Python combine two dictionaries

5 Ways to Merge Dictionaries in Python

Different ways to merge Python dictionaries along with the new operators in Python 3.9

In Python, a dictionary is a data structure that contains elements in the form of a key-value pair where keys are used to access the values of the dictionary. Python dictionaries are unordered and mutable i.e. the elements of the dictionaries can be changed.

In this article, we will explore five different ways to merge two or more dictionaries, along with a crude way.

For this article, let us create two dictionaries d1 and d2 which we want to concatenate into a single dictionary:

d1 = 'Canada': 'Ottawa', 
'United States': 'Washington D. C.'>

d2 = 'Malaysia': 'Kuala Lumpur'>

The Crude Way

You can merge two dictionaries by iterating over the key-value pairs of the second dictionary with the first one.

d3 = d1.copy()
for key, value in d2.items(): 
d3Python combine two dictionaries = value 

print(d3)

Output:

 'Canada': 'Ottawa', 
'United States': 'Washington D. C.', 
'France': 'Paris', 
'Malaysia': 'Kuala Lumpur'>

Now, let us see cleaner and better ways of merging the dictionaries:

Method 1: Using the update method

Dictionary has a method update() which merges the dictionary with the items from the other dictionary in place and overwrites existing keys.

d4 = d1.copy()
d4.update(d2)

print(d4)

Output:
 'Canada': 'Ottawa', 
'United States': 'Washington D. C.', 
'France': 'Paris', 
'Malaysia': 'Kuala Lumpur'>

The update method modifies the current dictionary. So you might want to create a copy of the dictionary before operating on the dictionary.

Method 2: Using the unpacking operator

We can merge dictionaries in one line by simply using the unpacking operator (**).

Output:
 'Canada': 'Ottawa', 
'United States': 'Washington D. C.', 
'France': 'Paris', 
'Malaysia': 'Kuala Lumpur'>

We can also merge multiple dictionaries using this method.

Method 3: Using collections.ChainMap

This is, perhaps, the least known method to merge dictionaries.
ChainMap class from the Collections module groups multiple dictionaries in a single view.

from collections import ChainMap
d6 = ChainMap(d1, d2)

print(d6)

Output:
ChainMap( 'India': 'Delhi', 
'United States': 'Washington D. C.'>, 
 'Malaysia': 'Kuala Lumpur'>)

This method returns an object of the ChainMap class. We can, still, use this object as we would use any other dictionary. e.g. d6[’India’] will return ‘Delhi’ .

However, in the case of the same keys in two dictionaries, this method will return the value of the first dictionary, unlike the other methods which return the value from the second dictionary.

x = 
y = 

z = ChainMap(x, y)
z['B']

# outputs 2

Method 4: Unpacking the second dictionary

We can merge the dictionaries by unpacking the second dictionary.

Output:
 'Canada': 'Ottawa', 
'United States': 'Washington D. C.', 
'France': 'Paris', 
'Malaysia': 'Kuala Lumpur'>

However, this method only works if the keys of the second dictionary are strings.

Output:
TypeError: keyword arguments must be strings

Method 5: Using the merge operator

Python 3.9 has introduced the merge operator (|) in the dict class.
Using the merge operator, we can combine dictionaries in a single line of code.

Output:
 'Canada': 'Ottawa', 
'United States': 'Washington D. C.', 
'France': 'Paris', 
'Malaysia': 'Kuala Lumpur'>

We can also merge the dictionaries in place by using the update operator (|=).

Resources

The code snippets used in this article can be found on my GitHub page.

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Merge Dictionaries in Python: 8 Standard Methods (with code)

A dictionary is one of the fundamental and most used data structures in python programming. It is not an uncommon situation where you wish to combine two or more dictionaries. In this article, we will study various ways to merge two dictionaries in Python in different situations along with the code.

But before that, let us have a brief look at dictionaries in python below.

What is Dictionary in Python?

A dictionary is an unordered collection of data elements stored in key-value pairs, unlike any other data structure holding the data as a single value element. You can create a dictionary by placing the key-value pair inside the curly brackets(<>) separated by a comma (,).

