two = < . "books": ["Python", "DSA"], . "college": "NSEC", . >>>> dict_three = < . "city": "Kolkata", . "country": "India" . >>>> dict_one.update(dict_two) >>> dict_one >>> dict_one.update(dict_three) >>> dict_one

Let’s explore some other options.

How to Merge Dictionaries in Python Using the ** operator

You can use the double-asterisk (**) method to unpack or expand a dictionary like this:

>>> dict_one = < . "id": 1, . "name": "Ashutosh", . >>>> dict_two = < . "books": ["Python", "DSA"] . "college": "NSEC", . >>>> dict_three = < . "city": "Kolkata", . "country": "India" . >>>> merged_dict = <**dict_one, **dict_two, **dict_three>>>> merged_dict

Using the ** operator to merge the dictionaries doesn’t affect any of the dictionaries.

How to Merge Dictionaries in Python Using the | Operator

Starting with Python 3.9, we can use the Union ( | ) operator to merge two or more dictionaries.

>>> dict_one = < . "id": 1, . "name": "Ashutosh", . >>>> dict_two = < . "books": ["Python", "DSA"], . "college": "NSEC", . >>>> dict_three = < . "city": "Kolkata", . "country": "India" . >>>> merged_dict = dict_one | dict_two | dict_three >>> merged_dict

This is the most convenient method available for merging dictionaries in Python.

Conclusion

We have explored several different methods for merging dictionaries. If you have Python 3.9 or above, you should use the | operator. But if you use older versions of Python, you can still use the other methods discussed above.

Ashutosh Krishna

Application Developer at Thoughtworks India

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