Moreover, the key and value elements are separated by placing a semi-colon(:) between them. Dictionary in python is ordered, changeable, and does not allow duplicates. That means the dictionary cannot have two items with the same key; hence, dictionary keys are immutable.

sample_dict = < "Language_1": "Python", "Langauge_2": "C++", "Language_3": "Java" > print(sample_dict)

'Language_1': 'Python', 'Langauge_2': 'C++', 'Language_3': 'Java'>

When is the need to merge dictionaries?

Dictionaries are used to store key-value pairs. They can prove exceptionally useful to store data that has to be indexed. For example, if you were to keep a record of all the passwords used on a device, you would implement a dictionary that keeps a track of all the different password values to the corresponding keys of all the apps they are used for.

But what happens when you need to combine the password records of two or more devices you own? This is the situation where we need it. We merge two dictionaries when we need to store two different parts of a common database.

How to Merge Dictionaries in Python?

Below are the 8 unique methods by which you can concatenate two dictionaries in python:

1) Using update() method

You can merge two dictionaries in python using the update() method. Using the update method, the first dictionary is merged with the other one by overwriting it. Hence, no new dictionary is created, and it returns None. If both dictionaries contain the same key and different values, then the final output will overwrite the value of the latter dictionary.

dict_1 = 'John': 15, 'Rick': 10, 'Misa' : 12 > dict_2 = 'Bonnie': 18,'Rick': 20,'Matt' : 16 > dict_1.update(dict_2) print('Updated dictionary:') print(dict_1)

'John': 15, 'Rick': 20, 'Misa': 12, 'Bonnie': 18, 'Matt': 16>

2) Using merge(|) operator

You can merge two dictionaries using the | operator. It is a very convenient method to merge dictionaries; however, it is only used in the python 3.9 version or more.

def Merge(dict_1, dict_2): result = dict_1 | dict_2 return result # Driver code dict_1 = 'John': 15, 'Rick': 10, 'Misa' : 12 > dict_2 = 'Bonnie': 18,'Rick': 20,'Matt' : 16 > dict_3 = Merge(dict_1, dict_2) print(dict_3)

'John': 15, 'Rick': 20, 'Misa': 12, 'Bonnie': 18, 'Matt': 16>

3) Using ** operator

The simplest way to concatenate two dictionaries in python is by using the unpacking operator(**). By applying the «**» operator to the dictionary, it expands its content being the collection of key-value pairs.

For example, if you apply «**» to dict_1 as shown below, the output will collect key-value pairs stored inside dict_1.

dict_1 = 'John': 15, 'Rick': 10, 'Misa': 12> print(dict(**dict_1))

Now, to combine two dictionaries using «**», we will make use of an additional dictionary to store the final output. The content of dict_1 and dict_2 will expand using the «**» operator and combine to form dict_3.

Similar to the update method, if both dictionaries has the same key with different values, then the final output will contain the value of the second dictionary. Take a look at the below example for understanding the working of the «**» operator.

dict_1 = 'John': 15, 'Rick': 10, 'Misa' : 12 > dict_2 = 'Bonnie': 18,'Rick': 20,'Matt' : 16 > dict_3 = **dict_1,**dict_2> print(dict_3)

'John': 15, 'Rick': 20, 'Misa': 12, 'Bonnie': 18, 'Matt': 16>

You can also merge three dictionaries at the same time using the “**” operator, as shown in the below example.

dict_1 = 'John': 15, 'Rick': 10, 'Misa' : 12 > dict_2 = 'Bonnie': 18,'Rick': 20,'Matt' : 16 > dict_3 = 'Stefan': 19, 'Riya': 14, 'Lora': 17> dict_4 = **dict_1,**dict_2, **dict_3> print (dict_4)

'John': 15, 'Rick': 20, 'Misa': 12, 'Bonnie': 18, 'Matt': 16, 'Stefan': 19, 'Riya': 14, 'Lora': 17>

4) Unpacking the second dictionary

Unpacking the second dictionary using the «**» operator will help you merge two dictionaries and get your final output. However, this method is not highly used and recommended because it will only work if the keys of the second dictionary are compulsorily strings. If any int value is encountered, then it will raise the «TypeError» method.

dict_1='John': 15, 'Rick': 10, 'Misa' : 12 > dict_2='Bonnie': 18,'Rick': 20,'Matt' : 16 > dict_3=dict(dict_1,**dict_2) print (dict_3)

'John': 15, 'Rick': 20, 'Misa': 12, 'Bonnie': 18, 'Matt': 16>

5) Using collection.ChainMap() method

This is one of the least known methods to merge two dictionaries in python. Using collection.ChainMap() method, you have to make use of the collection module from the ChainMap library which will help you to group multiple dictionaries in a single view.

If both dictionaries contain the same key/s, then the value of the first dictionary is fetched in the final output. Note that we will make use of the “import..from..” syntax to import the collection module as shown in the below example:

from collections import ChainMap dict_1='John': 15, 'Rick': 10, 'Misa' : 12 > dict_2='Bonnie': 18,'Rick': 20,'Matt' : 16 > dict_3 = ChainMap(dict_1, dict_2) print(dict_3) print(dict(dict_3))

ChainMap('John': 15, 'Rick': 10, 'Misa': 12>, 'Bonnie': 18, 'Rick': 20, 'Matt': 16>) 'Bonnie': 18, 'Rick': 10, 'Matt': 16, 'John': 15, 'Misa': 12>

6) Using itertools.chain()

The iterator returns the element from the first iterable until it’s empty and then jumps to the next iterable. Basically, it will handle the consecutive sequence as a single sequence.

As the dictionary is also iterable, we can use the chain() function from itertools class and merge two dictionaries. The return type of this method will be an object, and hence, we can convert the dictionary using the dict() constructor.

import itertools dict_1='John': 15, 'Rick': 10, 'Misa': 12> dict_2='Bonnie': 18, 'Rick': 20, 'Matt': 16> dict_3=itertools.chain(dict_1.items(),dict_2.items()) #Returns an iterator object print (dict_3) print(dict(dict_3))

itertools.chain object at 0x000001EB8E312520> 'John': 15, 'Rick': 20, 'Misa': 12, 'Bonnie': 18, 'Matt': 16>

7) Using dictionary comprehension

We can combine two dictionaries in python using dictionary comprehension. Here, we also use the for loop to iterate through the dictionary items and merge them to get the final output. If both dictionaries have common keys, then the final output using this method will contain the value of the second dictionary.

dict_1='John': 15, 'Rick': 10, 'Misa': 12> dict_2='Bonnie': 18, 'Rick': 20, 'Matt': 16> dict_3=for d in (dict_1,dict_2) for k,v in d.items()> print (dict_3)

'John': 15, 'Rick': 20, 'Misa': 12, 'Bonnie': 18, 'Matt': 16>

8) Add values of common keys

In all of the above methods, if both the dictionary contains the different data values of the same keys, the values in the final output were getting overridden. What if you wish to preserve all the value in the final output? For this purpose, you can merge the dictionaries such that it adds the values of the common keys in the list and return the final output.

In this method, we will also make use of for loop to traverse through the keys and values in the dictionary inside the function. Later, we will call the function to get the final output as merged dictionaries as shown below:

dict_1 = 'John': 15, 'Rick': 10, 'Misa': 12> dict_2 = 'Bonnie': 18, 'Rick': 20, 'Matt': 16> def mergeDictionary(dict_1, dict_2): dict_3 = **dict_1, **dict_2> for key, value in dict_3.items(): if key in dict_1 and key in dict_2: dict_3Python combine two dictionaries = [value , dict_1Python combine two dictionaries] return dict_3 dict_3 = mergeDictionary(dict_1, dict_2) print(dict_3)

'John': 15, 'Rick': [20, 10], 'Misa': 12, 'Bonnie': 18, 'Matt': 16>

Conclusion

Working with multiple dictionaries is one of the common scenarios while programming in python. But combining one or more dictionaries is often an unpleasant task if you are unaware of the techniques to do the same. Therefore, we have presented various methods to merge dictionaries in python.

Still, if you face any difficulty, get in touch with live python tutors to clear your doubt.

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